Add breaking change for exception type

docs/core/compatibility/6.0.md

@@ -18,4 +18,5 @@ If you're migrating an app to .NET 6.0, the breaking changes listed here might a
 
 - [Selected TableLayoutSettings properties throw InvalidEnumArgumentException](windows-forms/6.0/tablelayoutsettings-apis-throw-invalidenumargumentexception.md)
 - [NotifyIcon.Text maximum text length increased](windows-forms/6.0/notifyicon-text-max-text-length-increased.md)
+- [Some APIs throw ArgumentNullException](windows-forms/6.0/apis-throw-argumentnullexception.md)
 - [TreeNodeCollection.Item throws exception if node is assigned elsewhere](windows-forms/6.0/treenodecollection-item-throws-argumentexception.md)

docs/core/compatibility/toc.yml

@@ -28,6 +28,8 @@
           href: aspnet-core/6.0/kestrel-log-message-attributes-changed.md
       - name: Windows Forms
         items:
+        - name: APIs throw ArgumentNullException
+          href: windows-forms/6.0/apis-throw-argumentnullexception.md
         - name: NotifyIcon.Text maximum text length increased
           href: windows-forms/6.0/notifyicon-text-max-text-length-increased.md
         - name: TableLayoutSettings properties throw InvalidEnumArgumentException
@@ -574,6 +576,8 @@
       items:
       - name: .NET 6.0
         items:
+        - name: APIs throw ArgumentNullException
+          href: windows-forms/6.0/apis-throw-argumentnullexception.md
         - name: NotifyIcon.Text maximum text length increased
           href: windows-forms/6.0/notifyicon-text-max-text-length-increased.md
         - name: TableLayoutSettings properties throw InvalidEnumArgumentException

docs/core/compatibility/windows-forms/6.0/apis-throw-argumentnullexception.md

@@ -0,0 +1,47 @@
+---
+title: "Breaking change: Some APIs throw ArgumentNullException"
+description: Learn about the breaking change in .NET 6.0 where some APIs validate arguments and now throw an ArgumentNullException.
+ms.date: 01/29/2021
+---
+# Some APIs throw ArgumentNullException
+
+Some APIs now validate input parameters and throw an <xref:System.ArgumentNullException> where previously they threw a <xref:System.NullReferenceException>, if invoked with `null` input arguments.
+
+## Change description
+
+In previous .NET versions, the affected APIs throw a <xref:System.NullReferenceException> if invoked with an argument that's `null`.
+
+Starting in .NET 6.0, the affected APIs throw an <xref:System.ArgumentNullException> if invoked with an argument that's `null`.
+
+## Reason for change
+
+Throwing <xref:System.ArgumentNullException> conforms to .NET Runtime behavior. It provides a better debug experience by clearly communicating which argument caused the exception.
+
+## Version introduced
+
+.NET 6.0
+
+## Recommended action
+
+- Review and, if necessary, update your code to prevent passing `null` input arguments to the affected APIs.
+- If your code handles <xref:System.NullReferenceException>, replace or add an additional handler for <xref:System.ArgumentNullException>.
+
+## Affected APIs
+
+The following table lists the affected properties:
+
+| Property | Version changed |
+|-|-|-|-|
+| <xref:System.Windows.Forms.TreeNodeCollection.Item(System.Int32)?displayProperty=fullName> | Preview 1 |
+
+<!--
+
+### Affected APIs
+
+- `P:System.Windows.Forms.TreeNodeCollection.Item(System.Int32)`
+
+### Category
+
+Windows Forms
+
+-->