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ComDefaultInterfaceAttribute Class

Specifies a default interface to expose to COM. This class cannot be inherited.

Namespace:  System.Runtime.InteropServices
Assembly:  mscorlib (in mscorlib.dll)

[AttributeUsageAttribute(AttributeTargets::Class, Inherited = false)]
public ref class ComDefaultInterfaceAttribute sealed : public Attribute

Compilers do not necessarily respect the order in which interfaces are implemented. For example, suppose you have the following code:

interface IBase {}interface IDerived : IBase {}class MyClass : IBase, IDerived {}

The compiler emits the implemented interfaces as IDervied, IBase. The ordering of interfaces is important when using the ClassInterfaceAttribute class with [ClassInterface(ClassInterfaceType.None)] to expose an explicit interface as the default interface. Regardless of the specified ClassInterfaceType, use the ComDefaultInterfaceAttribute attribute to specify which interface should be recognized as the default.

Note that the type decorated by the ComDefaultInterfaceAttribute must actually derive from the interface specified as the default interface.


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0