Update default param val attr (dotnet#9356)

Author: Rageking8

snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample.cs

@@ -0,0 +1,18 @@
+//<snippet1>
+using System;
+using System.Runtime.InteropServices;
+
+public class Program
+{
+    public static void MethodWithDefaultParam([Optional, DefaultParameterValue("DEFAULT_PARAM_VALUE")] string str)
+    {
+        Console.WriteLine($"The passed value is: {str}");
+    }
+
+    public static void Main()
+    {
+        MethodWithDefaultParam(); // The passed value is: DEFAULT_PARAM_VALUE
+        MethodWithDefaultParam("NEW_VALUE"); // The passed value is: NEW_VALUE
+    }    
+}
+//</snippet1>

snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample1.cs

@@ -0,0 +1,14 @@
+//<snippet2>
+using System.Runtime.InteropServices;
+
+public class Program
+{
+    public static void MethodWithObjectDefaultAttr1([Optional, DefaultParameterValue(123)] object obj) {} // OK
+    public static void MethodWithObjectDefaultAttr2([Optional, DefaultParameterValue("abc")] object obj) {} // OK
+    public static void MethodWithObjectDefaultAttr3([Optional, DefaultParameterValue(null)] object? obj) {} // OK
+
+    public static void MethodWithObjectDefaultParam1(object obj = 123) {} // CS1763
+    public static void MethodWithObjectDefaultParam2(object obj = "abc") {} // CS1763
+    public static void MethodWithObjectDefaultParam3(object obj? = null) {} // OK
+}
+//</snippet2>

snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample2.cs

@@ -64,16 +64,17 @@
 ## Remarks  
  The <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> attribute allows you to specify a default parameter value in a language that does not otherwise support default parameters.  After you apply this attribute to your code, languages that support default parameters can use the specified value as a default parameter.  
   
- Note that the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> does not add support for default parameters to languages that do not support this feature.  For example, if you use the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> with a method written in C#, which does not support default parameters, you cannot use the default parameter when calling the method from C#.  You can use the default parameter only when calling the method in a language such as C++ that does support the feature.  
-  
  The <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> is particularly useful to specify default parameters for methods of a COM interop interface.  
   
-   
-  
 ## Examples  
- The following code example demonstrates how to apply the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> attribute to a parameter of a method written in C#.  
+
+The following code example demonstrates how to apply the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> attribute to a parameter of a method written in C#. The snippet also uses the <xref:System.Runtime.InteropServices.OptionalAttribute> attribute to enable the method to be called without any arguments.
+  
+:::code language="csharp" source="~/snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample1.cs" id="Snippet1":::
   
- :::code language="csharp" source="~/snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample.cs" id="Snippet1":::  
+There are some behavior differences between [C# optional arguments](/dotnet/csharp/programming-guide/classes-and-structs/named-and-optional-arguments#optional-arguments) and using <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> with <xref:System.Runtime.InteropServices.OptionalAttribute>. The following code snippet demonstrates these differences.
+  
+:::code language="csharp" source="~/snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample2.cs" id="Snippet2":::
   
  ]]></format>
     </remarks>
@@ -132,12 +133,10 @@
 ## Remarks  
  Use this constructor to apply the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> attribute to a parameter written in a language such as Microsoft Visual C# that does not support default parameters.  
   
-   
-  
 ## Examples  
- The following code example demonstrates how to apply the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> attribute to a parameter of a method written in C#.  
+The following code example demonstrates how to apply the <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> attribute to a parameter of a method written in C#. The <xref:System.Runtime.InteropServices.OptionalAttribute> attribute is also used to enable the method to be called without any arguments.
   
- :::code language="csharp" source="~/snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample.cs" id="Snippet1":::  
+:::code language="csharp" source="~/snippets/csharp/System.Runtime.InteropServices/DefaultParameterValueAttribute/Overview/sample1.cs" id="Snippet1":::
   
  ]]></format>
         </remarks>