C# Gdi中的绘制只发生一次

我正在c#中使用这个winapi库，并试图用它创建一个屏幕截图应用程序。它与常规的winapi完全相似。问题是，即使我正在更新窗口，绘制也只发生一次。我曾经用WiAPI做过一次C++，有完全相同的问题，但我似乎找不到解决的办法。有人知道这有什么问题吗？ 谢谢

内部类程序
{
静态public int virtualScreenLeft=User32Methods.GetSystemMetrics（SystemMetrics.SM_XVIRTUALSCREEN）；
静态公共int virtualScreenTop=User32Methods.GetSystemMetrics（SystemMetrics.SM_YVIRTUALSCREEN）；
静态public int virtualScreenRight=User32Methods.GetSystemMetrics（SystemMetrics.SM_CXVIRTUALSCREEN）；
静态公共int-virtualScreenBottom=User32Methods.GetSystemMetrics（SystemMetrics.SM_-CYVIRTUALSCREEN）；
静态公共int virtualScreenWidth=virtualScreenRight-virtualScreenLeft；
静态公共int virtualScreenHeight=virtualScreenBottom-virtualScreenTop；
静态公共bool mousePressed=false；
静态公共NetCoreEx.Geometry.Point lastMousePos=new NetCoreEx.Geometry.Point（）；
静态公共NetCoreEx.Geometry.Point currentMousePos=new NetCoreEx.Geometry.Point（）；
静态公共NetCoreEx.Geometry.Rectangle=new NetCoreEx.Geometry.Rectangle（）；
静态公共IntPtr bmp=captureVirtualScreen（）；
静态IntPtr captureVirtualScreen（）
{
//获取一个与屏幕兼容的DC
IntPtr hdc=Gdi32Methods.CreateCompatibleDC（IntPtr.Zero）；
//在内存中制作bmp以存储捕获
IntPtr bitmap=Gdi32Methods.CreateCompatibleBitmap（User32Methods.GetDC（IntPtr.Zero），
虚拟CreenWidth、虚拟CreenHeight）；
//加入他们
GDI32方法。选择对象（hdc、位图）；
//从屏幕复制到我的位图
Gdi32Methods.BitBlt（hdc、virtualScreenLeft、virtualScreenTop、，
虚拟CreenWidth，虚拟CreenHeight，
GetDC（IntPtr.Zero）、0、0、BitBltFlags.SRCCOPY）；
//删除DC
GDI32方法。删除DC（hdc）；
//返回位图
返回位图；
}
静态int Main（字符串[]args）
{
var instanceHandle=Kernel32Methods.GetModuleHandle（IntPtr.Zero）；
var wc=新的WindowClassEx
{
Size=（uint）封送处理.SizeOf（），
ClassName=“MainWindow”，
CursorHandle=User32Helpers.LoadCursor（IntPtr.Zero，SystemCursor.IDC_箭头），
IconHandle=User32Helpers.LoadIcon（IntPtr.Zero、SystemIcon.IDI_应用程序），
Styles=WindowClassStyles.CS_HREDRAW | WindowClassStyles.CS_VREDRAW，
BackgroundBrushHandle=新IntPtr（（int）StockObject.WHITE_BRUSH），
WindowProc=WindowProc，
InstanceHandle=InstanceHandle
};
var resReg=User32Methods.RegisterClassEx（ref-wc）；
如果（resReg==0）
{
Console.Error.WriteLine（“注册失败”）；
返回-1；
}
var hwnd=User32Methods.CreateWindowEx（WindowExStyles.WS_EX_左），
wc.ClassName，“；）”，WindowStyles.WS_弹出窗口| WindowStyles.WS_可见，
virtualScreenLeft，virtualScreenTop，
虚拟CreenWidth，虚拟CreenHeight，
IntPtr.Zero，IntPtr.Zero，instanceHandle，IntPtr.Zero）；
if（hwnd==IntPtr.Zero）
{
Console.Error.WriteLine（“窗口创建失败”）；
返回-1；
}
User32Methods.ShowWindow（hwnd，ShowWindowCommands.SW_SHOWNORMAL）；
User32Methods.UpdateWindow（hwnd）；
消息消息；
国际关系；
while（（res=User32Methods.GetMessage（out-msg，IntPtr.Zero，0，0））！=0）
{
User32Methods.TranslateMessage（参考消息）；
User32Methods.DispatchMessage（ref msg）；
User32Methods.UpdateWindow（hwnd）；
}
返回res；
}
专用静态IntPtr WindowProc（IntPtr hwnd、uint umsg、IntPtr wParam、IntPtr LPRAM）
{
var msg=（WM）umsg；
开关（msg）
{
案例WM.KEYDOWN：
{
开关（wParam.ToInt32（））
{
//27-电子稳定控制系统的标志
案例27：
{
User32Methods.PostQuitMessage（0）；
打破
}
}
打破
}
案例WM.LBUTTONDOWN：
{
鼠标按下=真；
User32Methods.GetCursorPos（out lastMousePos）；
User32Methods.GetCursorPos（out-currentMousePos）；
打破
}
case WM.MOUSEMOVE：
{
如果（鼠标按下）
User32Methods.GetCursorPos（out-currentMousePos）；
打破
}
案例WM.LBUTTONUP：
{
鼠标按下=错误；
打破
}
案例WM.CLOSE：
{
User32Methods.PostQuitMessage（0）；
打破
}
案例WM.PAINT：
{
IntPtr bm=新的IntPtr（）；
    internal class Program
    {
        static public int virtualScreenLeft = User32Methods.GetSystemMetrics(SystemMetrics.SM_XVIRTUALSCREEN);
        static public int virtualScreenTop = User32Methods.GetSystemMetrics(SystemMetrics.SM_YVIRTUALSCREEN);
        static public int virtualScreenRight = User32Methods.GetSystemMetrics(SystemMetrics.SM_CXVIRTUALSCREEN);
        static public int virtualScreenBottom = User32Methods.GetSystemMetrics(SystemMetrics.SM_CYVIRTUALSCREEN);
        static public int virtualScreenWidth = virtualScreenRight - virtualScreenLeft;
        static public int virtualScreenHeight = virtualScreenBottom - virtualScreenTop;

        static public bool mousePressed = false;
        static public NetCoreEx.Geometry.Point lastMousePos = new NetCoreEx.Geometry.Point();
        static public NetCoreEx.Geometry.Point currentMousePos = new NetCoreEx.Geometry.Point();
        static public NetCoreEx.Geometry.Rectangle rect = new NetCoreEx.Geometry.Rectangle();

        static public IntPtr bmp = captureVirtualScreen();

        static IntPtr captureVirtualScreen()
        {
            // Get a DC compat. w/ the screen
            IntPtr hdc = Gdi32Methods.CreateCompatibleDC(IntPtr.Zero);

            // Make a bmp in memory to store the capture in
            IntPtr bitmap =  Gdi32Methods.CreateCompatibleBitmap(User32Methods.GetDC(IntPtr.Zero) , 
                                                                 virtualScreenWidth, virtualScreenHeight);

            // Join them
            Gdi32Methods.SelectObject(hdc, bitmap);

            // Copy from the screen to my bitmap
            Gdi32Methods.BitBlt(hdc, virtualScreenLeft, virtualScreenTop,
                                     virtualScreenWidth, virtualScreenHeight, 
                                     User32Methods.GetDC(IntPtr.Zero), 0, 0, BitBltFlags.SRCCOPY);

            // Delete the DC
            Gdi32Methods.DeleteDC(hdc);

            // Return the bitmap
            return bitmap;
        }
        

        static int Main(string[] args)
        {
            var instanceHandle = Kernel32Methods.GetModuleHandle(IntPtr.Zero);

            var wc = new WindowClassEx
            {
                Size = (uint)Marshal.SizeOf<WindowClassEx>(),
                ClassName = "MainWindow",
                CursorHandle = User32Helpers.LoadCursor(IntPtr.Zero, SystemCursor.IDC_ARROW),
                IconHandle = User32Helpers.LoadIcon(IntPtr.Zero, SystemIcon.IDI_APPLICATION),
                Styles = WindowClassStyles.CS_HREDRAW | WindowClassStyles.CS_VREDRAW,
                BackgroundBrushHandle = new IntPtr((int)StockObject.WHITE_BRUSH),
                WindowProc = WindowProc,
                InstanceHandle = instanceHandle
            };

            var resReg = User32Methods.RegisterClassEx(ref wc);
            if (resReg == 0)
            {
                Console.Error.WriteLine("registration failed");
                return -1;
            }

            var hwnd = User32Methods.CreateWindowEx(WindowExStyles.WS_EX_LEFT,
                wc.ClassName, ";)", WindowStyles.WS_POPUP | WindowStyles.WS_VISIBLE,
                virtualScreenLeft, virtualScreenTop,
                virtualScreenWidth, virtualScreenHeight,
                IntPtr.Zero, IntPtr.Zero, instanceHandle, IntPtr.Zero);

            if (hwnd == IntPtr.Zero)
            {
                Console.Error.WriteLine("window creation failed");
                return -1;
            }

            User32Methods.ShowWindow(hwnd, ShowWindowCommands.SW_SHOWNORMAL);
            User32Methods.UpdateWindow(hwnd);

            Message msg;
            int res;
            while ((res = User32Methods.GetMessage(out msg, IntPtr.Zero, 0, 0)) != 0)
            {
                User32Methods.TranslateMessage(ref msg);
                User32Methods.DispatchMessage(ref msg);
                User32Methods.UpdateWindow(hwnd);
            }

            return res;
        }
        
        private static IntPtr WindowProc(IntPtr hwnd, uint umsg, IntPtr wParam, IntPtr lParam)
        {
            var msg = (WM)umsg;
            switch (msg)
            {
                case WM.KEYDOWN:
                    {
                        switch (wParam.ToInt32())
                        {
                            // 27 - flag for esc
                            case 27:
                                {
                                    User32Methods.PostQuitMessage(0);
                                    break;
                                }
                        }
                        break;
                    }
                case WM.LBUTTONDOWN:
                    {

                        mousePressed = true;
                        User32Methods.GetCursorPos(out lastMousePos);
                        User32Methods.GetCursorPos(out currentMousePos);

                        break;
                    }
                case WM.MOUSEMOVE:
                    {
                        if (mousePressed)
                            User32Methods.GetCursorPos(out currentMousePos);
                        break;
                    }
                case WM.LBUTTONUP:
                    {
                        mousePressed = false;
                        break;
                    }
                case WM.CLOSE:
                    {
                        User32Methods.PostQuitMessage(0);
                        break;
                    }
                case WM.PAINT:
                    {

                        IntPtr bm = new IntPtr();
                        PaintStruct ps;
                        
                        var hdc = User32Methods.BeginPaint(hwnd, out ps);

                        IntPtr hdcMem = Gdi32Methods.CreateCompatibleDC(hdc);
                        IntPtr hbmOld = Gdi32Methods.SelectObject(hdcMem, bmp);
                        
                        Gdi32Methods.GetObject(bmp, 0, bm);

                        Gdi32Methods.BitBlt(hdc, virtualScreenLeft, virtualScreenTop,
                                            virtualScreenWidth, virtualScreenHeight,
                                            hdcMem, 0, 0, BitBltFlags.SRCCOPY);


                        Gdi32Methods.SelectObject(hdcMem, hbmOld);
                        Gdi32Methods.DeleteDC(hdcMem);

                        rect.Left = lastMousePos.X;
                        rect.Top = lastMousePos.Y;
                        rect.Right = currentMousePos.X;
                        rect.Bottom = lastMousePos.Y;

                        User32Methods.FillRect(hdc, ref rect, 
                         Gdi32Helpers.GetStockObject(StockObject.WHITE_BRUSH));

                        User32Methods.EndPaint(hwnd, ref ps);
                        break;
                    }
            }
            return User32Methods.DefWindowProc(hwnd, umsg, wParam, lParam);
        }
    }