C# 4 and .NET 4

Using a CoM Component from a .neT Client ❘ 707
}
*message = tmp;
return S_OK;
STDMETHODIMP CCOMDemo::Add(LONG val1, LONG val2, LONG* result)
{
*result = val1 + val2;
return S_OK;
}
STDMETHODIMP CCOMDemo::Sub(LONG val1, LONG val2, LONG* result)
{
*result = val1 - val2;
return S_OK;
}
code snippet COMServer/COMDemo.cpp
Now, you can build the component. The build process also confi gures the component in the registry.
Creating a runtime Callable Wrapper
You can now use the COM component from within .NET. To make this possible, you must create a
runtime callable wrapper (RCW). Using the RCW, the .NET client sees a .NET object instead of the COM
component; there is no need to deal with the COM characteristics because this is done by the wrapper. An
RCW hides the IUnknown and IDispatch interfaces (see Figure 26 - 9) and deals itself with the reference
counts of the COM object.
IUnknown
IDispatch
.NET
IMath
RCW
IWelcome
COM
Client
IWelcome
IMath
Object
figure 26-9
The RCW can be created by using the command - line utility tlbimp or by using Visual Studio.
Starting the command
tlbimp COMServer.dll /out:Interop.COMServer.dll
creates the fi le Interop.COMServer.dll , which contains a .NET assembly with the wrapper class. In
this generated assembly, you can fi nd the namespace COMWrapper with the class CCOMDemoClass and the
interfaces CCOMDemo , IMath , and IWelcome . The name of the namespace can be changed by using options
of the tlbimp utility. The option /namespace allows you to specify a different namespace, and with
/asmversion you can defi ne the version number of the assembly.
Another important option of this command-line utility is /keyfile, which is used for
assigning a strong name to the generated assembly. Strong names are discussed in Chapter 18.
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