解决C#全屏幕截图的实现方法

今天一位同事想写一个全屏幕截图的代码。当然要实现的第一步是能够获取整个屏幕的位图，记得Win32 API的CreateDC, BitBlt等函数可以使用。于是上网查了下，果然屏幕截图用这些函数。但winform已经可以把API都忘记了，所以得寻找一个无Win32 API的实现方式。综合了网上的实现，以及自己的一些设计，实现思路如下：
1. 开始截图时，创建一个与屏幕大小一样的位图，然后用Graphics.CopyFromScreen()把屏幕位图拷贝到该位图上。这是很关键的一步，这样所有的操作就都可以在该位图上进行了，而无实际屏幕无关了。 

Code
int width = Screen.PrimaryScreen.Bounds.Width;
int height = Screen.PrimaryScreen.Bounds.Height;
Bitmap bmp = new Bitmap(width, height);
using (Graphics g = Graphics.FromImage(bmp)) {
    g.CopyFromScreen(0, 0, 0, 0, new Size(width, height));
}

2. 接下来为了方便在这之上进行截图，有一个很重要的设计实现方式：用全屏幕窗体代替现有真实屏幕，这样就可以把截图过程的所有操作都在那个窗体上实现（该窗体设置成无边框，高宽等于屏幕大小即可），另外为了显示掩蔽效果（只能正常显示选择的部分屏幕内容，而其实部分用一个如半透明层覆盖），就添加一层半透明位置位图。具体代码如下：

Code
public partial class FullScreenForm : Form {
    private Rectangle rectSelected = Rectangle.Empty;
    private bool isClipping = false;
    private Bitmap screen;
    private Bitmap coverLayer = null;
    private Color coverColor;
    private Brush rectBrush = null;
    private Bitmap resultBmp = null;
    public FullScreenForm(Bitmap screen) {
        InitializeComponent();
        int width = Screen.PrimaryScreen.Bounds.Width;
        int height = Screen.PrimaryScreen.Bounds.Height;
        coverLayer = new Bitmap(width, height);
        coverColor = Color.FromArgb(50, 200, 0, 0);
        rectBrush = new SolidBrush(coverColor);
        using (Graphics g = Graphics.FromImage(coverLayer)) {
            g.Clear(coverColor);
        }
        this.Bounds = new Rectangle(0, 0, width, height);
        this.screen = screen;
        this.DoubleBuffered = true;
    }
    protected override void OnMouseDown(MouseEventArgs e) {
        if (e.Button == MouseButtons.Left) {
            isClipping = true;
            rectSelected.Location = e.Location;
        }
        else if (e.Button == MouseButtons.Right) {
            this.DialogResult = DialogResult.OK;
        }
    }
    protected override void OnMouseMove(MouseEventArgs e) {
        if (e.Button == MouseButtons.Left && isClipping) {
            rectSelected.Width = e.X - rectSelected.X;
            rectSelected.Height = e.Y - rectSelected.Y;

            this.Invalidate();
        }
    }
    protected override void OnMouseUp(MouseEventArgs e) {
        if (e.Button == MouseButtons.Left && isClipping) {
            rectSelected.Width = e.X - rectSelected.X;
            rectSelected.Height = e.Y - rectSelected.Y;
            this.Invalidate();
            resultBmp = new Bitmap(rectSelected.Width, rectSelected.Height);
            using (Graphics g = Graphics.FromImage(resultBmp)) {
                g.DrawImage(screen,new Rectangle(0, 0, rectSelected.Width, rectSelected.Height), rectSelected, GraphicsUnit.Pixel);
            }
            this.DialogResult = DialogResult.OK;
        }
    }
    protected override void OnPaint(PaintEventArgs e) {
        Graphics g = e.Graphics;
        g.DrawImage(screen, 0, 0);
        g.DrawImage(coverLayer, 0, 0);
        PaintRectangle();
    }
    protected override void OnPaintBackground(PaintEventArgs e) {

    }
    protected override void OnKeyDown(KeyEventArgs e) {
        if (e.KeyCode == Keys.Escape) {
            this.DialogResult = DialogResult.Cancel;
        }
    }
    private void PaintRectangle() {
        using (Graphics g = Graphics.FromImage(coverLayer)) {
            g.Clear(coverColor);
            GraphicsPath path = new GraphicsPath();
            path.AddRectangle(this.Bounds);
            path.AddRectangle(rectSelected);
            g.FillPath(rectBrush, path);
            g.DrawRectangle(Pens.Blue, rectSelected);
        }
    }
    public Bitmap ResultBitmap {
        get { return resultBmp; }
    }
}

上面的代码都很容易看明白，这里有一个技巧就是GraphicsPath，它自动会形成一个中空的区域。上面的实现很容易扩展：多区域截图，多裁判截图等都很容易实现。