PInvoke CreateDesktop

Question

Hi,

I am trying to PInvoke CreateDesktop in a way that passes the flag to inherit the desktop by child processes. The declaration is as follows:

[DllImport("user32", EntryPoint = "CreateDesktopW", CharSet = CharSet.Unicode, SetLastError = true)]
        public static extern IntPtr CreateDesktop(string lpszDesktop, IntPtr lpszDevice, IntPtr pDevmode, int dwFlags,
                                                  int dwDesiredAccess, [MarshalAs(UnmanagedType.LPStruct)] SECURITY_ATTRIBUTES lpsa);

        [StructLayout(LayoutKind.Sequential)]
        public struct SECURITY_ATTRIBUTES
        {
            public int nLength;
            public IntPtr lpSecurityDescriptor;
            public int bInheritHandle;
        }

And I use it as follows:

Win32.SECURITY_ATTRIBUTES sa = new Win32.SECURITY_ATTRIBUTES();
            sa.nLength = Marshal.SizeOf(sa);
            sa.bInheritHandle = 1;
            testDesktopHandle = Win32.CreateDesktop(name, IntPtr.Zero, IntPtr.Zero, 0, Win32.GENERIC_ALL, sa);

And it unfortunately doesn't work, I get the following error:

System.Runtime.InteropServices.MarshalDirectiveException: Cannot marshal 'parameter #6': Invalid managed/unmanaged type combination (this value type must be paired with Struct).

Any ideas what I am doing wrong?

Answer 1

Try changing parameter #6 to

static extern IntPtr CreateDesktop(..., [In] ref SECURITY_ATTRIBUTES lpsa);

(This compiles and doesn't throw an exception at runtime, but I've tested it only with bogus arguments.)

Compare with C++ declaration of CreateDesktop:

HDESK WINAPI CreateDesktop(..., __in_opt LPSECURITY_ATTRIBUTES lpsa);
                                  ↑      ↑ ↑
                                  [In] ref SECURITY_ATTRIBUTES lpsa

LP stands for "long pointer", i.e. LPSECURITY_ATTRIBUTES is a pointer to a SECURITY_ATTRIBUTES struct. So in C# you need to pass your struct instance (value type) by reference.

Answer 2

Consider using the following prototype instead:

    [DllImport("user32", EntryPoint = "CreateDesktopW", CharSet = CharSet.Unicode, SetLastError = true)]
    public static extern IntPtr CreateDesktop(string lpszDesktop, IntPtr lpszDevice, IntPtr pDevmode, int dwFlags,
                                              int dwDesiredAccess, IntPtr lpsa);

Then to call it just create a pinned handle:

  GCHandle handle = GCHandle.Alloc(myStruct);
  try {
   IntPtr pinnedAddress = handle.AddrOfPinnedObject();
  }
  finally {
   handle.Free();
  }

This works VERY well for calling PInvoke'd methods with structs.