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Clarify Optional Parameters and Nullable Optional Parameter Behavior in C#. #44408

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Clarify Optional Parameters and Nullable Optional Parameter Behavior in C#. #44408

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f55c861
Resolved Bug 44404.
Jan 16, 2025
033dc16
Fixed typo.
Jan 16, 2025
5a6213e
Resolved comments.
Jan 17, 2025
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7 changes: 4 additions & 3 deletions docs/csharp/programming-guide/classes-and-structs/named-and-optional-arguments.md
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Expand Up @@ -70,13 +70,14 @@ The following code implements the examples from this section along with some add

## Optional arguments

The definition of a method, constructor, indexer, or delegate can specify its parameters are required or optional. Any call must provide arguments for all required parameters, but can omit arguments for optional parameters.
The definition of a method, constructor, indexer, or delegate can specify its parameters are required or optional. Any call must provide arguments for all required parameters, but can omit arguments for optional parameters. Additionally, nullable reference types (`T?`) implicitly have `null` as their default value.
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The new sentence emphasizes the cause of the confusion, rather than removing it. We need to make a clear distinction between an optional parameter and the default value for a type.


Each optional parameter has a default value as part of its definition. If no argument is sent for that parameter, the default value is used. A default value must be one of the following types of expressions:

- a constant expression;
- an expression of the form `new ValType()`, where `ValType` is a value type, such as an [enum](../../language-reference/builtin-types/enum.md) or a [struct](../../language-reference/builtin-types/struct.md);
- an expression of the form [default(ValType)](../../language-reference/operators/default.md), where `ValType` is a value type.
- For nullable value types or nullable reference types (`T?`), the default value is always `null`.
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This isn't correct. All of these declarations are legal. They conform to the first three bullet points:

public struct S
{
}

public class C {
    void M(int? size = 5){} // a constant
    void M(string? msg = "Hello World"){} // constant
    void M(S someStruct = new S()){} // new expression, assuming `S` is a `struct`
    void M(object? o = default) { } // OK default expression
    void M(int size, S someStruct = default) {} // OK default expression addition parameter needed
    void M(S? someStruct = default) {} // OK default expression
}


Optional parameters are defined at the end of the parameter list, after any required parameters. If the caller provides an argument for any one of a succession of optional parameters, it must provide arguments for all preceding optional parameters. Comma-separated gaps in the argument list aren't supported. For example, in the following code, instance method `ExampleMethod` is defined with one required and two optional parameters.

Expand All @@ -85,7 +86,7 @@ Optional parameters are defined at the end of the parameter list, after any requ
The following call to `ExampleMethod` causes a compiler error, because an argument is provided for the third parameter but not for the second.

```csharp
//anExample.ExampleMethod(3, ,4);
// anExample.ExampleMethod(3, ,4);
```

However, if you know the name of the third parameter, you can use a named argument to accomplish the task.
Expand All @@ -99,7 +100,7 @@ IntelliSense uses brackets to indicate optional parameters, as shown in the foll
![Screenshot showing IntelliSense quick info for the ExampleMethod method.](./media/named-and-optional-arguments/optional-examplemethod-parameters.png)

> [!NOTE]
> You can also declare optional parameters by using the .NET <xref:System.Runtime.InteropServices.OptionalAttribute> class. `OptionalAttribute` parameters do not require a default value. However, if a default value is desired, take a look at <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> class.
> You can also declare optional parameters by using the .NET <xref:System.Runtime.InteropServices.OptionalAttribute> class. `OptionalAttribute` parameters do not require a default value. However, if a default value is desired, take a look at <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> class. Additionally, nullable types (`T?`) implicitly default to `null` without needing to use these attributes.
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The additional sentence here mixes an optional parameter with the default value for a type.


### Example

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