C# 4 and .NET 4

Using a .neT Component from a CoM Client ❘ 719
#import "./DotNetComponent/bin/debug/DotnetServer.tlb" named_guids
using namespace std;
using namespace DotnetComponent;
code snippet COMClient/COMClient.cpp
In the _tmain() method, the fi rst thing to do before any other COM call is the initialization of COM
with the API call CoInitialize() . CoInitialize() creates and enters an STA for the thread. The
variable spWelcome is of type IWelcomePtr , which is a smart pointer. The smart pointer method
CreateInstance() accepts the ProgID as an argument to create the COM object by using the COM API
CoCreateInstance() . The operator - > is overridden with the smart pointer, so that you can invoke the
methods of the COM object such as Greeting() :
int _tmain(int argc, _TCHAR* argv[])
{
HRESULT hr;
hr = CoInitialize(NULL);
try
{
IWelcomePtr spWelcome;
// CoCreateInstance()
hr = spWelcome.CreateInstance("Wrox.DotnetComponent");
cout < < spWelcome- > Greeting("Bill") < < endl;
The second interface supported by your .NET component is IMath , and there is also a smart pointer
that wraps the COM interface: IMathPtr . You can directly assign one smart pointer to another as in
spMath = spWelcome; . In the implementation of the smart pointer (the = operator is overridden), the
QueryInterface() method is called. With a reference to the IMath interface, you can call the Add() method.
IMathPtr spMath;
spMath = spWelcome; // QueryInterface()
long result = spMath- > Add(4, 5);
cout < < "result:" < < result < < endl;
}
If an HRESULT error value is returned by the COM object (this is done by the CCW that returns HRESULT
errors if the .NET component generates exceptions), the smart pointer wraps the HRESULT errors and
generates _com_error exceptions instead. Errors are handled in the catch block. At the end of the
program, the COM DLLs are closed and unloaded using CoUninitialize() :
catch (_com_error & e)
{
cout < < e.ErrorMessage() < < endl;
}
CoUninitialize();
return 0;
}
Now you can run the application, and you will get outputs from the Greeting() and the Add() methods
to the console. You can also try to debug into the smart pointer class, where you can see the COM API
calls directly.
If you get an exception stating that the component cannot be found, check if the
same version of the assembly that is confi gured in the registry is installed in the global
assembly cache.
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