C# 4 and .NET 4

708 ❘ ChaPTer 26 interOp
An RCW can also be created by using Visual Studio. To create a simple sample application, create a C#
console project. In Solution Explorer, add a reference to the COM server by selecting the COM tab in the
Add Reference dialog, and scroll down to the entry COMServer 1.0 Type Library . Here are listed all
COM objects that are confi gured in the registry. Selecting a COM component from the list creates an RCW
class. With Visual Studio 2010, this wrapper class can be created in the main assembly of the project by
setting the property Embed Interop Types to true , which is the default. Setting it to false creates a
separate interop assembly that needs to be deployed with the application.
using the rCW
After creating the wrapper class, you can write the code for the application to instantiate and access the
component. Because of the custom attributes in the C++ fi le, the generated namespace of the RCW class is
Wrox.ProCSharp.COMInterop.Server . Add this namespace, as well as the namespace System.Runtime
.InteropServices , to the declarations. From the namespace System.Runtime.InteropServices , the
Marshal class will be used to release the COM object:
using System;
using System.Runtime.InteropServices;
using Wrox.ProCSharp.Interop.Server
namespace Wrox.ProCSharp.Interop.Client
{
class Program
{
[STAThread]
static void Main()
{
code snippet DotnetClient/Program.cs
Now, the COM component can be used similarly to a .NET class. obj is a variable of type COMDemo . COMDemo
is a .NET interface that offers the methods of both the IWelcome and IMath interfaces. However, it is also
possible to cast to a specifi c interface such as IWelcome . With a variable that is declared as type IWelcome ,
the method Greeting() can be called.
var obj = new COMDemo();
IWelcome welcome = obj;
Console.WriteLine(welcome.Greeting("Stephanie"));
A l t ho u g h COMDemo is an interface, you can instantiate new objects of type COMDemo.
Contrary to normal interfaces, you can do this with wrapped COM interfaces.
If the object — as in this case — offers multiple interfaces, a variable of the other interface can be declared,
and by using a simple assignment with the cast operator, the wrapper class does a QueryInterface() with
the COM object to return the second interface pointer. With the I Math variable, the methods of the IMath
interface can be called.
IMath math;
math = (IMath)welcome;
int x = math.Add(4, 5);
Console.WriteLine(x);
If the COM object should be released before the garbage collector cleans up the object, the static method
Marshal.ReleaseComObject() invokes the Release() method of the component, so that the component
can destroy itself and free up memory:
}
}
}
Marshal.ReleaseComObject(math);
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