调用vb6 COM dll函数时，C#GetProcAddress返回0

我有一个vb6 COM dll，里面有一个类LoginClass和一个函数LogiUser。我需要从C#动态调用这个vb6 COM dll。我尝试在C#代码下面动态访问它，但即使在LoadLibrary返回指针之后，GetProcAddress仍返回0。

static class NativeMethods
{
    [DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern IntPtr LoadLibrary(string dllToLoad);
    [DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
    public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);
    [DllImport("kernel32")]
    public static extern bool FreeLibrary(IntPtr hModule);
}
class COMCommands
{    
 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
 private delegate string Login(string userName, string Password, bool Result);
 public string CallLoginCommand(string UserName, string Password, ref bool Result)
 {
  IntPtr pDll = NativeMethods.LoadLibrary(@"D:'MyCOMdll.dll");
  IntPtr pAddressOfFunctionToCall = NativeMethods.GetProcAddress(pDll, "LoginUser");
  Login CallLogin = (Login)Marshal.GetDelegateForFunctionPointer(pAddressOfFunctionToCall, typeof(Login));
  string theResult = CallLogin(UserName, Password, Result);
  bool result = NativeMethods.FreeLibrary(pDll);
  return theResult;
 }
}