Question

I made a very small application that captures the screen inside games using SlimDX.
(I press left click to capture)

The capture works (atleast when I click on the form itself) but as soon as I click on firefox or any other application, I get this exception :

A callback was made on a garbage collected delegate of type ‘CaptureScreen!CaptureScreen.Form1+WinEventDelegate::Invoke’. This may cause application crashes, corruption and data loss. When passing delegates to unmanaged code, they must be kept alive by the managed application until it is guaranteed that they will never be called.

at this line in my program.cs:

Application.Run(new Form1());

My Form1.cs (the designer itself has no controls)

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Drawing.Imaging;
    using System.IO;
    using System.Linq;
    using System.Runtime.InteropServices;
    using System.Text;
    using System.Windows.Forms;
    using Microsoft.DirectX.Direct3D;

    namespace CaptureScreen
    {
        public partial class Form1 : Form
        {
            private const uint WINEVENT_OUTOFCONTEXT = 0;
            private const uint EVENT_SYSTEM_FOREGROUND = 3;
            private const int WH_MOUSE_LL = 14;
            private const int WM_LBUTTONDOWN = 513;

            delegate void WinEventDelegate(IntPtr hWinEventHook, uint eventType, IntPtr hwnd,         int idObject, int idChild, uint dwEventThread, uint dwmsEventTime);

            IntPtr m_hhook;

            [DllImport("user32.dll")]
            static extern bool UnhookWinEvent(IntPtr hWinEventHook);
            [DllImport("user32.dll")]
            static extern IntPtr SetWinEventHook(uint eventMin, uint eventMax, IntPtr                 hmodWinEventProc, WinEventDelegate lpfnWinEventProc, uint idProcess, uint         idThread, uint dwFlags);
            [DllImport("user32.dll")]
            static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

            public Form1()
            {
                m_hhook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND,         IntPtr.Zero, WinEventProc, 0, 0, WINEVENT_OUTOFCONTEXT);

                hookProc = new HookProc(LowLevelMouseProc);
                hook = SetWindowsHookEx(WH_MOUSE_LL, hookProc, GetModuleHandle(null), 0);

                InitializeComponent();
            }

            private void Form1_FormClosing(object sender, FormClosingEventArgs e)
            {
                UnhookWinEvent(m_hhook);
                UnhookWindowsHookEx(hook);
            }

            void WinEventProc(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int         idChild, uint dwEventThread, uint dwmsEventTime)
            {
                if (eventType == EVENT_SYSTEM_FOREGROUND)
                {
                    StringBuilder sb = new StringBuilder(500);
                    GetWindowText(hwnd, sb, sb.Capacity);
                }
            }

            [DllImport("kernel32.dll")]
            static extern IntPtr GetModuleHandle(string moduleName);

            [DllImport("user32.dll")]
            static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint         dwThreadId);

            [DllImport("user32.dll")]
            public static extern int UnhookWindowsHookEx(IntPtr hhook);

            [DllImport("user32.dll")]
            static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, uint wParam, IntPtr lParam);
            delegate IntPtr HookProc(int nCode, uint wParam, IntPtr lParam);

            [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
            public static extern IntPtr GetForegroundWindow();

            private HookProc hookProc;
            private IntPtr hook;

            IntPtr LowLevelMouseProc(int nCode, uint wParam, IntPtr lParam)
            {
                if (nCode >= 0 && (IntPtr)wParam == (IntPtr)WM_LBUTTONDOWN)
                {
                    CaptureScreen();
                }
                return CallNextHookEx(IntPtr.Zero, nCode, wParam, lParam);
            }

            private void CaptureScreen()
            {
                StreamReader reader = new StreamReader(Path.GetFullPath("../../Counter.txt"));
                string currentpic = reader.ReadLine();
                if (string.IsNullOrEmpty(currentpic))
                    currentpic = "0";
                reader.Close();

                Bitmap bitmap = Direct3DCapture.CaptureWindow(GetForegroundWindow());
                bitmap.Save(Path.GetFullPath("../../ScreenCapture/Test" + currentpic + ".gif"),         ImageFormat.Gif);

                StreamWriter writer = new StreamWriter(Path.GetFullPath("../../Counter.txt"));
                writer.Write((int.Parse(currentpic)) + 1);
                writer.Close();
            }

            public readonly uint DWM_EC_DISABLECOMPOSITION = 0;
            public readonly uint DWM_EC_ENABLECOMPOSITION = 1;
            [DllImport("dwmapi.dll", EntryPoint = "DwmEnableComposition")]
            protected static extern uint Win32DwmEnableComposition(uint uCompositionAction);
        }
    }

the class that captures the screen can be found here:
http://spazzarama.wordpress.com/2009/02/07/screencapture-with-direct3d/

Any idea on how I can fix this?

Answer 1

Your problem is that you are just passing WinEventProc to SetWinEventHook, which will implicitly create a delegate that is eligible to be GCed once the current method exits (if not sooner!) You are seeing the consequences of that fact.

You will need to create a new member of Form1 of type WinEventDelegate, and use that as the parameter:

private WinEventDelegate winEventProc;

and then make use of it in your call to SetWinEventHook:

this.winEventProc = new WinEventDelegate(WinEventProc);
m_hhook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND, IntPtr.Zero, this.winEventProc, 0, 0, WINEVENT_OUTOFCONTEXT);

That should ensure that your delegate stays alive as long as you need.