CA1408: Do not use AutoDual ClassInterfaceType

Cause

A Component Object Model (COM) visible type is marked with the ClassInterfaceAttribute attribute set to the AutoDual value of ClassInterfaceType.

Rule Description

Types that use a dual interface enable clients to bind to a specific interface layout. Any changes in a future version to the layout of the type or any base types will break COM clients that bind to the interface. By default, if the ClassInterfaceAttribute attribute is not specified, a dispatch-only interface is used.

Unless marked otherwise, all public nongeneric types are visible to COM; all nonpublic and generic types are invisible to COM.

How to Fix Violations

To fix a violation of this rule, change the value of the ClassInterfaceAttribute attribute to the None value of ClassInterfaceType and explicitly define the interface.

When to Suppress Warnings

Do not suppress a warning from this rule unless it is certain that the layout of the type and its base types will not change in a future version.

Example

The following example shows a class that violates the rule and a re-declaration of the class to use an explicit interface.

Imports System
Imports System.Runtime.InteropServices

<Assembly: ComVisibleAttribute(True)>
Namespace InteroperabilityLibrary

   ' This violates the rule.
   <ClassInterfaceAttribute(ClassInterfaceType.AutoDual)> _ 
   Public Class DualInterface
      Public Sub SomeSub
      End Sub
   End Class

   Public Interface IExplicitInterface
      Sub SomeSub
   End Interface

   <ClassInterfaceAttribute(ClassInterfaceType.None)> _ 
   Public Class ExplicitInterface
      Implements IExplicitInterface

      Public Sub SomeSub Implements IExplicitInterface.SomeSub
      End Sub

   End Class

End Namespace

using System;
using System.Runtime.InteropServices;

[assembly: ComVisible(true)]
namespace InteroperabilityLibrary
{
   // This violates the rule.
   [ClassInterface(ClassInterfaceType.AutoDual)]
   public class DualInterface
   {
      public void SomeMethod() {}
   }

   public interface IExplicitInterface
   {
      void SomeMethod();
   }

   [ClassInterface(ClassInterfaceType.None)]
   public class ExplicitInterface : IExplicitInterface
   {
      public void SomeMethod() {}
   }
}

Related Rules

CA1403: Auto layout types should not be COM visible

CA1412: Mark ComSource Interfaces as IDispatch

See Also

Concepts

Introducing the Class Interface

Qualifying .NET Types for Interoperation

Other Resources

Interoperating with Unmanaged Code