Dealing with DllImport (Part 1 of 2)

Learning to program for the .NET Framework has been an interesting experience. I use the term "interesting" loosely: it has sometimes been amazingly easy, and sometimes unexpectedly frustrating.

For instance, I was writing a simple chat application with Windows Forms in C# and thought it would be good to grab the users' workstation and login names to identify each participant, rather than generate yet another CB-type handle like the instant messenger programs do. "Simple," I thought, "I'll just call GetUserName and GetComputerName." But when I searched the .NET Framework documentation for those Win32 API functions, I couldn't find them.

After a little bit of browsing, I gave up. "Well," I thought, "I can always use P/Invoke," meaning the Platform Invocation facility in .NET used to call DLL functions. "That shouldn't take more than a couple of minutes to set up."

I looked up the first Win32 API function:

    BOOL GetUserName(
     LPTSTR lpBuffer,  // name buffer
     LPDWORD nSize     // size of name buffer
   );

The GetUserName function is implemented in advapi32.dll, so I added the following code to my C# program:

    using System.Runtime.InteropServices; 
   //...
   [DllImport("advapi32.dll")]
   public static extern bool GetUserName( _
      Byte[] lpBuffer, int nSize);
   //...
   Byte[] b=new Byte[100];
   int n=100;
   bool rc=GetUserName(b, n); //kaboom!

Here System.Runtime.InteropServices is the namespace that supports DllImport, and DllImport is an attribute that means you're importing an unmanaged function from a DLL.

When I ran that code, I discovered that it threw a System.NullReferenceException when it tried to call GetUserName.

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