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PrimaryInteropAssemblyAttribute Class

Indicates that the attributed assembly is a primary interop assembly.

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

[AttributeUsageAttribute(AttributeTargets.Assembly, Inherited = false, AllowMultiple = true)]
public sealed class PrimaryInteropAssemblyAttribute : Attribute

You can apply this attribute to assemblies, although the Type Library Importer (Tlbimp.exe) typically applies it for you when it imports a type library.

An interop assembly contains metadata that describes existing COM types, which are often already described in a COM type library. Tlbimp.exe produces interop assemblies from COM type libraries. Interop assemblies typically only contain metadata (no code). Primary interop assemblies are provided by the same publisher as the type library they describe, and provide the official definitions of the types defined with that type library. Primary interop assemblies are always signed by their publisher to ensure uniqueness.

You can generate a primary interop assembly from a type library in the following ways:

  • Run TlbImp.exe with the /primary option from the command line.

  • Apply the PrimaryInteropAssemblyAttribute at design time.

To specify a primary interop assembly in managed source code, you must apply the System.Runtime.InteropServices.GuidAttribute and PrimaryInteropAssemblyAttribute to the assembly at design time. The GuidAttribute on the primary interop assembly identifies the LIBID of the type library and the PrimaryInteropAssemblyAttribute identifies the version of the particular type library for which this assembly is the primary interop assembly. The PrimaryInteropAssemblyAttribute can appear multiple times if the assembly is the primary interop assembly for multiple versions of the same type library.

When using the types defined in a type library, always reference the primary interop assembly for that type library, rather than reimporting or redefining the types themselves. For guidelines and procedures on how to produce or use primary interop assemblies, see Primary Interop Assemblies. For a detailed description of the type library importing process, see Type Library to Assembly Conversion Summary.


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

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, 1.1, 1.0