//namespace to be included
using System.Drawing.Imaging;

//Define Image & graphics variable.
private static Bitmap bmpScreenShot;
private static Graphics graphicScreenShot;


//The button click event.
private void button_Capture_Click(object sender, EventArgs e)
        {
            //To hide the form, otherwise it will be there in screen shot.
            this.Hide();

            //setting the bitmap object to the size of the screen.
            bmpScreenShot = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height, PixelFormat.Format32bppArgb);

            //creating a graphic object from that bitmap.
            graphicScreenShot = Graphics.FromImage(bmpScreenShot);

            //Taking the screen shot from the top left corner to bottom right corner.
            graphicScreenShot.CopyFromScreen(Screen.PrimaryScreen.Bounds.X, Screen.PrimaryScreen.Bounds.Y, 0, 0, Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);

             //Saving the screenshot in the defined format at the specified location.
            bmpScreenShot.Save("C:\\test.bmp", ImageFormat.Bmp);

            //Again showing the form.
            this.Show();
        }