The .NET runtime supports bidirectional interop with unmanaged code via COM, COM+, and native Win32 API calls. The BCL provides a set of classes and attributes that support this, including precise control of managed object lifetime, and the option of creating user-defined custom marshallers to handle specific interop situations.

For more information, see the following namespaces:

System.Runtime.InteropServices
System.Runtime.InteropServices.CustomMarshalers
System.Runtime.InteropServices.Expando
Microsoft.Win32.Interop
            Microsoft.Win32.Interop.Trident

Important types in other namespaces include System.Buffer.