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Applying Interop Attributes

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The System.Runtime.InteropServices namespace provides three categories of interop-specific attributes: those applied by you at design time, those applied by COM interop tools and APIs during the conversion process, and those applied either by you or COM interop.

If you are unfamiliar with the task of applying attributes to managed code, see Extending Metadata Using Attributes. Like other custom attributes, you can apply interop-specific attributes to types, methods, properties, parameters, fields, and other members.

Design-Time Attributes

You can adjust the outcome of the conversion process performed by COM interop tools and APIs by using design-time attributes. The following table describes the attributes that you can apply to your managed source code. COM interop tools, on occasion, might also apply the attributes described in this table.

Attribute Description
AutomationProxyAttribute Specifies whether the type should be marshalled using the Automation marshaller or a custom proxy and stub.
ClassInterfaceAttribute Controls the type of interface generated for a class.
CoClassAttribute Identifies the CLSID of the original coclass imported from a type library.

COM interop tools typically apply this attribute.
ComImportAttribute Indicates that a coclass or interface definition was imported from a COM type library. The runtime uses this flag to know how to activate and marshal the type. This attribute prohibits the type from being exported back to a type library.

COM interop tools typically apply this attribute.
ComRegisterFunctionAttribute Indicates that a method should be called when the assembly is registered for use from COM, so that user-written code can be executed during the registration process.
ComSourceInterfacesAttribute Identifies interfaces that are sources of events for the class.

COM interop tools can apply this attribute.
ComUnregisterFunctionAttribute Indicates that a method should be called when the assembly is unregistered from COM, so that user-written code can execute during the process.
ComVisibleAttribute Renders types invisible to COM when the attribute value equals false. This attribute can be applied to an individual type or to an entire assembly to control COM visibility. By default, all managed, public types are visible; the attribute is not needed to make them visible.
DispIdAttribute Specifies the COM dispatch identifier (DISPID) of a method or field. This attribute contains the DISPID for the method, field, or property it describes.

COM interop tools can apply this attribute.
ComDefaultInterfaceAttribute Indicates the default interface for a COM class implemented in .NET.

COM interop tools can apply this attribute.
FieldOffsetAttribute Indicates the physical position of each field within a class when used with the StructLayoutAttribute, and the LayoutKind is set to Explicit.
GuidAttribute Specifies the globally unique identifier (GUID) of a class, interface, or an entire type library. The string passed to the attribute must be a format that is an acceptable constructor argument for the type System.Guid.

COM interop tools can apply this attribute.
IDispatchImplAttribute Indicates which IDispatch interface implementation the common language runtime uses when exposing dual interfaces and dispinterfaces to COM.
InAttribute Indicates that data should be marshalled in to the caller. Can be used to attribute parameters.
InterfaceTypeAttribute Controls how a managed interface is exposed to COM clients (Dual, IUnknown-derived, or IDispatch only).

COM interop tools can apply this attribute.
LCIDConversionAttribute Indicates that an unmanaged method signature expects an LCID parameter.

COM interop tools can apply this attribute.
MarshalAsAttribute Indicates how the data in fields or parameters should be marshalled between managed and unmanaged code. The attribute is always optional because each data type has default marshalling behavior.

COM interop tools can apply this attribute.
OptionalAttribute Indicates that a parameter is optional.

COM interop tools can apply this attribute.
OutAttribute Indicates that the data in a field or parameter must be marshalled from a called object back to its caller.
PreserveSigAttribute Suppresses the HRESULT or retval signature transformation that normally takes place during interoperation calls. The attribute affects marshalling as well as type library exporting.

COM interop tools can apply this attribute.
ProgIdAttribute Specifies the ProgID of a .NET class. Can be used to attribute classes.
StructLayoutAttribute Controls the physical layout of the fields of a class.

COM interop tools can apply this attribute.

Conversion-Tool Attributes

The following table describes attributes that COM interop tools apply during the conversion process. You do not apply these attributes at design time.

Attribute Description
ComAliasNameAttribute Indicates the COM alias for a parameter or field type. Can be used to attribute parameters, fields, or return values.
ComConversionLossAttribute Indicates that information about a class or interface was lost when it was imported from a type library to an assembly.
ComEventInterfaceAttribute Identifies the source interface and the class that implements the methods of the event interface.
ImportedFromTypeLibAttribute Indicates that the assembly was originally imported from a COM type library. This attribute contains the type library definition of the original type library.
TypeLibFuncAttribute Contains the FUNCFLAGS that were originally imported for this function from the COM type library.
TypeLibTypeAttribute Contains the TYPEFLAGS that were originally imported for this type from the COM type library.
TypeLibVarAttribute Contains the VARFLAGS that were originally imported for this variable from the COM type library.

See also