PInvoke CreateDesktop

Question

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?

it is always worth checking http://www.pinvoke.net/ for these type of problems. — Ian Ringrose, Sep 21 '09 at 14:17
Unless the reason you're here is because you got this exception from using a declaration you got from pinvoke.net... *(raises hand)* — Daniel Earwicker, Sep 12 '14 at 09:52
@IanRingrose pinvoke.net declaration of `CreateDesktop` function is still bugged. It works only if one uses class to declare `SECURITY_ATTRIBUTES` and passes `null` as it's value. — Lightman, Jun 05 '17 at 11:45

dtb · Accepted Answer · 2009-09-16T17:43:09.927

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.

score 1 · Answer 2 · answered Sep 16 '09 at 17:35

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.

PInvoke CreateDesktop

2 Answers2

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