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Introduction to COM Interop (Visual Basic)

The Component Object Model (COM) lets an object 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) offer many advantages.

Interoperability with COM, or COM interop, enables you to use existing COM objects from .NET apps.

By using .NET to create COM components, you can use registration-free COM interop. This lets you control which DLL version is enabled when more than one version is installed on a computer, and lets end users use XCOPY or FTP to copy your application to an appropriate directory on their computer where it can be run. For more information, see Registration-Free COM Interop.

Managed Code and Data

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

Assemblies

An assembly is the primary building block of a .NET 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 the following:

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

For more information about assemblies and assembly manifests, see Assemblies in .NET.

Importing and Exporting Type Libraries

Visual Studio 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 about Tlbimp and Tlbexp, see Tlbimp.exe (Type Library Importer) and Tlbexp.exe (Type Library Exporter).

Interop Assemblies

Interop assemblies are .NET assemblies that 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 marshalling.

Interoperability Marshalling

All .NET applications share a set of common types that enable interoperability of objects, regardless of the programming language that is used. The parameters and return values of COM objects sometimes use data types that differ from those used in managed code. Interoperability marshalling 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.

See also

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