M>      [DllImport("coredll.dll", EntryPoint = "GetModuleFileName", SetLastError = true)]
M>        internal static extern Int32 GetModuleFileName(object hModule, Char[] lpFileName, int nBufferSize);


Народ, зачем так сложно да еще и интероп приплетать?

[c#]
public string GetApplicationPath()
{
   path =  Path.GetDirectoryName(Assembly.GetExecutingAssembly().GetModules()[0].FullyQualifiedName); 
   if(path[path.Length-1] != '\\')
      path += '\\';
   return path;
}