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Introduction to COM Interop

The Component Object Model (COM) allows an object to expose its functionality to other components and to host applications. While COM objects have been fundamental to Windows programming for many years, applications designed for the common language runtime (CLR) used by the .NET Platform offer many advantages.

.NET Platform applications will eventually replace those developed with COM. Until then, you may need to use or create COM objects with Visual Studio .NET. Interoperability with COM, or COM interop, enables you to use existing COM objects while transitioning to the .NET Platform at your own pace.

Managed Code and Data

Code developed for the .NET Platform is referred to as managed code, and contains metadata that is used by the common language runtime (CLR). Data used by .NET applications is called managed data because the .NET runtime manages data-related tasks such as allocating and reclaiming memory, and type checking. By default, Visual Basic .NET uses managed code and data, but you can access the unmanaged code and data of COM objects using interop assemblies (described later in this topic).


An assembly is the primary building block of a .NET Framework application. It is a collection of functionality that is built, versioned, and deployed as a single implementation unit containing one or more files. Each assembly contains an assembly manifest.

Type Libraries and Assembly Manifests

Type libraries describe characteristics of COM objects, such as member names and data types. Assembly manifests perform the same function for .NET applications. They include information about:

  • Assembly identity, version, culture, and digital signature
  • Files that make up the assembly implementation
  • Types and resources that make up the assembly, including those that are exported from it
  • Compile-time dependencies on other assemblies
  • Permissions required for the assembly to run properly

For more information on assemblies and assembly manifests, see Assemblies.

Importing and Exporting Type Libraries

Visual Studio .NET contains a utility, Tlbimp, that lets you import information from a type library into a .NET application. You can generate type libraries from assemblies by using the Tlbexp utility.

For information on Tlbimp and Tlbexp, see Type Library Importer (Tlbimp.exe) and Type Library Exporter (Tlbexp.exe).

Interop Assemblies

Interop assemblies are .NET assemblies that act as a bridge between managed and unmanaged code, mapping COM object members to equivalent .NET managed members. Interop assemblies created by Visual Basic .NET handle many of the details of working with COM objects, such as interoperability marshaling.

Interoperability Marshaling

All .NET applications share a set of common types that allow interoperability of objects, regardless of the programming language used. The parameters and return values of COM objects sometimes use data types that are different than those used in managed code. Interoperability marshaling is the process of packaging parameters and return values into equivalent data types as they move to and from COM objects. For more information, see Interop Marshaling Overview.

See Also

COM Interop | Walkthrough: Implementing Inheritance with COM Objects | Interoperating with Unmanaged Code | Troubleshooting Interoperability | Assemblies | Type Library Importer (Tlbimp.exe) | Type Library Exporter (Tlbexp.exe) | Interop Marshaling Overview