C# 读取摄像头老是显示黑色

public bool IsConnected { get; set; }
        //保存无符号句柄
        private IntPtr lwndC;
        //保存管理指示器
        private IntPtr mControlPtr;
        //摄像头显示宽度
        private int mWidth;
        //摄像头显示高度
        private int mHeight;
        public Camera(IntPtr handle, int width, int height)
        {
            //显示视频控件的句柄
            mControlPtr = handle;
            //视频宽度
            mWidth = width;
            //视频高度
            mHeight = height;
        }
        /// <summary>
        /// 打开视频设备
        /// </summary>
        public void StartWebCam()
        {
            byte[] lpszName = new byte[100];
            byte[] lpszVer = new byte[100];
            CameraWindowsAPI.capGetDriverDescriptionA(0, lpszName, 100, lpszVer, 100);
            this.lwndC = CameraWindowsAPI.capCreateCaptureWindowA(lpszName, CameraWindowsAPI.WS_CHILD | CameraWindowsAPI.WS_VISIBLE, 0, 0, mWidth, mHeight, mControlPtr, 0);
            if (CameraWindowsAPI.SendMessage(lwndC, CameraWindowsAPI.WM_CAP_DRIVER_CONNECT, 0, 0))
            {
                IsConnected = true;
                CameraWindowsAPI.SendMessage(lwndC, CameraWindowsAPI.WM_CAP_SET_PREVIEWRATE, 1, 0);
                CameraWindowsAPI.SendMessage(lwndC, CameraWindowsAPI.WM_CAP_SET_PREVIEW, true, 0);

                CameraWindowsAPI.SendMessage(lwndC, CameraWindowsAPI.WM_CAP_DRIVER_CONNECT, 0, 0);
            }
            else
            {
                IsConnected = false;
            }
        }
        /// <summary>
        /// 关闭视频设备
        /// </summary>
        public void CloseWebcam()
        {
            CameraWindowsAPI.SendMessage(lwndC, CameraWindowsAPI.WM_CAP_DRIVER_DISCONNECT, 0, 0);
        }
        ///   <summary>  
        ///   拍照
        ///   </summary>  
        ///   <param   name="path">要保存bmp文件的路径</param>  
        public void GrabImage(IntPtr hWndC, string path)
        {
            IntPtr hBmp = Marshal.StringToHGlobalAnsi(path);
            CameraWindowsAPI.SendMessage(lwndC, CameraWindowsAPI.WM_CAP_SAVEDIB, 0, hBmp.ToInt32());
        }