diff --git a/Directory.Build.props b/Directory.Build.props
index 5b17ca697b..f36acfd7a1 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -128,4 +128,7 @@
   </ItemGroup>
 
   <Import Condition="Exists('User.Directory.Build.props')" Project="User.Directory.Build.props" />
+  <PropertyGroup>
+    <DefineConstants Condition="'$(DO_SAFE_COLLECTION_WRAPPER)'!='false'">$(DefineConstants);DO_SAFE_COLLECTION_WRAPPER</DefineConstants>
+  </PropertyGroup>
 </Project>
diff --git a/Orm/Xtensive.Orm.Tests.Core/Helpers/TopologicalSorterTest.cs b/Orm/Xtensive.Orm.Tests.Core/Helpers/TopologicalSorterTest.cs
index cc257e8ef0..8f931b5f33 100644
--- a/Orm/Xtensive.Orm.Tests.Core/Helpers/TopologicalSorterTest.cs
+++ b/Orm/Xtensive.Orm.Tests.Core/Helpers/TopologicalSorterTest.cs
@@ -63,7 +63,7 @@ private static void InternalPerformanceTest(int nodeCount, int averageConnection
                 List<Node<int, int>> removedEdges;
                 var result = TopologicalSorter.SortToList(nodes, out removedEdges);
                 if (!allowLoops)
-                    Assert.AreEqual(nodeCount, result.Count);
+                    Assert.AreEqual(nodeCount, result.Count());
             }
             GC.GetTotalMemory(true);
         }
diff --git a/Orm/Xtensive.Orm/Core/Extensions/ListExtensions.cs b/Orm/Xtensive.Orm/Core/Extensions/ListExtensions.cs
index d50fdcb846..a26fc585b1 100644
--- a/Orm/Xtensive.Orm/Core/Extensions/ListExtensions.cs
+++ b/Orm/Xtensive.Orm/Core/Extensions/ListExtensions.cs
@@ -8,7 +8,8 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-
+using System.Linq;
+using System.Runtime.CompilerServices;
 
 namespace Xtensive.Core
 {
@@ -119,5 +120,25 @@ public static void EnsureIndexIsValid(this IList list, int index)
       if (index < 0 || index >= list.Count)
         throw new IndexOutOfRangeException(Strings.ExIndexOutOfRange);
     }
+
+#if DO_SAFE_COLLECTION_WRAPPER
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static IReadOnlyList<T> AsSafeWrapper<T>(this List<T> list) => list.AsReadOnly();
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static IReadOnlyList<T> AsSafeWrapper<T>(this IReadOnlyList<T> list) => Array.AsReadOnly(list.ToArray());
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static IReadOnlyList<T> AsSafeWrapper<T>(this T[] array) => Array.AsReadOnly(array);
+#else
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static IReadOnlyList<T> AsSafeWrapper<T>(this List<T> list) => list;
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static IReadOnlyList<T> AsSafeWrapper<T>(this IReadOnlyList<T> list) => list;
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static IReadOnlyList<T> AsSafeWrapper<T>(this T[] array) => array;
+#endif
   }
-}
\ No newline at end of file
+}
diff --git a/Orm/Xtensive.Orm/Linq/ExpressionVisitor.cs b/Orm/Xtensive.Orm/Linq/ExpressionVisitor.cs
index 3ec6657269..10b3400c27 100644
--- a/Orm/Xtensive.Orm/Linq/ExpressionVisitor.cs
+++ b/Orm/Xtensive.Orm/Linq/ExpressionVisitor.cs
@@ -18,7 +18,7 @@ namespace Xtensive.Linq
   /// </summary>
   public abstract class ExpressionVisitor : ExpressionVisitor<Expression>
   {
-    protected override ReadOnlyCollection<Expression> VisitExpressionList(ReadOnlyCollection<Expression> expressions)
+    protected override IReadOnlyList<Expression> VisitExpressionList(ReadOnlyCollection<Expression> expressions)
     {
       bool isChanged = false;
       var results = new List<Expression>(expressions.Count);
@@ -28,7 +28,7 @@ protected override ReadOnlyCollection<Expression> VisitExpressionList(ReadOnlyCo
         results.Add(p);
         isChanged |= !ReferenceEquals(expression, p);
       }
-      return isChanged ? results.AsReadOnly() : expressions;
+      return isChanged ? results.AsSafeWrapper() : expressions;
     }
 
     /// <summary>
@@ -38,8 +38,8 @@ protected override ReadOnlyCollection<Expression> VisitExpressionList(ReadOnlyCo
     /// <returns>Visit result.</returns>
     protected virtual ElementInit VisitElementInitializer(ElementInit initializer)
     {
-      ReadOnlyCollection<Expression> arguments = VisitExpressionList(initializer.Arguments);
-      if (arguments!=initializer.Arguments) {
+      var arguments = VisitExpressionList(initializer.Arguments);
+      if (arguments != initializer.Arguments) {
         return Expression.ElementInit(initializer.AddMethod, arguments);
       }
       return initializer;
@@ -50,7 +50,7 @@ protected virtual ElementInit VisitElementInitializer(ElementInit initializer)
     /// </summary>
     /// <param name="original">The original element initializer list.</param>
     /// <returns>Visit result.</returns>
-    protected virtual ReadOnlyCollection<ElementInit> VisitElementInitializerList(ReadOnlyCollection<ElementInit> original)
+    protected virtual IReadOnlyList<ElementInit> VisitElementInitializerList(ReadOnlyCollection<ElementInit> original)
     {
       var results = new List<ElementInit>();
       bool isChanged = false;
@@ -60,7 +60,7 @@ protected virtual ReadOnlyCollection<ElementInit> VisitElementInitializerList(Re
         results.Add(p);
         isChanged |= !ReferenceEquals(originalIntializer, p);
       }
-      return isChanged ? results.AsReadOnly() : original;
+      return isChanged ? results.AsSafeWrapper() : original;
     }
 
     /// <inheritdoc/>
@@ -253,7 +253,7 @@ protected virtual MemberMemberBinding VisitMemberMemberBinding(MemberMemberBindi
     /// </summary>
     /// <param name="original">The original binding list.</param>
     /// <returns>Visit result.</returns>
-    protected virtual ReadOnlyCollection<MemberBinding> VisitBindingList(ReadOnlyCollection<MemberBinding> original)
+    protected virtual IReadOnlyList<MemberBinding> VisitBindingList(ReadOnlyCollection<MemberBinding> original)
     {
       var results = new List<MemberBinding>();
       bool isChanged = false;
@@ -263,7 +263,7 @@ protected virtual ReadOnlyCollection<MemberBinding> VisitBindingList(ReadOnlyCol
         results.Add(p);
         isChanged |= !ReferenceEquals(originalBinding, p);
       }
-      return isChanged ? results.AsReadOnly() : original;
+      return isChanged ? results.AsSafeWrapper() : original;
     }
 
     protected virtual MemberListBinding VisitMemberListBinding(MemberListBinding binding)
diff --git a/Orm/Xtensive.Orm/Linq/ExpressionVisitor{TResult}.cs b/Orm/Xtensive.Orm/Linq/ExpressionVisitor{TResult}.cs
index b9900b549c..c749422741 100644
--- a/Orm/Xtensive.Orm/Linq/ExpressionVisitor{TResult}.cs
+++ b/Orm/Xtensive.Orm/Linq/ExpressionVisitor{TResult}.cs
@@ -9,6 +9,7 @@
 using System.Collections.ObjectModel;
 using System.Linq.Expressions;
 using Xtensive.Reflection;
+using Xtensive.Core;
 
 
 
@@ -142,14 +143,14 @@ protected virtual TResult Visit(Expression e)
     /// </summary>
     /// <param name="expressions">The expression list.</param>
     /// <returns>Visit result.</returns>
-    protected virtual ReadOnlyCollection<TResult> VisitExpressionList(ReadOnlyCollection<Expression> expressions)
+    protected virtual IReadOnlyList<TResult> VisitExpressionList(ReadOnlyCollection<Expression> expressions)
     {
       var results = new List<TResult>(expressions.Count);
       for (int i = 0, n = expressions.Count; i < n; i++) {
         var p = Visit(expressions[i]);
         results.Add(p);
       }
-      return results.AsReadOnly();
+      return results.AsSafeWrapper();
     }
 
     /// <summary>
diff --git a/Orm/Xtensive.Orm/Orm/Building/Builders/AssociationBuilder.cs b/Orm/Xtensive.Orm/Orm/Building/Builders/AssociationBuilder.cs
index db882628e5..bb8e4da57d 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Builders/AssociationBuilder.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Builders/AssociationBuilder.cs
@@ -80,7 +80,7 @@ public static void BuildReversedAssociation(BuildingContext context, Association
       AssociationInfo existing;
       if (!context.Model.Associations.TryGetValue(association.Name, out existing)) {
         context.Model.Associations.Add(association);
-        association.Ancestors.AddRange(field.Associations);
+        association.AddAncestors(field.Associations);
 
         var associationsToRemove = field.Associations
           .Where(a => a.TargetType == association.TargetType)
diff --git a/Orm/Xtensive.Orm/Orm/Building/Builders/ModelBuilder.cs b/Orm/Xtensive.Orm/Orm/Building/Builders/ModelBuilder.cs
index 3c6050ea40..571daa2606 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Builders/ModelBuilder.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Builders/ModelBuilder.cs
@@ -239,7 +239,7 @@ private void PreprocessAssociations()
               .ToList();
             if (pairedAssociations.Count > 0) {
               foreach (var paired in pairedAssociations) {
-                paired.First.Ancestors.AddRange(interfaceAssociations);
+                paired.First.AddAncestors(interfaceAssociations);
                 if (paired.First.TargetType.IsInterface || typesWithProcessedInheritedAssociations.Contains(paired.First.TargetType))
                   AssociationBuilder.BuildReversedAssociation(context, paired.First, paired.Second);
                 else {
@@ -288,11 +288,13 @@ bool associationFilter(AssociationInfo a)
             var interfaceAssociationsToRemove = interfaceAssociations
               .Where(ia => ia.OwnerType != association.OwnerType)
               .ToList();
-            association.Ancestors.AddRange(interfaceAssociationsToRemove);
+            association.AddAncestors(interfaceAssociationsToRemove);
             foreach (var interfaceAssociation in interfaceAssociationsToRemove)
               interfaceAssociations.Remove(interfaceAssociation);
           }
-          refField.Associations.AddRange(interfaceAssociations);
+          if (interfaceAssociations.Count > 0) {
+            refField.Associations.AddRange(interfaceAssociations);
+          }
           foreach (var association in inheritedAssociations) {
             if (!refField.Associations.Contains(association.Name))
               refField.Associations.Add(association);
diff --git a/Orm/Xtensive.Orm/Orm/Building/Builders/TypeBuilder.cs b/Orm/Xtensive.Orm/Orm/Building/Builders/TypeBuilder.cs
index 0db467dc80..eeb9c8719d 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Builders/TypeBuilder.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Builders/TypeBuilder.cs
@@ -8,6 +8,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
+
 using Xtensive.Core;
 using Xtensive.Orm.Building.Definitions;
 using Xtensive.Orm.Building.DependencyGraph;
@@ -52,7 +53,7 @@ public TypeInfo BuildType(TypeDef typeDef)
           MappingSchema = typeDef.MappingSchema,
           HasVersionRoots = TypeHelper.GetInterfacesUnordered(typeDef.UnderlyingType)
             .Any(static type => type == typeof(IHasVersionRoots)),
-          Validators = validators.AsReadOnly(),
+          Validators = validators.AsSafeWrapper(),
         };
 
         if (typeDef.StaticTypeId != null) {
@@ -234,6 +235,16 @@ private FieldInfo BuildDeclaredField(TypeInfo type, FieldDef fieldDef)
     {
       BuildLog.Info(nameof(Strings.LogBuildingDeclaredFieldXY), type.Name, fieldDef.Name);
 
+      var validators = fieldDef.Validators;
+
+      if (fieldDef.IsStructure && DeclaresOnValidate(fieldDef.ValueType)) {
+        validators.Add(new StructureFieldValidator());
+      }
+
+      if (fieldDef.IsEntitySet && DeclaresOnValidate(fieldDef.ValueType)) {
+        validators.Add(new EntitySetFieldValidator());
+      }
+
       var fieldInfo = new FieldInfo(type, fieldDef.Attributes) {
         UnderlyingProperty = fieldDef.UnderlyingProperty,
         Name = fieldDef.Name,
@@ -244,17 +255,9 @@ private FieldInfo BuildDeclaredField(TypeInfo type, FieldDef fieldDef)
         Length = fieldDef.Length,
         Scale = fieldDef.Scale,
         Precision = fieldDef.Precision,
-        Validators = fieldDef.Validators,
+        Validators = validators,
       };
 
-      if (fieldInfo.IsStructure && DeclaresOnValidate(fieldInfo.ValueType)) {
-        fieldInfo.Validators.Add(new StructureFieldValidator());
-      }
-
-      if (fieldInfo.IsEntitySet && DeclaresOnValidate(fieldInfo.ValueType)) {
-        fieldInfo.Validators.Add(new EntitySetFieldValidator());
-      }
-
       type.Fields.Add(fieldInfo);
 
       if (fieldInfo.IsEntitySet) {
diff --git a/Orm/Xtensive.Orm/Orm/Building/BuildingContext.cs b/Orm/Xtensive.Orm/Orm/Building/BuildingContext.cs
index e911e81c5c..a4835f2fea 100644
--- a/Orm/Xtensive.Orm/Orm/Building/BuildingContext.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/BuildingContext.cs
@@ -62,12 +62,12 @@ public sealed class BuildingContext
     /// <summary>
     /// Gets all available <see cref="IModule"/> implementations.
     /// </summary>
-    public ICollection<IModule> Modules { get; private set; }
+    public IReadOnlyList<IModule> Modules { get; }
 
     /// <summary>
     /// Gets all available <see cref="IModule2"/> implementations.
     /// </summary>
-    public ICollection<IModule2> Modules2 { get; private set; }
+    public IReadOnlyList<IModule2> Modules2 { get; }
 
     internal ModelDefBuilder ModelDefBuilder { get; set; }
 
diff --git a/Orm/Xtensive.Orm/Orm/Building/Definitions/FieldDef.cs b/Orm/Xtensive.Orm/Orm/Building/Definitions/FieldDef.cs
index 3fb5d381fd..b7814e0c2a 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Definitions/FieldDef.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Definitions/FieldDef.cs
@@ -287,7 +287,7 @@ public string PairTo
     /// <summary>
     /// Gets of <see cref="IPropertyValidator"/> instances associated with this field.
     /// </summary>
-    public List<IPropertyValidator> Validators { get; private set; }
+    public List<IPropertyValidator> Validators { get; } = new();
 
     internal bool IsDeclaredAsNullable
     {
@@ -325,7 +325,6 @@ internal FieldDef(Type valueType, Validator validator)
       if ((valueType.IsClass || valueType.IsInterface) && !IsStructure)
         attributes |= FieldAttributes.Nullable;
       ValueType = valueType;
-      Validators = new List<IPropertyValidator>();
 
       // Nullable<T>
       if (valueType.IsNullable()) {
diff --git a/Orm/Xtensive.Orm/Orm/Building/Definitions/TypeDef.cs b/Orm/Xtensive.Orm/Orm/Building/Definitions/TypeDef.cs
index 48c4c8fd18..071193139b 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Definitions/TypeDef.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Definitions/TypeDef.cs
@@ -113,7 +113,7 @@ public NodeCollection<TypeDef> Implementors
     /// <summary>
     /// Gets <see cref="IObjectValidator"/> instances associated with this type.
     /// </summary>
-    public List<IObjectValidator> Validators { get; private set; }
+    public List<IObjectValidator> Validators { get; } = new();
 
     /// <summary>
     /// Gets or sets the type discriminator value.
@@ -219,8 +219,6 @@ internal TypeDef(ModelDefBuilder builder, Type type, Validator validator)
       implementors = IsInterface
         ? new NodeCollection<TypeDef>(this, "Implementors")
         : NodeCollection<TypeDef>.Empty;
-
-      Validators = new List<IObjectValidator>();
     }
   }
 }
\ No newline at end of file
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/IHasNestedNodes.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/IHasNestedNodes.cs
index 50d1354035..283995835e 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/IHasNestedNodes.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/IHasNestedNodes.cs
@@ -11,10 +11,10 @@ namespace Xtensive.Orm.Internals.Prefetch
 {
   internal interface IHasNestedNodes
   {
-    ReadOnlyCollection<BaseFieldNode> NestedNodes { get; }
+    IReadOnlyList<BaseFieldNode> NestedNodes { get; }
 
     IReadOnlyCollection<Key> ExtractKeys(object target);
 
-    IHasNestedNodes ReplaceNestedNodes(ReadOnlyCollection<BaseFieldNode> nestedNodes);
+    IHasNestedNodes ReplaceNestedNodes(IReadOnlyList<BaseFieldNode> nestedNodes);
   }
 }
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/KeyExtractorNode.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/KeyExtractorNode.cs
index 9b3af6a0e6..9eb36884c4 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/KeyExtractorNode.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/KeyExtractorNode.cs
@@ -15,7 +15,7 @@ internal class KeyExtractorNode<T> : Node, IHasNestedNodes
   {
     public Func<T, IReadOnlyCollection<Key>> KeyExtractor { get; }
 
-    public ReadOnlyCollection<BaseFieldNode> NestedNodes { get; }
+    public IReadOnlyList<BaseFieldNode> NestedNodes { get; }
 
     IReadOnlyCollection<Key> IHasNestedNodes.ExtractKeys(object target)
     {
@@ -27,10 +27,8 @@ public IReadOnlyCollection<Key> ExtractKeys(T target)
       return KeyExtractor.Invoke(target);
     }
 
-    public IHasNestedNodes ReplaceNestedNodes(ReadOnlyCollection<BaseFieldNode> nestedNodes)
-    {
-      return new KeyExtractorNode<T>(KeyExtractor, nestedNodes);
-    }
+    public IHasNestedNodes ReplaceNestedNodes(IReadOnlyList<BaseFieldNode> nestedNodes) =>
+      new KeyExtractorNode<T>(KeyExtractor, nestedNodes);
 
     public override Node Accept(NodeVisitor visitor)
     {
@@ -42,7 +40,7 @@ protected override string GetDescription()
       return $"KeyExtraction<{typeof(T).Name}>";
     }
 
-    public KeyExtractorNode(Func<T, IReadOnlyCollection<Key>> extractor, ReadOnlyCollection<BaseFieldNode> nestedNodes)
+    public KeyExtractorNode(Func<T, IReadOnlyCollection<Key>> extractor, IReadOnlyList<BaseFieldNode> nestedNodes)
       : base("*")
     {
       ArgumentValidator.EnsureArgumentNotNull(extractor, nameof(extractor));
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeAggregator.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeAggregator.cs
index d239cce2a7..f5a7c1efbd 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeAggregator.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeAggregator.cs
@@ -7,6 +7,7 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
+using Xtensive.Core;
 
 namespace Xtensive.Orm.Internals.Prefetch
 {
@@ -26,7 +27,7 @@ public static IList<KeyExtractorNode<T>> Aggregate(IEnumerable<KeyExtractorNode<
       return result;
     }
 
-    public override ReadOnlyCollection<BaseFieldNode> VisitNodeList(ReadOnlyCollection<BaseFieldNode> nodes)
+    public override IReadOnlyList<BaseFieldNode> VisitNodeList(IReadOnlyList<BaseFieldNode> nodes)
     {
       var result = new List<BaseFieldNode>();
       foreach (var group in nodes.Where(n => n!=null).GroupBy(n => n.Path)) {
@@ -36,11 +37,11 @@ public override ReadOnlyCollection<BaseFieldNode> VisitNodeList(ReadOnlyCollecti
           result.Add(node);
         else {
           var nodeToVisit = (BaseFieldNode) container.ReplaceNestedNodes(
-            new ReadOnlyCollection<BaseFieldNode>(group.Cast<IHasNestedNodes>().SelectMany(c => c.NestedNodes).ToList()));
+            group.Cast<IHasNestedNodes>().SelectMany(c => c.NestedNodes).ToList().AsSafeWrapper());
           result.Add((BaseFieldNode) Visit(nodeToVisit));
         }
       }
-      return new ReadOnlyCollection<BaseFieldNode>(result);
+      return result.AsSafeWrapper();
     }
 
     // Constructor
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeBuilder.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeBuilder.cs
index ebc5ad3fad..d0321d4432 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeBuilder.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeBuilder.cs
@@ -1,4 +1,4 @@
-// Copyright (C) 2012-2020 Xtensive LLC.
+// Copyright (C) 2012-2020 Xtensive LLC.
 // This code is distributed under MIT license terms.
 // See the License.txt file in the project root for more information.
 // Created by: Denis Krjuchkov
@@ -119,9 +119,9 @@ private IEnumerable<BaseFieldNode> VisitMemberAccess(MemberExpression access)
       return EnumerableUtils.One(result);
     }
 
-    private static ObjectModel.ReadOnlyCollection<BaseFieldNode> WrapNodes(IEnumerable<BaseFieldNode> nodes)
+    private static IReadOnlyList<BaseFieldNode> WrapNodes(IEnumerable<BaseFieldNode> nodes)
     {
-      return nodes.ToList().AsReadOnly();
+      return nodes.ToList().AsSafeWrapper();
     }
 
     private IEnumerable<BaseFieldNode> VisitChildren(Expression expression)
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeVisitor.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeVisitor.cs
index ce371ec875..ec98bc9a94 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeVisitor.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/NodeVisitor.cs
@@ -4,7 +4,9 @@
 // Created by: Alexis Kochetov
 // Created:    2011.01.14
 
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using Xtensive.Core;
 
 namespace Xtensive.Orm.Internals.Prefetch
 {
@@ -17,7 +19,7 @@ public virtual Node Visit(Node node)
       return node.Accept(this);
     }
 
-    public virtual ReadOnlyCollection<BaseFieldNode> VisitNodeList(ReadOnlyCollection<BaseFieldNode> nodes)
+    public virtual IReadOnlyList<BaseFieldNode> VisitNodeList(IReadOnlyList<BaseFieldNode> nodes)
     {
       BaseFieldNode[] list = null;
       var index = 0;
@@ -35,9 +37,7 @@ public virtual ReadOnlyCollection<BaseFieldNode> VisitNodeList(ReadOnlyCollectio
         }
         index++;
       }
-      return list==null
-        ? nodes
-        : new ReadOnlyCollection<BaseFieldNode>(list);
+      return list?.AsSafeWrapper() ?? nodes;
     }
 
     public virtual Node VisitKeyExtractorNode<T>(KeyExtractorNode<T> keyExtractorNode)
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/ReferenceNode.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/ReferenceNode.cs
index 5bceb8dc84..13577e3796 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/ReferenceNode.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/ReferenceNode.cs
@@ -15,7 +15,7 @@ internal sealed class ReferenceNode : BaseFieldNode, IHasNestedNodes
   {
     public TypeInfo ReferenceType { get; private set; }
 
-    public ReadOnlyCollection<BaseFieldNode> NestedNodes { get; private set; }
+    public IReadOnlyList<BaseFieldNode> NestedNodes { get; private set; }
 
     public IReadOnlyCollection<Key> ExtractKeys(object target)
     {
@@ -30,10 +30,8 @@ public IReadOnlyCollection<Key> ExtractKeys(object target)
         : new[] {referenceKey};
     }
 
-    public IHasNestedNodes ReplaceNestedNodes(ReadOnlyCollection<BaseFieldNode> nestedNodes)
-    {
-      return new ReferenceNode(Path, Field, ReferenceType, NestedNodes);
-    }
+    public IHasNestedNodes ReplaceNestedNodes(IReadOnlyList<BaseFieldNode> nestedNodes) =>
+      new ReferenceNode(Path, Field, ReferenceType, NestedNodes);
 
     public override Node Accept(NodeVisitor visitor)
     {
@@ -42,7 +40,7 @@ public override Node Accept(NodeVisitor visitor)
 
     // Constructors
 
-    public ReferenceNode(string path, FieldInfo field, TypeInfo referenceType, ReadOnlyCollection<BaseFieldNode> nestedNodes)
+    public ReferenceNode(string path, FieldInfo field, TypeInfo referenceType, IReadOnlyList<BaseFieldNode> nestedNodes)
       : base(path, field)
     {
       ReferenceType = referenceType;
diff --git a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/SetNode.cs b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/SetNode.cs
index 5650ef97a9..8d00ff0ac4 100644
--- a/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/SetNode.cs
+++ b/Orm/Xtensive.Orm/Orm/Internals/Prefetch/Nodes/SetNode.cs
@@ -14,7 +14,7 @@ namespace Xtensive.Orm.Internals.Prefetch
 {
   internal class SetNode : BaseFieldNode, IHasNestedNodes
   {
-    public ReadOnlyCollection<BaseFieldNode> NestedNodes { get; }
+    public IReadOnlyList<BaseFieldNode> NestedNodes { get; }
 
     public TypeInfo ElementType { get; }
 
@@ -30,7 +30,7 @@ public IReadOnlyCollection<Key> ExtractKeys(object target)
       return fetchedKeys.ToArray(fetchedKeys.Count);
     }
 
-    public IHasNestedNodes ReplaceNestedNodes(ReadOnlyCollection<BaseFieldNode> nestedNodes) =>
+    public IHasNestedNodes ReplaceNestedNodes(IReadOnlyList<BaseFieldNode> nestedNodes) =>
       new SetNode(Path, Field, ElementType, NestedNodes);
 
     public override Node Accept(NodeVisitor visitor) => visitor.VisitSetNode(this);
@@ -38,7 +38,7 @@ public IHasNestedNodes ReplaceNestedNodes(ReadOnlyCollection<BaseFieldNode> nest
     // Constructors
 
 
-    public SetNode(string path, FieldInfo field, TypeInfo elementType, ReadOnlyCollection<BaseFieldNode> nestedNodes)
+    public SetNode(string path, FieldInfo field, TypeInfo elementType, IReadOnlyList<BaseFieldNode> nestedNodes)
       : base(path, field)
     {
       ElementType = elementType;
diff --git a/Orm/Xtensive.Orm/Orm/Linq/Translator.Materialization.cs b/Orm/Xtensive.Orm/Orm/Linq/Translator.Materialization.cs
index 0da819fec2..080980bc39 100644
--- a/Orm/Xtensive.Orm/Orm/Linq/Translator.Materialization.cs
+++ b/Orm/Xtensive.Orm/Orm/Linq/Translator.Materialization.cs
@@ -89,9 +89,8 @@ private static ProjectionExpression Optimize(ProjectionExpression origin)
       if (usedColumns.Count == 0)
         usedColumns.Add(0);
       if (usedColumns.Count < origin.ItemProjector.DataSource.Header.Length) {
-        var usedColumnsArray = usedColumns.ToArray();
-        var resultProvider = new SelectProvider(originProvider, usedColumnsArray);
-        var itemProjector = origin.ItemProjector.Remap(resultProvider, usedColumnsArray);
+        var resultProvider = new SelectProvider(originProvider, usedColumns);
+        var itemProjector = origin.ItemProjector.Remap(resultProvider, usedColumns);
         var result = origin.Apply(itemProjector);
         return result;
       }
diff --git a/Orm/Xtensive.Orm/Orm/Model/AssociationInfo.cs b/Orm/Xtensive.Orm/Orm/Model/AssociationInfo.cs
index e30f4cc7e5..3d84508a72 100644
--- a/Orm/Xtensive.Orm/Orm/Model/AssociationInfo.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/AssociationInfo.cs
@@ -5,6 +5,7 @@
 // Created:    2008.07.02
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using Xtensive.Core;
 using Xtensive.Tuples.Transform;
@@ -62,10 +63,8 @@ public TypeInfo AuxiliaryType
     /// Gets or sets ancestor association.
     /// </summary>
     /// <value>The ancestor.</value>
-    public NodeCollection<AssociationInfo> Ancestors
-    {
-      get { return ancestors; }
-    }
+    public NodeCollection<AssociationInfo> Ancestors =>
+      ancestors ??= new NodeCollection<AssociationInfo>(this, "Ancestors");
 
     /// <summary>
     /// Gets the underlying index for this instance.
@@ -194,6 +193,24 @@ public Tuple ExtractForeignKey(TypeInfo type, Tuple tuple)
 
       throw new InvalidOperationException(Strings.ExCanNotExtractForeignKey);
     }
+    
+    internal void AddAncestors(IReadOnlyList<AssociationInfo> associations)
+    {
+      if (associations.Count > 0) {
+        Ancestors.AddRange(associations); 
+      }
+    }
+
+    public override void Lock(bool recursive)
+    {
+      base.Lock(recursive);
+      if (!recursive)
+        return;
+      if (ancestors is null || ancestors.Count == 0)
+        ancestors = NodeCollection<AssociationInfo>.Empty;
+      else
+        ancestors.Lock(false);
+    }
 
     /// <inheritdoc/>
     public override void UpdateState()
@@ -245,7 +262,6 @@ public AssociationInfo(FieldInfo ownerField, TypeInfo targetType, Multiplicity m
       Multiplicity = multiplicity;
       OnOwnerRemove = onOwnerRemove;
       OnTargetRemove = onTargetRemove;
-      ancestors = new NodeCollection<AssociationInfo>(this, "Ancestors");
     }
   }
-}
\ No newline at end of file
+}
diff --git a/Orm/Xtensive.Orm/Orm/Model/ColumnIndexMap.cs b/Orm/Xtensive.Orm/Orm/Model/ColumnIndexMap.cs
index 9e49ed67c6..9d859e1db5 100644
--- a/Orm/Xtensive.Orm/Orm/Model/ColumnIndexMap.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/ColumnIndexMap.cs
@@ -6,6 +6,7 @@
 
 using System;
 using System.Collections.Generic;
+using Xtensive.Core;
 
 
 namespace Xtensive.Orm.Model
diff --git a/Orm/Xtensive.Orm/Orm/Model/FieldInfo.cs b/Orm/Xtensive.Orm/Orm/Model/FieldInfo.cs
index b15369e0e1..1f8701ccc4 100644
--- a/Orm/Xtensive.Orm/Orm/Model/FieldInfo.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/FieldInfo.cs
@@ -51,7 +51,6 @@ public sealed class FieldInfo : MappedNode,
     private TypeInfo declaringType;
     private FieldInfo parent;
     private ColumnInfo column;
-    private NodeCollection<AssociationInfo> associations;
     private Type itemType;
     private string originalName;
     internal SegmentTransform valueExtractor;
@@ -60,7 +59,6 @@ public sealed class FieldInfo : MappedNode,
     private int fieldId;
     private int? cachedHashCode;
 
-    private IList<IPropertyValidator> validators;
     private Segment<int> mappingInfo;
 
     #region IsXxx properties
@@ -545,24 +543,22 @@ public ColumnInfo Column
     /// <param name="targetType"></param>
     public AssociationInfo GetAssociation(TypeInfo targetType)
     {
-      if (associations.Count == 0)
-        return null;
-
-      if (associations.Count == 1)
-        return associations[0];
+      switch (Associations.Count) {
+        case 0:
+          return null;
+        case 1:
+          return Associations[0];
+      }
 
       var ordered = IsLocked
-        ? associations
-        : associations.Reorder();
+        ? Associations
+        : Associations.Reorder();
 
       return ordered.FirstOrDefault(
         a => a.TargetType.UnderlyingType.IsAssignableFrom(targetType.UnderlyingType));
     }
 
-    public NodeCollection<AssociationInfo> Associations
-    {
-      get { return associations; }
-    }
+    public NodeCollection<AssociationInfo> Associations { get; private set; }
 
     /// <summary>
     /// Gets or sets field's adapter index.
@@ -582,14 +578,7 @@ public int AdapterIndex
     /// Gets <see cref="IPropertyValidator"/> instances
     /// associated with this field.
     /// </summary>
-    public IList<IPropertyValidator> Validators
-    {
-      get { return validators; }
-      internal set {
-        EnsureNotLocked();
-        validators = value;
-      }
-    }
+    public IReadOnlyList<IPropertyValidator> Validators { get; init; }
 
     /// <summary>
     /// Gets value indicating if this field
@@ -658,7 +647,7 @@ public override void UpdateState()
       columns?.Clear();           // To prevent event handler leak
       columns = null;
 
-      HasImmediateValidators = validators.Count > 0 && validators.Any(v => v.IsImmediate);
+      HasImmediateValidators = Validators.Count > 0 && Validators.Any(v => v.IsImmediate);
 
       CreateMappingInfo();
     }
@@ -669,16 +658,14 @@ public override void Lock(bool recursive)
       base.Lock(recursive);
       if (!recursive)
         return;
-      validators = Array.AsReadOnly(validators.ToArray());
       Fields.Lock(true);
-      if (column != null)
-        column.Lock(true);
-      if (associations.Count > 1) {
-        var sorted = associations.Reorder();
-        associations = new NodeCollection<AssociationInfo>(associations.Owner, associations.Name);
-        associations.AddRange(sorted);
+      column?.Lock(true);
+      if (Associations.Count > 1) {
+        var sorted = Associations.Reorder();
+        Associations = new NodeCollection<AssociationInfo>(Associations.Owner, Associations.Name);
+        Associations.AddRange(sorted);
       }
-      associations.Lock(false);
+      Associations?.Lock(false);
     }
 
     private void CreateMappingInfo()
@@ -778,9 +765,11 @@ public FieldInfo Clone()
         defaultValue = defaultValue,
         defaultSqlExpression = defaultSqlExpression,
         DeclaringField = DeclaringField,
-        Validators = Validators.Select(v => v.CreateNew()).ToList(),
+        Validators = Validators.Select(v => v.CreateNew()).ToArray(),
       };
-      clone.Associations.AddRange(associations);
+      if (Associations.Count > 0) {
+        clone.Associations.AddRange(Associations);
+      }
       return clone;
     }
 
@@ -805,10 +794,8 @@ private FieldInfo(TypeInfo declaringType, TypeInfo reflectedType, FieldAttribute
       Attributes = attributes;
       this.declaringType = declaringType;
       this.reflectedType = reflectedType;
-      Fields = IsEntity || IsStructure
-        ? new FieldInfoCollection(this, "Fields")
-        : FieldInfoCollection.Empty;
-      associations = new NodeCollection<AssociationInfo>(this, "Associations");
+      Fields = IsEntity || IsStructure ? new FieldInfoCollection(this, "Fields") : FieldInfoCollection.Empty;
+      Associations = IsEntity || IsEntitySet ? new NodeCollection<AssociationInfo>(this, "Associations") : NodeCollection<AssociationInfo>.Empty;
     }
   }
-}
\ No newline at end of file
+}
diff --git a/Orm/Xtensive.Orm/Orm/Model/HierarchyInfo.cs b/Orm/Xtensive.Orm/Orm/Model/HierarchyInfo.cs
index 7ced941373..cf8643f398 100644
--- a/Orm/Xtensive.Orm/Orm/Model/HierarchyInfo.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/HierarchyInfo.cs
@@ -6,6 +6,7 @@
 
 using System;
 using System.Collections.Generic;
+using Xtensive.Core;
 
 namespace Xtensive.Orm.Model
 {
@@ -47,7 +48,7 @@ public override void UpdateState()
       Key.UpdateState();
       var list = new List<TypeInfo>(Root.AllDescendants.Count + 1) {Root};
       list.AddRange(Root.AllDescendants);
-      Types = list.AsReadOnly();
+      Types = list.AsSafeWrapper();
       if (Types.Count == 1)
         InheritanceSchema = InheritanceSchema.ConcreteTable;
       if (TypeDiscriminatorMap != null)
@@ -67,16 +68,16 @@ public override void Lock(bool recursive)
     // Constructors
 
     /// <summary>
-    /// 	Initializes a new instance of this class.
+    ///     Initializes a new instance of this class.
     /// </summary>
     /// <param name="root">The hierarchy root.</param>
     /// <param name="key">The key info.</param>
     /// <param name="inheritanceSchema">The inheritance schema.</param>
     /// <param name="typeDiscriminatorMap">The type discriminator map.</param>
     public HierarchyInfo(
-      TypeInfo root, 
-      KeyInfo key, 
-      InheritanceSchema inheritanceSchema, 
+      TypeInfo root,
+      KeyInfo key,
+      InheritanceSchema inheritanceSchema,
       TypeDiscriminatorMap typeDiscriminatorMap)
     {
       Root = root;
diff --git a/Orm/Xtensive.Orm/Orm/Model/IndexInfo.cs b/Orm/Xtensive.Orm/Orm/Model/IndexInfo.cs
index 370095e3be..23c5633102 100644
--- a/Orm/Xtensive.Orm/Orm/Model/IndexInfo.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/IndexInfo.cs
@@ -35,7 +35,7 @@ public sealed class IndexInfo : MappedNode
     private readonly IndexInfo declaringIndex;
     private double fillFactor;
     private string shortName;
-    private ReadOnlyCollection<ColumnInfo> columns;
+    private IReadOnlyList<ColumnInfo> columns;
     private TupleDescriptor tupleDescriptor;
     private TupleDescriptor keyTupleDescriptor;
     private IReadOnlyList<TypeInfo> filterByTypes;
@@ -339,7 +339,7 @@ public override void UpdateState()
       var lazy = new List<int>();
       var regular = new List<int>();
 
-      for (int i = 0; i < columns.Count; i++) {
+      for (int i = 0, count = columns.Count; i < count; i++) {
         var item = columns[i];
         if (item.IsPrimaryKey || item.IsSystem)
           system.Add(i);
@@ -393,7 +393,7 @@ private void CreateColumns()
       var result = new List<ColumnInfo>(keyColumns.Count + valueColumns.Count);
       result.AddRange(keyColumns.Select(static pair => pair.Key));
       result.AddRange(valueColumns);
-      columns = result.AsReadOnly();
+      columns = result.AsSafeWrapper();
     }
 
 
diff --git a/Orm/Xtensive.Orm/Orm/Model/PartialIndexFilterInfo.cs b/Orm/Xtensive.Orm/Orm/Model/PartialIndexFilterInfo.cs
index 8de3e95f09..b37d3f18d9 100644
--- a/Orm/Xtensive.Orm/Orm/Model/PartialIndexFilterInfo.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/PartialIndexFilterInfo.cs
@@ -32,12 +32,12 @@ public LambdaExpression Expression
       }
     }
 
-    private IList<FieldInfo> fields;
+    private IReadOnlyList<FieldInfo> fields;
 
     /// <summary>
     /// Fields used in <see cref="Expression"/>.
     /// </summary>
-    public IList<FieldInfo> Fields
+    public IReadOnlyList<FieldInfo> Fields
     {
       get { return fields; }
       set
@@ -56,9 +56,6 @@ public override void Lock(bool recursive)
         throw Exceptions.NotInitialized("Expression");
       if (Fields==null)
         throw Exceptions.NotInitialized("Fields");
-      fields = fields is List<FieldInfo> list
-        ? list.AsReadOnly()
-        : (IList<FieldInfo>) fields.ToList().AsReadOnly();
       base.Lock(recursive);
     }
   }
diff --git a/Orm/Xtensive.Orm/Orm/Model/TypeIndexInfoCollection.cs b/Orm/Xtensive.Orm/Orm/Model/TypeIndexInfoCollection.cs
index 2911d4b34c..8c51be8372 100644
--- a/Orm/Xtensive.Orm/Orm/Model/TypeIndexInfoCollection.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/TypeIndexInfoCollection.cs
@@ -8,6 +8,7 @@
 using System.Diagnostics;
 using System.Linq;
 using System.Collections.Generic;
+using Xtensive.Core;
 
 namespace Xtensive.Orm.Model
 {
@@ -39,7 +40,7 @@ public IReadOnlyList<IndexInfo> RealPrimaryIndexes
       get {
         return IsLocked 
           ? realPrimaryIndexes
-          : FindRealPrimaryIndexes(PrimaryIndex).AsReadOnly();
+          : FindRealPrimaryIndexes(PrimaryIndex).AsSafeWrapper();
       }
     }
 
@@ -76,8 +77,8 @@ public override void UpdateState()
     {
       base.UpdateState();
       primaryIndex = FindPrimaryIndex();
-      realPrimaryIndexes = FindRealPrimaryIndexes(primaryIndex).AsReadOnly();
-      indexesContainingAllData = FindIndexesContainingAllData().AsReadOnly();
+      realPrimaryIndexes = FindRealPrimaryIndexes(primaryIndex).AsSafeWrapper();
+      indexesContainingAllData = FindIndexesContainingAllData().AsSafeWrapper();
     }
 
     private IndexInfo GetIndex(IEnumerable<FieldInfo> fields)
@@ -118,7 +119,7 @@ public IReadOnlyList<IndexInfo> GetIndexesContainingAllData()
     {
       return IsLocked
         ? indexesContainingAllData
-        : FindIndexesContainingAllData().AsReadOnly();
+        : FindIndexesContainingAllData().AsSafeWrapper();
     }
 
     private List<IndexInfo> FindIndexesContainingAllData()
diff --git a/Orm/Xtensive.Orm/Orm/Model/TypeInfo.cs b/Orm/Xtensive.Orm/Orm/Model/TypeInfo.cs
index cb3521f44e..f2b41d653f 100644
--- a/Orm/Xtensive.Orm/Orm/Model/TypeInfo.cs
+++ b/Orm/Xtensive.Orm/Orm/Model/TypeInfo.cs
@@ -610,7 +610,7 @@ public IEnumerable<AssociationInfo> GetTargetAssociations()
         if (!IsLocked) {
           return result;
         }
-        targetAssociations = result.ToList().AsReadOnly();
+        targetAssociations = result.ToList().AsSafeWrapper();
       }
       return targetAssociations;
     }
@@ -625,7 +625,7 @@ public IEnumerable<AssociationInfo> GetOwnerAssociations()
         if (!IsLocked) {
           return result;
         }
-        ownerAssociations = result.ToList().AsReadOnly();
+        ownerAssociations = result.ToList().AsSafeWrapper();
       }
       return ownerAssociations;
     }
@@ -737,7 +737,7 @@ public override void UpdateState()
         }
       }
 
-      HasValidators = Validators.Count > 0 || fields.Any(f => f.HasValidators);
+      HasValidators = Validators.Count > 0 || fields.Any(static f => f.HasValidators);
 
       // Selecting master parts from paired associations & single associations
       var associations = model.Associations.Find(this)
@@ -798,7 +798,7 @@ public override void UpdateState()
 
       var sortedRemovalSequence = sequence.Where(a => a.Ancestors.Count > 0).ToList();
       if (sortedRemovalSequence.Count == 0) {
-        removalSequence = sequence.AsReadOnly();
+        removalSequence = sequence.AsSafeWrapper();
       }
       else {
         var sequenceSize = sequence.Count;
@@ -806,7 +806,7 @@ public override void UpdateState()
           sortedRemovalSequence.Capacity = sequenceSize;
         }
         sortedRemovalSequence.AddRange(sequence.Where(a => a.Ancestors.Count == 0));
-        removalSequence = sortedRemovalSequence.AsReadOnly();
+        removalSequence = sortedRemovalSequence.AsSafeWrapper();
       }
     }
 
diff --git a/Orm/Xtensive.Orm/Orm/Providers/Persister.cs b/Orm/Xtensive.Orm/Orm/Providers/Persister.cs
index 6d6733d023..f36c539733 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/Persister.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/Persister.cs
@@ -1,4 +1,4 @@
-// Copyright (C) 2012 Xtensive LLC.
+// Copyright (C) 2012 Xtensive LLC.
 // All rights reserved.
 // For conditions of distribution and use, see license.
 // Created by: Denis Krjuchkov
@@ -93,11 +93,10 @@ private SqlPersistTask CreateRemoveTask(PersistAction action, bool validateVersi
       }
     }
 
-    private ICollection<PersistRequest> GetOrBuildRequest(StorageNode node, PersistRequestBuilderTask task)
+    private IReadOnlyList<PersistRequest> GetOrBuildRequest(StorageNode node, PersistRequestBuilderTask task)
     {
       var cache = node.PersistRequestCache;
-      ICollection<PersistRequest> result;
-      if (cache.TryGetValue(task, out result))
+      if (cache.TryGetValue(task, out var result))
         return result;
       result = requestBuilder.Build(node, task);
       return cache.TryAdd(task, result) ? result : cache[task];
diff --git a/Orm/Xtensive.Orm/Orm/Providers/Requests/PersistRequestBuilder.cs b/Orm/Xtensive.Orm/Orm/Providers/Requests/PersistRequestBuilder.cs
index 2ef5b920a5..36210481bf 100644
--- a/Orm/Xtensive.Orm/Orm/Providers/Requests/PersistRequestBuilder.cs
+++ b/Orm/Xtensive.Orm/Orm/Providers/Requests/PersistRequestBuilder.cs
@@ -6,6 +6,7 @@
 
 using System;
 using System.Collections.Generic;
+using Xtensive.Core;
 using Xtensive.Orm.Configuration;
 using Xtensive.Orm.Model;
 using Xtensive.Sql;
@@ -23,7 +24,7 @@ public class PersistRequestBuilder : DomainBoundHandler
     private ProviderInfo providerInfo;
     private StorageDriver driver;
 
-    internal ICollection<PersistRequest> Build(StorageNode node, PersistRequestBuilderTask task)
+    internal IReadOnlyList<PersistRequest> Build(StorageNode node, PersistRequestBuilderTask task)
     {
       var context = new PersistRequestBuilderContext(task, node.Mapping, node.Configuration);
       List<PersistRequest> result;
@@ -52,14 +53,14 @@ internal ICollection<PersistRequest> Build(StorageNode node, PersistRequestBuild
         }
         var batchRequest = CreatePersistRequest(batch, bindings, node.Configuration);
         batchRequest.Prepare();
-        return new List<PersistRequest> {batchRequest}.AsReadOnly();
+        return new List<PersistRequest> { batchRequest }.AsSafeWrapper();
       }
 
       foreach (var item in result) {
         item.Prepare();
       }
 
-      return result.AsReadOnly();
+      return result.AsSafeWrapper();
     }
 
     protected virtual List<PersistRequest> BuildInsertRequest(PersistRequestBuilderContext context)
diff --git a/Orm/Xtensive.Orm/Orm/Rse/Providers/Compilable/SelectProvider.cs b/Orm/Xtensive.Orm/Orm/Rse/Providers/Compilable/SelectProvider.cs
index 181e0054d4..5cfd06f4d0 100644
--- a/Orm/Xtensive.Orm/Orm/Rse/Providers/Compilable/SelectProvider.cs
+++ b/Orm/Xtensive.Orm/Orm/Rse/Providers/Compilable/SelectProvider.cs
@@ -44,17 +44,7 @@ protected override string ParametersToString()
     public SelectProvider(CompilableProvider provider, IReadOnlyList<int> columnIndexes)
       : base(ProviderType.Select, provider)
     {
-      switch (columnIndexes) {
-        case int[] indexArray:
-          ColumnIndexes = Array.AsReadOnly(indexArray);
-          break;
-        case List<int> indexList:
-          ColumnIndexes = indexList.AsReadOnly();
-          break;
-        default:
-          ColumnIndexes = columnIndexes;
-          break;
-      }
+      ColumnIndexes = columnIndexes.AsSafeWrapper();
 
       Initialize();
     }
diff --git a/Orm/Xtensive.Orm/Orm/Rse/Transformation/ColumnMappingInspector.cs b/Orm/Xtensive.Orm/Orm/Rse/Transformation/ColumnMappingInspector.cs
index 257716a6a2..d346556d93 100644
--- a/Orm/Xtensive.Orm/Orm/Rse/Transformation/ColumnMappingInspector.cs
+++ b/Orm/Xtensive.Orm/Orm/Rse/Transformation/ColumnMappingInspector.cs
@@ -78,7 +78,7 @@ protected override Provider VisitSelect(SelectProvider provider)
       mappings[provider] = newMappings;
       return source == provider.Source
         ? provider
-        : new SelectProvider(source, indexColumns.ToArray());
+        : new SelectProvider(source, indexColumns);
     }
 
     /// <inheritdoc/>
@@ -122,7 +122,7 @@ protected override Provider VisitJoin(JoinProvider provider)
     {
       // split
 
-      SplitMappings(provider, out var leftMapping, out var rightMapping);
+      var (leftMapping, rightMapping) = SplitMappings(provider);
 
       leftMapping = Merge(leftMapping, provider.EqualIndexes.Select(p => p.First));
       rightMapping = Merge(rightMapping, provider.EqualIndexes.Select(p => p.Second));
@@ -148,7 +148,7 @@ protected override Provider VisitJoin(JoinProvider provider)
 
     protected override Provider VisitPredicateJoin(PredicateJoinProvider provider)
     {
-      SplitMappings(provider, out var leftMapping, out var rightMapping);
+      var (leftMapping, rightMapping) = SplitMappings(provider);
 
       leftMapping.AddRange(mappingsGatherer.Gather(provider.Predicate,
         provider.Predicate.Parameters[0]));
@@ -190,7 +190,7 @@ protected override Provider VisitApply(ApplyProvider provider)
     {
       // split
 
-      SplitMappings(provider, out var leftMapping, out var rightMapping);
+      var (leftMapping, rightMapping) = SplitMappings(provider);
 
       var applyParameter = provider.ApplyParameter;
       var currentOuterUsages = new List<int>();
@@ -217,12 +217,12 @@ protected override Provider VisitApply(ApplyProvider provider)
       _ = outerColumnUsages.Remove(applyParameter);
 
       var pair = OverrideRightApplySource(provider, newRightProvider, rightMapping);
-      if (pair.First == null) {
+      if (pair.compilableProvider == null) {
         rightMapping = mappings[provider.Right];
       }
       else {
-        newRightProvider = pair.First;
-        rightMapping = pair.Second;
+        newRightProvider = pair.compilableProvider;
+        rightMapping = pair.mapping;
       }
       RestoreMappings(oldMappings);
 
@@ -400,9 +400,8 @@ private static CompilableProvider BuildSetOperationSource(CompilableProvider pro
       return new SelectProvider(provider, columns);
     }
 
-    protected virtual Pair<CompilableProvider, List<int>> OverrideRightApplySource(ApplyProvider applyProvider,
-      CompilableProvider provider, List<int> requestedMapping) =>
-      new Pair<CompilableProvider, List<int>>(provider, requestedMapping);
+    protected virtual (CompilableProvider compilableProvider, List<int> mapping) OverrideRightApplySource(ApplyProvider applyProvider, CompilableProvider provider, List<int> requestedMapping) =>
+      (provider, requestedMapping);
 
     #endregion
 
@@ -441,20 +440,21 @@ private static List<int> MergeMappings(Provider originalLeft, List<int> leftMap,
       return result;
     }
 
-    private void SplitMappings(BinaryProvider provider, out List<int> leftMapping, out List<int> rightMapping)
+    private (List<int> leftMapping, List<int> rightMapping) SplitMappings(BinaryProvider provider)
     {
       var binaryMapping = mappings[provider];
-      leftMapping = new List<int>(binaryMapping.Count);
+      var leftMapping = new List<int>(binaryMapping.Count);
       var leftCount = provider.Left.Header.Length;
       var index = 0;
       while (index < binaryMapping.Count && binaryMapping[index] < leftCount) {
         leftMapping.Add(binaryMapping[index]);
         index++;
       }
-      rightMapping = new List<int>(binaryMapping.Count - index);
+      var rightMapping = new List<int>(binaryMapping.Count - index);
       for (var i = index; i < binaryMapping.Count; i++) {
         rightMapping.Add(binaryMapping[i] - leftCount);
       }
+      return (leftMapping, rightMapping);
     }
 
     private void RegisterOuterMapping(ApplyParameter parameter, int value)
@@ -508,7 +508,7 @@ private void VisitJoin(ref List<int> leftMapping, ref CompilableProvider left, r
     private Dictionary<Provider, List<int>> ReplaceMappings(Provider firstNewKey, List<int> firstNewValue)
     {
       var oldMappings = mappings;
-      mappings = new Dictionary<Provider, List<int>> {{firstNewKey, firstNewValue}};
+      mappings = new() { { firstNewKey, firstNewValue } };
       return oldMappings;
     }
 
diff --git a/Orm/Xtensive.Orm/Orm/Rse/Transformation/RedundantColumnRemover.cs b/Orm/Xtensive.Orm/Orm/Rse/Transformation/RedundantColumnRemover.cs
index 9ef551e717..39fe411f27 100644
--- a/Orm/Xtensive.Orm/Orm/Rse/Transformation/RedundantColumnRemover.cs
+++ b/Orm/Xtensive.Orm/Orm/Rse/Transformation/RedundantColumnRemover.cs
@@ -27,13 +27,13 @@ internal sealed class RedundantColumnRemover : ColumnMappingInspector
         && methodInfo.GetParameters()[1].ParameterType.GetGenericTypeDefinition() == WellKnownTypes.FuncOfTArgTResultType)
       .CachedMakeGenericMethod(WellKnownOrmTypes.Tuple, WellKnownOrmTypes.Tuple);
 
-    protected override Pair<CompilableProvider, List<int>> OverrideRightApplySource(ApplyProvider applyProvider, CompilableProvider provider, List<int> requestedMapping)
+    protected override (CompilableProvider, List<int>) OverrideRightApplySource(ApplyProvider applyProvider, CompilableProvider provider, List<int> requestedMapping)
     {
       var currentMapping = mappings[applyProvider.Right];
       if (currentMapping.SequenceEqual(requestedMapping))
         return base.OverrideRightApplySource(applyProvider, provider, requestedMapping);
       var selectProvider = new SelectProvider(provider, requestedMapping.ToArray());
-      return new Pair<CompilableProvider, List<int>>(selectProvider, requestedMapping);
+      return (selectProvider, requestedMapping);
     }
 
     protected override Provider VisitRaw(RawProvider provider)
diff --git a/Orm/Xtensive.Orm/Orm/StorageNode.cs b/Orm/Xtensive.Orm/Orm/StorageNode.cs
index 20ac4d09b1..8c339a341e 100644
--- a/Orm/Xtensive.Orm/Orm/StorageNode.cs
+++ b/Orm/Xtensive.Orm/Orm/StorageNode.cs
@@ -72,7 +72,7 @@ public sealed class StorageNode : ISessionSource
     /// </summary>
     internal ConcurrentDictionary<AssociationInfo, (CompilableProvider, Parameter<Xtensive.Tuples.Tuple>)> RefsToEntityQueryCache { get; }
     internal ConcurrentDictionary<SequenceInfo, object> KeySequencesCache { get; }
-    internal ConcurrentDictionary<PersistRequestBuilderTask, ICollection<PersistRequest>> PersistRequestCache { get; }
+    internal ConcurrentDictionary<PersistRequestBuilderTask, IReadOnlyList<PersistRequest>> PersistRequestCache { get; } = new();
 
     /// <inheritdoc/>
     public Session OpenSession() =>
@@ -127,7 +127,6 @@ internal StorageNode(Domain domain, NodeConfiguration configuration, ModelMappin
       EntitySetTypeStateCache = new ConcurrentDictionary<Xtensive.Orm.Model.FieldInfo, EntitySetTypeState>();
       RefsToEntityQueryCache = new ConcurrentDictionary<AssociationInfo, (CompilableProvider, Parameter<Xtensive.Tuples.Tuple>)>();
       KeySequencesCache = new ConcurrentDictionary<SequenceInfo, object>();
-      PersistRequestCache = new ConcurrentDictionary<PersistRequestBuilderTask, ICollection<PersistRequest>>();
 
       this.domain = domain;
       Configuration = configuration;
diff --git a/Orm/Xtensive.Orm/Reflection/InterfaceMapping.cs b/Orm/Xtensive.Orm/Reflection/InterfaceMapping.cs
index cf161f4969..548d13ad2f 100644
--- a/Orm/Xtensive.Orm/Reflection/InterfaceMapping.cs
+++ b/Orm/Xtensive.Orm/Reflection/InterfaceMapping.cs
@@ -10,6 +10,7 @@
 using ReflectionInterfaceMapping=System.Reflection.InterfaceMapping;
 using System.Linq;
 using System.Collections.Generic;
+using Xtensive.Core;
 
 namespace Xtensive.Reflection
 {
diff --git a/Orm/Xtensive.Orm/Reflection/TypeHelper.cs b/Orm/Xtensive.Orm/Reflection/TypeHelper.cs
index 011c19f2bb..02362d6e7e 100644
--- a/Orm/Xtensive.Orm/Reflection/TypeHelper.cs
+++ b/Orm/Xtensive.Orm/Reflection/TypeHelper.cs
@@ -732,15 +732,15 @@ public static IReadOnlyList<Type> GetInterfacesUnordered(Type type) =>
     /// <param name="type">The type to get the interfaces of.</param>
     [Obsolete("Use GetInterfacesOrderByInheritance instead")]
     public static Type[] GetInterfaces(this Type type) =>
-      OrderedInterfaces.GetOrAdd(type, t => t.GetInterfaces().OrderByInheritance().ToArray());
+      OrderedInterfaces.GetOrAdd(type, static t => t.GetInterfaces().OrderByInheritance().ToArray());
 
     /// <summary>
     /// Gets the interfaces of the specified type.
     /// Interfaces will be ordered from the very base ones to ancestors.
     /// </summary>
     /// <param name="type">The type to get the interfaces of.</param>
-    public static Type[] GetInterfacesOrderedByInheritance(this Type type) =>
-      OrderedInterfaces.GetOrAdd(type, t => t.GetInterfaces().OrderByInheritance().ToArray());
+    public static Type[] GetInterfacesOrderByInheritance(this Type type) =>
+      OrderedInterfaces.GetOrAdd(type, static t => t.GetInterfaces().OrderByInheritance().ToArray());
 
     /// <summary>
     /// Gets the sequence of type itself, all its base types and interfaces.
diff --git a/Orm/Xtensive.Orm/Sorting/Node.cs b/Orm/Xtensive.Orm/Sorting/Node.cs
index e07541db0d..b0900e21b1 100644
--- a/Orm/Xtensive.Orm/Sorting/Node.cs
+++ b/Orm/Xtensive.Orm/Sorting/Node.cs
@@ -10,7 +10,6 @@
 using System.Linq;
 using Xtensive.Core;
 
-
 namespace Xtensive.Sorting
 {
   /// <summary>
@@ -22,9 +21,9 @@ namespace Xtensive.Sorting
   public class Node<TNodeItem, TConnectionItem>
   {
     private List<NodeConnection<TNodeItem, TConnectionItem>> incomingConnections;
-    private ReadOnlyCollection<NodeConnection<TNodeItem, TConnectionItem>> incomingConnectionsReadOnlyList;
+    private IReadOnlyList<NodeConnection<TNodeItem, TConnectionItem>> incomingConnectionsReadOnlyList;
     private List<NodeConnection<TNodeItem, TConnectionItem>> outgoingConnections;
-    private ReadOnlyCollection<NodeConnection<TNodeItem, TConnectionItem>> outgoingConnectionsReadOnlyList;
+    private IReadOnlyList<NodeConnection<TNodeItem, TConnectionItem>> outgoingConnectionsReadOnlyList;
 
     /// <summary>
     /// Gets node item.
@@ -164,18 +163,14 @@ public IEnumerable<NodeConnection<TNodeItem, TConnectionItem>> RemoveConnections
 
     private void EnsureIncomingConnections()
     {
-      if (incomingConnectionsReadOnlyList==null) {
-        incomingConnections = new List<NodeConnection<TNodeItem, TConnectionItem>>();
-        incomingConnectionsReadOnlyList = incomingConnections.AsReadOnly();
-      }
+      incomingConnectionsReadOnlyList ??=
+        (incomingConnections = new List<NodeConnection<TNodeItem, TConnectionItem>>()).AsSafeWrapper();
     }
 
     private void EnsureOutgoingConnections()
     {
-      if (outgoingConnectionsReadOnlyList==null) {
-        outgoingConnections = new List<NodeConnection<TNodeItem, TConnectionItem>>();
-        outgoingConnectionsReadOnlyList = outgoingConnections.AsReadOnly();
-      }
+      outgoingConnectionsReadOnlyList ??=
+        (outgoingConnections = new List<NodeConnection<TNodeItem, TConnectionItem>>()).AsSafeWrapper();
     }
 
     // Constructors