我的C#窗体要拉一下才能刷出图形，怎么办啊

是想实现点一下画一个五角星图案，小白求助。
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

    int startX = 0;
    int  startY = 0;
    private void Form1_MouseDown(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            startX = e.X;
            startY = e.Y;
        }

        MessageBox.Show("坐标： " + Convert.ToString(startX) +"  ,  "+ Convert.ToString(startY));
    }

    protected override void OnPaint(PaintEventArgs e)
    {
       Point center = new Point(startX,startY);
      //绘制五角星
        int radius = 100;
        PointF[] pentagons = new PointF[] { new PointF(center.X, center.Y - radius),
            new PointF((float)(center.X + radius * Math.Sin(72 * Math.PI / 180)), (float)(center.Y - radius * Math.Cos(72 * Math.PI / 180))),
            new PointF((float)(center.X + radius * Math.Sin(36 * Math.PI / 180)), (float)(center.Y + radius * Math.Cos(36* Math.PI / 180))),
            new PointF((float)(center.X - radius * Math.Sin(36 * Math.PI / 180)),(float)( center.Y + radius * Math.Cos(36 * Math.PI / 180))),
            new PointF((float)(center.X - radius * Math.Sin(72 * Math.PI / 180)), (float)(center.Y - radius * Math.Cos(72 * Math.PI / 180))),
        };


        GraphicsPath path = new GraphicsPath(FillMode.Winding);
        path.AddLine(pentagons[0], pentagons[2]);
        path.AddLine(pentagons[2], pentagons[4]);
        path.AddLine(pentagons[4], pentagons[1]);
        path.AddLine(pentagons[1], pentagons[3]);
        path.AddLine(pentagons[3], pentagons[0]);
        path.CloseFigure();

        e.Graphics.FillPath(Brushes.Red, path);
        path.Dispose();

    }



}

}

在formload里调用下this.Refreash