C# 4 and .NET 4

Using a CoM Component from a .neT Client ❘ 709
Earlier you learned that the COM object is released as soon as the reference count is 0.
Marshal.ReleaseComObject() decrements the reference count by 1 by invoking the
Release() method. Because the RCW does just one call to AddRef() to increment
the reference count, a single call to Marshal.ReleaseComObject() is enough to release
the object no matter how many references to the RCW you keep.
After releasing the COM object using Marshal.ReleaseComObject() , you may not use any variable that
references the object. In the example, the COM object is released by using the variable math . The variable
welcome , which references the same object, cannot be used after releasing the object. Otherwise, you will
get an exception of type InvalidComObjectException .
Releasing COM objects when they are no longer needed is extremely important. COM
objects make use of the native memory heap, whereas .NET objects make use of the
managed memory heap. The garbage collector only deals with managed memory.
As you can see, with a runtime callable wrapper, a COM component can be used similarly to a .NET object.
using the Com server with dynamic language extensions
C# 4 includes an extension for using dynamic languages from C#. This is also an advantage for using COM
servers that offer the IDispatch interface. As you read earlier in the “ Dispatch Interfaces ” section, this
interface is resolved at runtime with the methods GetIdsOfNames() and Invoke() . With the dynamic
keyword and the help of a COM binder that is used behind the scenes, the COM component can be called
without creating an RCW object.
Declaring a variable of type dynamic and assigning a COM object to it uses the COM binder, and you can
invoke the methods of the default interface as shown. Creating an instance of the COM object without using
an RCW can be done by getting the Type object using Type.GetTypeFromProgID() , and instantiating
the COM object with the Activator.CreateInstance() method. You just don ’ t get IntelliSense with the
dynamic keyword, but can use the optional parameters that are very common with COM:
using System;
namespace Wrox.ProCSharp.Interop
{
class Program
{
static void Main()
{
Type t = Type.GetTypeFromProgID("COMServer.COMDemo");
dynamic o = Activator.CreateInstance(t);
Console.WriteLine(o.Greeting("Angela"));
}
}
}
code snippet DynamicDotnetClient/Program.cs
The dynamic language extensions of C# are explained in Chapter 12, “Dynamic
Language Extensions.”
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