CODEWITHSUNDEEP
c#.netinteroppinvoke
Grzenio
Grzenio

Reputation: 36649

PInvoke CreateDesktop

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?

Upvotes: 2

Views: 3946

Answers (2)

dtb
dtb

Reputation: 217313

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.

Upvotes: 6

csharptest.net
csharptest.net

Reputation: 64218

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.

Upvotes: 1

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