Why i keep getting error Extension methods must be defined in a non-generic static class?

Question

I created a new method:

private static void FillCircle(this Graphics g, Brush brush,
                              PointF center, float radius)
        {
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
            g.FillEllipse(brush, center.X - radius, center.Y - radius,
                                 radius + radius, radius + radius);
        }

But getting on the main form class name error:

namespace mws
{
    class DopplerRadar

The error is on the DopplerRadar

Severity Code Description Project File Line Suppression State Error CS1106 Extension method must be defined in a non-generic static class

If i change for example the FillCircle method to:

private void test()
{

}

I'm not getting any errors.

Answer 1

change

public class DopplerRadar

to

public static class DopplerRadar