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));
}

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; }
}
}