ComVisible

COMvisible refers to a .NET attribute used to control whether a type or an assembly is exposed to COM (Component Object Model) components. The attribute is implemented as ComVisibleAttribute, found in the System.Runtime.InteropServices namespace. It plays a key role in COM interop by signaling to COM clients whether the annotated code should be visible and accessible from unmanaged code.

Usage and scope

The attribute can be applied at two levels: the assembly level and the individual type level. When applied at the assembly level, it determines the default COM visibility for all types within the assembly. A common pattern is to set the assembly to false and then opt in specific types by applying ComVisible(true) to those types. Conversely, leaving the assembly visible by default allows types to be exposed unless explicitly restricted.

Example

using System.Runtime.InteropServices;

[assembly: ComVisible(false)]

[ComVisible(true)]

public class MyComVisibleClass

{

// members accessible from COM

}

Impact and related considerations

Marking a type as ComVisible(true) makes it accessible to COM clients, subject to additional interop settings such as GUIDs, interface types, and class interfaces. For types intended for COM, developers often assign GUIDs via GuidAttribute and may specify an interface type with InterfaceTypeAttribute. Exposing .NET types to COM typically requires registration of the assembly for COM interop, commonly using tools like regasm and the optional creation of a type library (TLB).

Notes on platforms

ComVisible is supported in .NET Framework and is available in modern .NET runtimes with COM interop capabilities. In .NET Core and later, COM interop remains possible on Windows, but deployment and registration considerations may differ from classic .NET Framework scenarios.