I need to draw a line on the form created in c#.

I want to add some detail and a fix to the code provided by Simon.

For example, add the following code to the form's constructor:

this.Paint += (sender, eventArgs) => {
    using (Pen pen = Pen(Color.FromArgb(255, 255, 0, 0))) {
        eventArgs.Graphics.DrawLine(pen, startX, startY, endX, endY);
    } //calls Pen.Dispose here, even if exception was thrown
};

Pay attention for the "using" statement. It's used to guarantee disposal of pen, no matter if exception was thrown or not. This construct is a syntactic sugar and is strictly equivalent to placing creation of the pen under try-finally block with disposal operation in the finally part.

Alternatively, you can re-use the pen and other resources. In this case, you should created them in the form's constructor and dispose in Form.Dispose, see http://msdn.microsoft.com/en-us/library/aw58wzka.aspx[^].

You need to initialize geometric parameters startX, startY, endX, endY prior to the call. If you ever need to change the position of the line, make the form fields. To change the position of line, you need to change the values of those fields. Now, how to trigger re-drawing of the picture? You need to cause sending the event WM_PAINT to the form. This is done using Form.Invalidate (inherited from Control.Invalidate. You can improve performance using Invalidate with parameters (Rectangle or Region) to invalidate just some part of the scene.

—SA