Fix clipboard GetData documentation to reflect NotSupportedException instead of null (#2150)

* Initial plan

* Fix clipboard GetData comments to reflect NotSupportedException instead of null

Co-authored-by: adegeo <67293991+adegeo@users.noreply.github.com>

* Change null check to type check for Person in ObsoleteGetDataExample

Co-authored-by: adegeo <67293991+adegeo@users.noreply.github.com>

* Use DirectCast instead of CType for Person type in VB example

Co-authored-by: adegeo <67293991+adegeo@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: adegeo <67293991+adegeo@users.noreply.github.com>

Author: Copilot

dotnet-desktop-guide/winforms/migration/snippets/clipboard-dataobject-net10/net/csharp/ObsoletePatterns.cs

@@ -20,7 +20,7 @@ public static void BrokenCustomTypeExample()
             Person person = new Person { Name = "John", Age = 30 };
             Clipboard.SetData("MyApp.Person", person);  // No data is stored
 
-            // Later attempts to retrieve the data return null
+            // Later attempts to retrieve the data return a NotSupportedException instance
             object data = Clipboard.GetData("MyApp.Person");
         }
         // </ObsoleteCustomType>
@@ -31,10 +31,9 @@ public static void ObsoleteGetDataExample()
             // Don't use - GetData() is obsolete in .NET 10
             object data = Clipboard.GetData("MyApp.Person");  // Obsolete method
 
-            // Always returns null on a custom object type
-            if (data != null)
+            // Returns a NotSupportedException instance for a custom object type
+            if (data is Person person)
             {
-                Person person = (Person)data;
                 Console.WriteLine($"Processing person: {person.Name}, Age: {person.Age}");
             }
         }

dotnet-desktop-guide/winforms/migration/snippets/clipboard-dataobject-net10/net/vb/ObsoletePatterns.vb

@@ -16,7 +16,7 @@ Namespace ClipboardExamples
             Dim person As New Person With {.Name = "John", .Age = 30}
             Clipboard.SetData("MyApp.Person", person)  ' No data is stored
 
-            ' Later attempts to retrieve the data return null
+            ' Later attempts to retrieve the data return a NotSupportedException instance
             Dim data As Object = Clipboard.GetData("MyApp.Person")
         End Sub
         ' </ObsoleteCustomType>
@@ -26,9 +26,9 @@ Namespace ClipboardExamples
             ' Don't use - GetData() is obsolete in .NET 10
             Dim data As Object = Clipboard.GetData("MyApp.Person")  ' Obsolete method
 
-            ' Always returns null on a custom object type
-            If data IsNot Nothing Then
-                Dim person As Person = CType(data, Person)
+            ' Returns a NotSupportedException instance for a custom object type
+            If TypeOf data Is Person Then
+                Dim person As Person = DirectCast(data, Person)
                 Console.WriteLine($"Processing person: {person.Name}, Age: {person.Age}")
             End If
         End Sub