Allow the model to store invalid data and give detailed exceptions about it

src/Hl7.Fhir.Base/ElementModel/ElementNodeComparator.cs

@@ -44,14 +44,14 @@ public static TreeComparisonResult IsEqualTo(this ITypedElement expected, ITyped
             foreach (var exp in childrenExp)
             {
                 if (!childrenActual.MoveNext())
-                    TreeComparisonResult.Fail(actual.Location, $"number of children was different");
+                    return TreeComparisonResult.Fail(actual.Location, $"number of children was different");
 
                 var result = exp.IsEqualTo(childrenActual.Current);
                 if (!result.Success)
                     return result;
             }
             if (childrenActual.MoveNext())
-                TreeComparisonResult.Fail(actual.Location, $"number of children was different");
+                return TreeComparisonResult.Fail(actual.Location, $"number of children was different");
 
             return TreeComparisonResult.OK;
         }

src/Hl7.Fhir.Base/Model/Base.cs

@@ -36,6 +36,7 @@ POSSIBILITY OF SUCH DAMAGE.
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.Diagnostics.CodeAnalysis;
+using System.Linq;
 using System.Threading;
 
 namespace Hl7.Fhir.Model;
@@ -46,8 +47,10 @@ public abstract partial class Base : IAnnotatable, INotifyPropertyChanged
     /// FHIR Type Name
     /// </summary>
     public virtual string TypeName => GetType().Name;
-
+    
     private Dictionary<string, object>? _overflow = null;
+
+    protected bool HasOverflow => _overflow is not null && _overflow.Any();
 
     /// <summary>
     /// A dictionary containing all elements that are not explicitly defined in the class.
@@ -130,7 +133,7 @@ public object this[string key]
     /// <returns><c>true</c> if the given value was set in the POCO or present in the overflow dictionary, <c>false</c> otherwise.
     /// For lists, this means they should not be empty.</returns>
     public virtual bool TryGetValue(string key, [NotNullWhen(true)] out object? value) =>
-        Overflow.TryGetValue(key, out value) && (value is not IReadOnlyList<Base> list || list.Count > 0);
+        Overflow.TryGetValue(key, out value);
 
     /// <summary>
     /// Enumerates all non-empty elements in the POCO and the overflow dictionary.