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.

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 System.Runtime.InteropServices.ICustomMarshaler 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 System.Runtime.InteropServices.ICustomMarshaler interface includes methods that provide conversion support, cleanup support, and information about the data to be marshaled.

Conversion

ICustomMarshaler.MarshalNativeToManaged(IntPtr)

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.

ICustomMarshaler.MarshalManagedToNative(object)

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

ICustomMarshaler.CleanUpNativeData(IntPtr)

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

ICustomMarshaler.CleanUpManagedData(object)

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

Size Information

ICustomMarshaler.GetNativeDataSize

Returns the size of the unmanaged data to be marshaled.

Implementing the GetInstance Method

In addition to implementing the System.Runtime.InteropServices.ICustomMarshaler interface, custom marshalers must implement a static method called GetInstance that accepts a string as a parameter and has a return type of System.Runtime.InteropServices.ICustomMarshaler. 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 System.Runtime.InteropServices.MarshalAsAttribute attribute to the parameter or field that is being marshaled.

You must also pass the UnmanagedType.CustomMarshaler enumeration value to the System.Runtime.InteropServices.MarshalAsAttribute constructor. In addition, you must specify the MarshalAsAttribute.MarshalType field with one of the following named parameters:

• MarshalAsAttribute.MarshalType (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.

• MarshalAsAttribute.MarshalCookie (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 System.Runtime.InteropServices.MarshalAsAttribute 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 ICustomMarshaler.MarshalNativeToManaged(IntPtr) and ICustomMarshaler.MarshalManagedToNative(object) 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 System.Runtime.InteropServices.MarshalAsAttribute 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 System.Runtime.InteropServices.MarshalAsAttribute attribute is the UnmanagedType.CustomMarshaler enumeration value UnmanagedType.CustomMarshaler.

The second parameter is the MarshalAsAttribute.MarshalType 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").