今天一位同事想寫一個全屏幕截圖的代碼。當然要實現的第一步是能夠獲取整個屏幕的位圖，記得Win32 API的CreateDC, BitBlt等函數可以使用。于是上網查了下，果然屏幕截圖用這些函數。但winform已經可以把API都忘記了，所以得尋找一個無Win32 API的實現方式。綜合了網上的實現，以及自己的一些設計，實現思路如下：
1. 開始截圖時，創建一個與屏幕大小一樣的位圖，然后用Graphics.CopyFromScreen()把屏幕位圖拷貝到該位圖上。這是很關鍵的一步，這樣所有的操作就都可以在該位圖上進行了，而無實際屏幕無關了。
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. 接下來為了方便在這之上進行截圖，有一個很重要的設計實現方式：用全屏幕窗體代替現有真實屏幕，這樣就可以把截圖過程的所有操作都在那個窗體上實現（該窗體設置成無邊框，高寬等于屏幕大小即可），另外為了顯示掩蔽效果（只能正常顯示選擇的部分屏幕內容，而其實部分用一個如半透明層覆蓋），就添加一層半透明位置位圖。具體代碼如下：
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; }
}
}
上面的代碼都很容易看明白，這里有一個技巧就是GraphicsPath，它自動會形成一個中空的區域。上面的實現很容易擴展：多區域截圖，多裁判截圖等都很容易實現。
