Managed C++ & P/Invoke in a web serice

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I am exploring using P/Invoke to call some legacy code from our customer
contact software. I have been trying some simple test scenarios and haven't
had success. Can someone point me to an example that shows how this can be
done? Here is a snippet of code I am trying:

Header file of C++ class for web service:

[DllImport("kernel32.dll", SetLastError=true)]
extern "C" bool GetComputerName(String *name, int *len);

Class file for web service:

String __gc* callKAPIClass::HelloWorld()
{
String *p = "1234567890123456789012345678901234567890";
int len = 30;
bool rc = true;

rc = GetComputerName(p, &len);
int rc2 = Marshal::GetLastWin32Error();

return p;
}

Unfortunately, I always get an error in GetComputerName().

Mark
.

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