Mark ComSource interfaces as IDispatch

TypeName	MarkComSourceInterfacesAsIDispatch
CheckId	CA1412
Category	Microsoft.Interoperability
Breaking Change	Breaking

Cause

A type is marked with the System.Runtime.InteropServices.ComSourceInterfacesAttribute attribute and at least one the specified interfaces is not marked with the System.Runtime.InteropServices.InterfaceTypeAttribute attribute set to System.Runtime.InteropServices.ComInterfaceType.InterfaceIsIDispatch.

Rule Description

ComSourceInterfacesAttribute is used to identify the event interfaces that a class exposes to COM clients. These interfaces must be exposed as InterfaceIsIDispatch to allow Visual Basic 6 COM clients to receive event notifications. By default, if an interface is not marked with the InterfaceTypeAttribute attribute, it is exposed as a dual interface.

How to Fix Violations

To fix a violation of this rule add or modify the InterfaceTypeAttribute attribute so that its value is set to InterfaceIsIDispatch for all the interfaces specified with the ComSourceInterfacesAttribute attribute.

When to Exclude Warnings

Do not exclude a warning from this rule.

Example

The following example shows a class where one of the interfaces violates the rule.

Imports Microsoft.VisualBasic
Imports System
Imports System.Runtime.InteropServices

<Assembly: ComVisibleAttribute(True)>
Namespace InteroperabilityLibrary

   ' This violates the rule for type EventSource.
   <InterfaceType(ComInterfaceType.InterfaceIsDual)> _ 
   Public Interface IEventsInterface
      Sub EventOne
      Sub EventTwo
   End Interface

   ' This satisfies the rule.
   <InterfaceType(ComInterfaceType.InterfaceIsIDispatch)> _ 
   Public Interface IMoreEventsInterface
      Sub EventThree
      Sub EventFour
   End Interface

   <ComSourceInterfaces( _
      "InteroperabilityLibrary.IEventsInterface" & _ 
      ControlChars.NullChar & _ 
      "InteroperabilityLibrary.IMoreEventsInterface")> _
   Public Class EventSource
      ' Event and method declarations.
   End Class

End Namespace

using System;
using System.Runtime.InteropServices;

[assembly: ComVisible(true)]
namespace InteroperabilityLibrary
{
   // This violates the rule for type EventSource.
   [InterfaceType(ComInterfaceType.InterfaceIsDual)]
   public interface IEventsInterface
   {
      void EventOne();
      void EventTwo();
   }

   // This satisfies the rule.
   [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
   public interface IMoreEventsInterface
   {
      void EventThree();
      void EventFour();
   }

   [ComSourceInterfaces(
      "InteroperabilityLibrary.IEventsInterface\0" + 
      "InteroperabilityLibrary.IMoreEventsInterface")]
   public class EventSource
   {
      // Event and method declarations.
   }
}

Related Rules

Do not use AutoDual ClassInterfaceType

See Also

Tasks

How to: Raise Events Handled by a COM Sink

Other Resources

Interoperating with Unmanaged Code