You can apply this attribute to assemblies, although the [<topic://cpgrfTypeLibraryImporterTlbimpexe>] 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 System.Runtime.InteropServices.PrimaryInteropAssemblyAttribute at design time.

To specify a primary interop assembly in managed source code, you must apply the System.Runtime.InteropServices.GuidAttribute and System.Runtime.InteropServices.PrimaryInteropAssemblyAttribute to the assembly at design time. The System.Runtime.InteropServices.GuidAttribute on the primary interop assembly identifies the LIBID of the type library and the System.Runtime.InteropServices.PrimaryInteropAssemblyAttribute identifies the version of the particular type library for which this assembly is the primary interop assembly. The System.Runtime.InteropServices.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 [<topic://cpconprimaryinteropassemblies>]. For a detailed description of the type library importing process, see [<topic://cpcontypelibrarytoassemblyconversionsummary>].