C# dll crushed (System.Drawing exception)

[ouzsharp]

The follwing class works well when used in a C# console application:

namespace ScreenShotDemo
{
    /// <summary>
    /// Provides functions to capture the entire screen, or a particular window, and save it to a file.
    /// </summary>
    public class ScreenCapture
    {
        /// <summary>
        /// Creates an Image object containing a screen shot of the entire desktop
        /// </summary>
        /// <returns></returns>
        public Image CaptureScreen()
        {
            return CaptureWindow(User32.GetDesktopWindow());
        }
        /// <summary>
        /// Creates an Image object containing a screen shot of a specific window
        /// </summary>
        /// <param name="handle">The handle to the window. (In windows forms, this is obtained by the Handle property)</param>
        /// <returns></returns>
        public Image CaptureWindow(IntPtr handle)
        {
            // get te hDC of the target window
            IntPtr hdcSrc = User32.GetWindowDC(handle);
            // get the size
            User32.RECT windowRect = new User32.RECT();
            User32.GetWindowRect(handle, ref windowRect);
            int width = windowRect.right - windowRect.left;
            int height = windowRect.bottom - windowRect.top;
            // create a device context we can copy to
            IntPtr hdcDest = GDI32.CreateCompatibleDC(hdcSrc);
            // create a bitmap we can copy it to,
            // using GetDeviceCaps to get the width/height
            IntPtr hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc, width, height);
            // select the bitmap object
            IntPtr hOld = GDI32.SelectObject(hdcDest, hBitmap);
            // bitblt over
            GDI32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, GDI32.SRCCOPY);
            // restore selection
            GDI32.SelectObject(hdcDest, hOld);
            // clean up 
            GDI32.DeleteDC(hdcDest);
            User32.ReleaseDC(handle, hdcSrc);
            // get a .NET image object for it
            Image img = Image.FromHbitmap(hBitmap);
            // free up the Bitmap object
            GDI32.DeleteObject(hBitmap);
            return img;
        }
        /// <summary>
        /// Captures a screen shot of a specific window, and saves it to a file
        /// </summary>
        /// <param name="handle"></param>
        /// <param name="filename"></param>
        /// <param name="format"></param>
        public void CaptureWindowToFile(IntPtr handle, string filename, ImageFormat format)
        {
            Image img = CaptureWindow(handle);
            img.Save(filename, format);
        }
        /// <summary>
        /// Captures a screen shot of the entire desktop, and saves it to a file
        /// </summary>
        /// <param name="filename"></param>
        /// <param name="format"></param>
        public void CaptureScreenToFile(string filename, ImageFormat format)
        {
            Image img = CaptureScreen();
            img.Save(filename, format);
        }
 
        /// <summary>
        /// Helper class containing Gdi32 API functions
        /// </summary>
        private class GDI32
        {
 
            public const int SRCCOPY = 0x00CC0020; // BitBlt dwRop parameter
            [DllImport("gdi32.dll")]
            public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest,
                int nWidth, int nHeight, IntPtr hObjectSource,
                int nXSrc, int nYSrc, int dwRop);
            [DllImport("gdi32.dll")]
            public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth,
                int nHeight);
            [DllImport("gdi32.dll")]
            public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
            [DllImport("gdi32.dll")]
            public static extern bool DeleteDC(IntPtr hDC);
            [DllImport("gdi32.dll")]
            public static extern bool DeleteObject(IntPtr hObject);
            [DllImport("gdi32.dll")]
            public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
        }
 
        /// <summary>
        /// Helper class containing User32 API functions
        /// </summary>
        private class User32
        {
            [StructLayout(LayoutKind.Sequential)]
            public struct RECT
            {
                public int left;
                public int top;
                public int right;
                public int bottom;
            }
            [DllImport("user32.dll")]
            public static extern IntPtr GetDesktopWindow();
            [DllImport("user32.dll")]
            public static extern IntPtr GetWindowDC(IntPtr hWnd);
            [DllImport("user32.dll")]
            public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);
            [DllImport("user32.dll")]
            public static extern IntPtr GetWindowRect(IntPtr hWnd, ref RECT rect);
        }
    }
}
 
 But when I use it in a dll for COM object, as in the following:
 
namespace Database_COMObject
{
    [Guid("694C1820-04B6-4988-928F-FD858B95C880")]
    public interface DBCOM_Interface
    {
        [DispId(1)]
        void Init(string userid, string password);
        [DispId(2)]
        bool ExecuteSelectCommand(string selCommand);
        [DispId(3)]
        bool NextRow();
        [DispId(4)]
        void ExecuteNonSelectCommand(string insCommand);
        [DispId(5)]
        string GetColumnData(int pos);
        [DispId(6)]
        string GetMessage(string msg);
        [DispId(7)]
        Image CaptureScreen();
        [DispId(8)]
        Image CaptureWindow(IntPtr handle);
        [DispId(9)]
        Image GetImage(int ms);
    }
 
    // Events interface Database_COMObjectEvents 
    [Guid("47C976E0-C208-4740-AC42-41212D3C34F0"),
    InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    public interface DBCOM_Events
    {
    }
 
 
    [Guid("9E5E5FB2-219D-4ee7-AB27-E4DBED8E123E"),
    ClassInterface(ClassInterfaceType.None),
    ComSourceInterfaces(typeof(DBCOM_Events))]
    public class DBCOM_Class : DBCOM_Interface
    {
       //-----------------------
        public const int SRCCOPY = 0x00CC0020; // BitBlt dwRop parameter
        [DllImport("gdi32.dll")]
        public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest,
            int nWidth, int nHeight, IntPtr hObjectSource,
            int nXSrc, int nYSrc, int dwRop);
        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth,
            int nHeight);
        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
        [DllImport("gdi32.dll")]
        public static extern bool DeleteDC(IntPtr hDC);
        [DllImport("gdi32.dll")]
        public static extern bool DeleteObject(IntPtr hObject);
        [DllImport("gdi32.dll")]
        public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
        //----------------------
        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;
        }
        [DllImport("user32.dll")]
        public static extern IntPtr GetDesktopWindow();
        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowDC(IntPtr hWnd);
        [DllImport("user32.dll")]
        public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);
        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowRect(IntPtr hWnd, ref RECT rect);
        //----------------------
        private SqlConnection myConnection = null;
        SqlDataReader myReader = null;
 
        public DBCOM_Class()
        {
        }
 
        public void Init(string userid, string password)
        {
            try
            {
                string myConnectString = "user id=" + userid + ";password=" + password +
                    ";Database=NorthWind;Server=SKYWALKER;Connect Timeout=30";
                myConnection = new SqlConnection(myConnectString);
                myConnection.Open();
                //MessageBox.Show("CONNECTED");
            }
            catch (Exception e)
            {
                MessageBox.Show(e.Message);
            }
        }
 
        public bool ExecuteSelectCommand(string selCommand)
        {
            if (myReader != null)
                myReader.Close();
 
            SqlCommand myCommand = new SqlCommand(selCommand);
            myCommand.Connection = myConnection;
            myCommand.ExecuteNonQuery();
            myReader = myCommand.ExecuteReader();
            return true;
        }
 
        public bool NextRow()
        {
            if (!myReader.Read())
            {
                myReader.Close();
                return false;
            }
            return true;
        }
 
        public string GetColumnData(int pos)
        {
            Object obj = myReader.GetValue(pos);
            if (obj == null) return "";
            return obj.ToString();
        }
 
        public void ExecuteNonSelectCommand(string insCommand)
        {
            SqlCommand myCommand = new SqlCommand(insCommand, myConnection);
            int retRows = myCommand.ExecuteNonQuery();
        }
 
 
        public string GetMessage(string msg)
        {
            return msg;
        }
        public Image GetImage(int ms)
        {
            return CaptureScreen();
        }
        /// <summary>
        /// Creates an Image object containing a screen shot of the entire desktop
        /// </summary>
        /// <returns></returns>
        public Image CaptureScreen()
        {
            return CaptureWindow(GetDesktopWindow());
        }
        /// <summary>
        /// Creates an Image object containing a screen shot of a specific window
        /// </summary>
        /// <param name="handle">The handle to the window. (In windows forms, this is obtained by the Handle property)</param>
        /// <returns></returns>
        public Image CaptureWindow(IntPtr handle)
        {
            // get te hDC of the target window
            IntPtr hdcSrc = User32.GetWindowDC(handle);
            // get the size
            User32.RECT windowRect = new User32.RECT();
            User32.GetWindowRect(handle, ref windowRect);
            int width = windowRect.right - windowRect.left;
            int height = windowRect.bottom - windowRect.top;
            // create a device context we can copy to
            IntPtr hdcDest = GDI32.CreateCompatibleDC(hdcSrc);
            // create a bitmap we can copy it to,
            // using GetDeviceCaps to get the width/height
            IntPtr hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc, width, height);
            // select the bitmap object
            IntPtr hOld = GDI32.SelectObject(hdcDest, hBitmap);
            // bitblt over
            GDI32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, GDI32.SRCCOPY);
            // restore selection
            GDI32.SelectObject(hdcDest, hOld);
            // clean up 
            GDI32.DeleteDC(hdcDest);
            User32.ReleaseDC(handle, hdcSrc);
            // get a .NET image object for it
            Image img = Image.FromHbitmap(hBitmap);
            // free up the Bitmap object
            GDI32.DeleteObject(hBitmap);
            return img;
        }
    }
 
    /// <summary>
    /// Helper class containing Gdi32 API functions
    /// </summary>
    public class GDI32
    {
 
        public const int SRCCOPY = 0x00CC0020; // BitBlt dwRop parameter
        [DllImport("gdi32.dll")]
        public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest,
            int nWidth, int nHeight, IntPtr hObjectSource,
            int nXSrc, int nYSrc, int dwRop);
        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth,
            int nHeight);
        [DllImport("gdi32.dll")]
        public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
        [DllImport("gdi32.dll")]
        public static extern bool DeleteDC(IntPtr hDC);
        [DllImport("gdi32.dll")]
        public static extern bool DeleteObject(IntPtr hObject);
        [DllImport("gdi32.dll")]
        public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
    }
 
    /// <summary>
    /// Helper class containing User32 API functions
    /// </summary>
    public class User32
    {
        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;
        }
        [DllImport("user32.dll")]
        public static extern IntPtr GetDesktopWindow();
        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowDC(IntPtr hWnd);
        [DllImport("user32.dll")]
        public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);
        [DllImport("user32.dll")]
        public static extern IntPtr GetWindowRect(IntPtr hWnd, ref RECT rect);
    }
    public class Class1
    {
    }
}
 
 

The "GetMessage()" method works well, but the "CaptureScreen()" method will cause "System.Drawing" exception:

NB: I called this from PHP

Fatal error: Uncaught exception 'com_exception' with message '<b>Source:</b> System.Drawing<br/><b>Description:</b> 使用されたパラメータが有効ではありません (The japanese characters mean: The used parameter(s) not valid).

Please do you have any idea?
Thanks in advance

[tlhintoq]

The "*GetMessage()*" method works well, but the "*CaptureScreen()*" method will cause "_System.Drawing_" exception:

public string GetMessage(string msg)
        {
            return msg;
        }

It would be kind of hard for GetMessage() to NOT work. All it does is return the message sent in. It basically serves no purpose at all since any method calling it already knew the value of 'msg'.

public Image CaptureWindow(IntPtr handle)

Put a breakpoint on this line. See what value is being passed in when it is called. I'll bet that 'handle' is null.

Since CaptureScreen() is passing out a value it receives from

[DllImport("user32.dll")]
            public static extern IntPtr GetDesktopWindow();

you are at the mercy of this DLL function. If it fails, then you don't have a value to pass out, so the next method in line fails.

All of your code assumes all the values received will be good. There is no error correction/compensation. If anything comes in that isn't valid it comes crashing down. You need to check that values passed into functions are what you expect and fail gracefully if they aren't.

[Plater]

I am a bit confused. Your .NET code is almost entirely DLLImports. You are making a .NET DLL that wraps unmanaged DLLImport calls...then wrapping it BACK to an COM DLL...?
Why not just make a plain COM DLL and bypass the .NET overhead for it?
I see you save the image to file but I believe there are methods to do that with hBitmap anyway?