C# 擷取螢幕任意座標像素顏色

透過 P/Invoke 呼叫 GDI32 / User32 API，可擷取桌面全螢幕畫面至 Bitmap，再以 GetPixel 取得滑鼠指標位置的顏色值。適用於拾色器、螢幕分析等工具開發。

核心 API 宣告

using System.Runtime.InteropServices;

public static class Win32Api
{
    [DllImport("gdi32.dll")] public static extern bool BitBlt(IntPtr hdcDest, int x, int y, int w, int h, IntPtr hdcSrc, int srcX, int srcY, int rop);
    [DllImport("gdi32.dll")] public static extern IntPtr CreateCompatibleBitmap(IntPtr hdc, int w, int h);
    [DllImport("gdi32.dll")] public static extern IntPtr CreateCompatibleDC(IntPtr hdc);
    [DllImport("gdi32.dll")] public static extern IntPtr SelectObject(IntPtr hdc, IntPtr hObj);
    [DllImport("gdi32.dll")] public static extern bool DeleteDC(IntPtr hdc);
    [DllImport("gdi32.dll")] public static extern bool DeleteObject(IntPtr hObj);
    [DllImport("user32.dll")] public static extern IntPtr GetDesktopWindow();
    [DllImport("user32.dll")] public static extern IntPtr GetDC(IntPtr hWnd);
    [DllImport("user32.dll")] public static extern int GetSystemMetrics(int nIndex);
    [DllImport("user32.dll")] public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);

    public static Bitmap CaptureDesktop()
    {
        IntPtr hDesk = GetDesktopWindow();
        IntPtr hdcSrc = GetDC(hDesk);
        IntPtr hdcDest = CreateCompatibleDC(hdcSrc);
        int w = GetSystemMetrics(0); // SM_CXSCREEN
        int h = GetSystemMetrics(1); // SM_CYSCREEN
        IntPtr hBmp = CreateCompatibleBitmap(hdcSrc, w, h);
        if (hBmp == IntPtr.Zero) return null;

        IntPtr hOld = SelectObject(hdcDest, hBmp);
        BitBlt(hdcDest, 0, 0, w, h, hdcSrc, 0, 0, 0x00CC0020); // SRCCOPY
        SelectObject(hdcDest, hOld);
        DeleteDC(hdcDest);
        ReleaseDC(hDesk, hdcSrc);

        Bitmap bmp = Image.FromHbitmap(hBmp);
        DeleteObject(hBmp);
        GC.Collect();
        return bmp;
    }
}

WinForms 整合範例

private Bitmap _screenBmp;

private void Label_MouseDown(object sender, MouseEventArgs e)
{
    _screenBmp = Win32Api.CaptureDesktop();
}

private void Label_MouseUp(object sender, MouseEventArgs e)
{
    if (_screenBmp == null) return;
    Color c = _screenBmp.GetPixel(Cursor.Position.X, Cursor.Position.Y);
    label1.BackColor = c;
    label1.Text = $"R:{c.R} G:{c.G} B:{c.B}  #{c.R:X2}{c.G:X2}{c.B:X2}";
    _screenBmp.Dispose();
    _screenBmp = null;
}

優化建議

• 效能：全螢幕截圖較耗時，建議僅截取滑鼠周圍小區域（如 10x10），或使用 Graphics.CopyFromScreen 替代 GDI API。
• Graphics.CopyFromScreen 範例：

Bitmap bmp = new Bitmap(1, 1);
using (Graphics g = Graphics.FromImage(bmp))
{
    g.CopyFromScreen(Cursor.Position, Point.Empty, new Size(1, 1));
}
Color c = bmp.GetPixel(0, 0);

• 多螢幕：GetSystemMetrics(0/1) 僅回傳主螢幕尺寸。多螢幕環境請用 Screen.AllScreens 計算虛擬桌面範圍。
• 高 DPI：啟用 Per-Monitor DPI Awareness 專案設定，避免座標縮放偏移。