Winform在控件内实现简单画笔功能

using System.Drawing;
using System.Windows.Forms;

namespace ZhuoHuiSchoolroom.ZhuoHuiClass
{
    /// <summary>
    /// 实现窗体内简单画笔功能
    /// </summary>
    public class DrawClass
    {
        private Color __Color = new Color();
        private Point __Start = new Point();
        private Point __End = new Point();
        private Form __frmMain = new Form();
        private PictureBox __picMain = new PictureBox();
        private Graphics __g = null;
        private Pen __p = null;

        public Color Color
        {
            get { return this.__Color; }
            set { this.__Color = value; }
        }
        public Point Start
        {
            get { return this.__Start; }
            set { this.__Start = value; }
        }
        public Point End
        {
            get { return this.__End; }
            set { this.__End = value; }
        }

        public DrawClass(Form frmMain)
        : this(frmMain, Color.Black)
        { }
        public DrawClass(PictureBox picMian)
        : this(picMian, Color.Red)
        { }

        public DrawClass(Form frmMain, Color c)
        {
            this.__frmMain = frmMain;
            this.__g = Graphics.FromHwnd(frmMain.Handle);
            this.__Color = c;
            this.__p = new Pen(this.__Color, );
            this.__frmMain.MouseDown += new MouseEventHandler(__frmMain_MouseDown);
            this.__frmMain.MouseMove += new MouseEventHandler(__frmMain_MouseMove);
            this.__frmMain.MouseUp += new MouseEventHandler(__frmMain_MouseUp);
        }

        public DrawClass(PictureBox frmMain, Color c)
        {
            this.__picMain = frmMain;
            this.__g = Graphics.FromHwnd(frmMain.Handle);
            this.__Color = c;
            this.__p = new Pen(this.__Color, );
            this.__picMain.MouseDown += new MouseEventHandler(__frmMain_MouseDown);
            this.__picMain.MouseMove += new MouseEventHandler(__frmMain_MouseMove);
            this.__picMain.MouseUp += new MouseEventHandler(__frmMain_MouseUp);
        }

        private void __frmMain_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                __End.X = e.X;
                __End.Y = e.Y;
                __g.DrawLine(this.__p, __Start, __End);
                __Start = __End;
            }
        }

        private void __frmMain_MouseUp(object sender, MouseEventArgs e)
        {

        }

        private void __frmMain_MouseDown(object sender, MouseEventArgs e)
        {
            this.__Start.X = e.X;
            this.__Start.Y = e.Y;
        }

    }
}

DrawClass draw = new ZhuoHuiClass.DrawClass(this.pic);                              //调用画笔功能