mscorlib [00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00] 1.0.5000.0 2.0.0.0 4.0.0.0 Gtk# is thread aware, but not thread safe; See the <link location="node:gtk-sharp/programming/threads">Gtk# Thread Programming</link> for details. System.Runtime.InteropServices.ComVisible(true)

A marshaler provides a bridge between the functionality of old and new interfaces. Custom marshaling provides the following benefits:

It enables client applications that were designed to work with an old interface to also work with servers that implement a new interface.

It enables client applications built to work with a new interface to work with servers that implement an old interface.

If you have an interface that introduces different marshaling behavior or that is exposed to the Component Object Model (COM) in a different way, you can design a custom marshaler instead of using the interop marshaler. By using a custom marshaler, you can minimize the distinction between new .NET Framework components and existing COM components.

For example, suppose that you are developing a managed interface called INew. When this interface is exposed to COM through a standard COM callable wrapper (CCW), it has the same methods as the managed interface and uses the marshaling rules built into the interop marshaler. Now suppose that a well-known COM interface called IOld already provides the same functionality as the INew interface. By designing a custom marshaler, you can provide an unmanaged implementation of IOld that simply delegates the calls to the managed implementation of the INew interface. Therefore, the custom marshaler acts as a bridge between the managed and unmanaged interfaces.

Custom marshalers are not invoked when calling from managed code to unmanaged code on a dispatch-only interface.

## Defining the Marshaling Type

Before you can build a custom marshaler, you must define the managed and unmanaged interfaces that will be marshaled. These interfaces commonly perform the same function but are exposed differently to managed and unmanaged objects.

A managed compiler produces a managed interface from metadata, and the resulting interface looks like any other managed interface. The following example shows a typical interface.

code reference: System.Runtime.InteropServices.ICustomMarshaler#1

You define the unmanaged type in Interface Definition Language (IDL) and compile it with the Microsoft Interface Definition Language (MIDL) compiler. You define the interface within a library statement and assign it an interface ID with the universal unique identifier (UUID) attribute, as the following example demonstrates.

`[uuid(9B2BAADA-0705-11D3-A0CD-00C04FA35826)] library OldLib { [uuid(9B2BAADD-0705-11D3-A0CD-00C04FA35826)] interface IOld : IUnknown HRESULT OldMethod(); }`

The MIDL compiler produces several output files. If the interface is defined in Old.idl, the output file Old_i.c defines a const variable with the interface identifier (IID) of the interface, as the following example demonstrates.

`const IID IID_IOld = {0x9B2BAADD,0x0705,0x11D3,{0xA0,0xCD,0x00,0xC0,0x4F,0xA3,0x58,0x26}};`

The Old.h file is also produced by MIDL. It contains a C++ definition of the interface that can be included in your C++ source code.

## Implementing the ICustomMarshaler Interface

Your custom marshaler must implement the interface to provide the appropriate wrappers to the runtime.

The following C# code displays the base interface that must be implemented by all custom marshalers.

code reference: System.Runtime.InteropServices.ICustomMarshaler#2

The interface includes methods that provide conversion support, cleanup support, and information about the data to be marshaled.

Type of operation

ICustomMarshaler method

Description

Conversion (from native to managed code)

Marshals a pointer to native data into a managed object. This method returns a custom runtime callable wrapper (RCW) that can marshal the unmanaged interface that is passed as an argument. The marshaler should return an instance of the custom RCW for that type.

Conversion (from managed to native code)

Marshals a managed object into a pointer to native data. This method returns a custom COM callable wrapper (CCW) that can marshal the managed interface that is passed as an argument. The marshaler should return an instance of the custom CCW for that type.

Cleanup (of native code)

Enables the marshaler to clean up the native data (the CCW) that is returned by the method.

Cleanup (of managed code)

Enables the marshaler to clean up the managed data (the RCW) that is returned by the method.

Information (about native code)

Returns the size of the unmanaged data to be marshaled.

## Conversion

Marshals a pointer to native data into a managed object. This method returns a custom runtime callable wrapper (RCW) that can marshal the unmanaged interface that is passed as an argument. The marshaler should return an instance of the custom RCW for that type.

Marshals a managed object into a pointer to native data. This method returns a custom COM callable wrapper (CCW) that can marshal the managed interface that is passed as an argument. The marshaler should return an instance of the custom CCW for that type.

## Cleanup

Enables the marshaler to clean up the native data (the CCW) that is returned by the method.

Enables the marshaler to clean up the managed data (the RCW) that is returned by the method.

## Size Information

Returns the size of the unmanaged data to be marshaled.

## Implementing the GetInstance Method

In addition to implementing the interface, custom marshalers must implement a static method called GetInstance that accepts a as a parameter and has a return type of . This static method is called by the common language runtime's COM interop layer to instantiate an instance of the custom marshaler. The string that is passed to GetInstance is a cookie that the method can use to customize the returned custom marshaler.

`static ICustomMarshaler *GetInstance(String *pstrCookie);`

## Applying MarshalAsAttribute

To use a custom marshaler, you must apply the attribute to the parameter or field that is being marshaled.

You must also pass the enumeration value to the constructor. In addition, you must specify the field with one of the following named parameters:

(required): The assembly-qualified name of the custom marshaler. The name should include the namespace and class of the custom marshaler. If the custom marshaler is not defined in the assembly it is used in, you must specify the name of the assembly in which it is defined.

You can use the field instead of the field. takes a type that is easier to specify.

(optional): A cookie that is passed to the custom marshaler. You can use the cookie to provide additional information to the marshaler. For example, if the same marshaler is used to provide a number of wrappers, the cookie identifies a specific wrapper. The cookie is passed to the GetInstance method of the marshaler.

The attribute identifies the custom marshaler so it can activate the appropriate wrapper. The common language runtime's interop service then examines the attribute and creates the custom marshaler the first time the argument (parameter or field) needs to be marshaled.

The runtime then calls the and methods on the custom marshaler to activate the correct wrapper to handle the call.

## Using a Custom Marshaler

When the custom marshaler is complete, you can use it as a custom wrapper for a particular type. The following example shows the definition of the IUserData managed interface:

code reference: System.Runtime.InteropServices.ICustomMarshaler#3

In the following example, the IUserData interface uses the NewOldMarshaler custom marshaler to enable unmanaged client applications to pass an IOld interface to the DoSomeStuff method. The managed description of the DoSomeStuff method takes an INew interface, as shown in the previous example, whereas the unmanaged version of DoSomeStuff takes an IOld interface pointer, as shown in the following example.

`[uuid(9B2BAADA-0705-11D3-A0CD-00C04FA35826)] library UserLib { [uuid(9B2BABCD-0705-11D3-A0CD-00C04FA35826)] interface IUserData : IUnknown HRESULT DoSomeStuff(IUnknown* pIOld); }`

The type library that is generated by exporting the managed definition of IUserData yields the unmanaged definition shown in this example instead of the standard definition. The attribute applied to the INew argument in the managed definition of the DoSomeStuff method indicates that the argument uses a custom marshaler, as the following example shows.

code reference: System.Runtime.InteropServices.ICustomMarshaler#4

code reference: System.Runtime.InteropServices.ICustomMarshaler#5

In the previous examples, the first parameter provided to the attribute is the enumeration value UnmanagedType.CustomMarshaler.

The second parameter is the field, which provides the assembly-qualified name of the custom marshaler. This name consists of the namespace and class of the custom marshaler (MarshalType="MyCompany.NewOldMarshaler").

Provides custom wrappers for handling method calls.

- Method
  1.0.5000.0
  2.0.0.0
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  System.Void `` To be added

  Performs necessary cleanup of the managed data when it is no longer needed.

  The managed object to be destroyed.
- Method
  1.0.5000.0
  2.0.0.0
  4.0.0.0
  System.Void `` To be added

  Performs necessary cleanup of the unmanaged data when it is no longer needed.

  A pointer to the unmanaged data to be destroyed.
- Method
  1.0.5000.0
  2.0.0.0
  4.0.0.0
  System.Int32 To be added

  Returns the size of the native data to be marshaled.

  The size, in bytes, of the native data.

- Method
  1.0.5000.0
  2.0.0.0
  4.0.0.0
  System.IntPtr `` To be added

  Converts the managed data to unmanaged data.

  A pointer to the COM view of the managed object.

  The managed object to be converted.
- Method
  1.0.5000.0
  2.0.0.0
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  System.Object `` To be added

  Converts the unmanaged data to managed data.

  An object that represents the managed view of the COM data.

  A pointer to the unmanaged data to be wrapped.