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COM visible type base types should be COM visible







Breaking Change


A COM visible type derives from a type that is not COM visible.

When a COM visible type adds members in a new version, it must abide by strict guidelines to avoid breaking COM clients that bind to the current version. A type that is invisible to COM presumes it does not need to adhere to these COM versioning rules when it adds new members. However, if a COM visible type derives from the COM invisible type and exposes a class interface of ClassInterfaceType.AutoDual or AutoDispatch (the default), all public members of the base type (unless they are specifically marked as COM invisible, which would be redundant) are exposed to COM. If the base type adds new members in a subsequent version, any COM clients that bind to the class interface of the derived type might break. COM visible types should derive only from COM visible types to reduce the possibility of breaking COM clients.

To fix a violation of this rule make the base types COM visible or the derived type COM invisible.

Do not suppress a warning from this rule.

The following example shows a type that violates the rule.

Imports System
Imports System.Runtime.InteropServices

<Assembly: ComVisibleAttribute(False)>
Namespace InteroperabilityLibrary

   <ComVisibleAttribute(False)> _ 
   Public Class BaseClass

      Sub SomeSub(valueOne As Integer)
      End Sub 

   End Class 

   ' This class violates the rule.
   <ComVisibleAttribute(True)> _ 
   Public Class DerivedClass
      Inherits BaseClass

      Sub AnotherSub(valueOne As Integer, valueTwo As Integer)
      End Sub 

   End Class 

End Namespace