An extension method to calculate the surface area (in squared pixels) of a single polygon represented by a GraphicsPath object

GraphicsPathExtensions.cs

public static class GraphicsPathExtensions
{
    public static float ComputeArea(this GraphicsPath graphicsPath)
    {
        var points = graphicsPath.PathPoints.ToList();
        //Add the first point as the last in order to close the figure and compute area properly.
        points.Add(points[0]);

        return Math.Abs(points.Take(points.Count - 1).Select((p, i) => p.X * points[i + 1].Y - p.Y * points[i + 1].X).Sum()) / 2;
    }
}