To create a dashed line, first create a Pen. Then set the Pen's DashStyle property.

For example, the following code draws two lines, one with DashStyle Dash and one with DashStyle DashDot.

using (Pen dashed_pen = new Pen(Color.Blue, 2))
{
    ...
    dashed_pen.DashStyle = DashStyle.Dash;
    e.Graphics.DrawString("Dash", this.Font, Brushes.Black, 10, y - 8);
    e.Graphics.DrawLine(dashed_pen, 100, y, 250, y);
    y += 20;

    dashed_pen.DashStyle = DashStyle.DashDot;
    e.Graphics.DrawString("DashDot", this.Font, Brushes.Black, 10, y - 8);
    e.Graphics.DrawLine(dashed_pen, 100, y, 250, y);
    y += 20;
    ...
}

Don't forget to use the Pen's Dispose method to free graphics resources. If you use the using statement as in this example, C# calls Dispose automatically for you.