private void Form1_Load(object sender,
System.EventArgs e)
{
// Get the full size of the form
//取得窗口的完整大小
            fullSize = SystemInformation
.PrimaryMonitorMaximizedWindowSize;
//其实也即时初始化一副BMP图
// Create a bitmap using full size
            bitmap = new Bitmap(fullSize.Width,
fullSize.Height);
// Create a Graphics object from Bitmap
//并由于创建一份GRAPHICS
            curGraphics = Graphics.FromImage(bitmap);
// Set background color as form's color
            curGraphics.Clear(this.BackColor);
// Create a new pen and brush as
// default pen and brush
            curPen = new Pen(Color.Black);
curBrush = new SolidBrush(Color.Black);
}
在DOWN的时候,即取得开始的位置
private void Form1_MouseDown(object sender,
System.Windows.Forms.MouseEventArgs e)
{
// Store the starting point of
// the rectangle and set the drag mode
// to true
            curX = e.X;
curY = e.Y;
dragMode = true;
}
每次移动,都是重新画,只是肉眼所不能分得清而已
private void Form1_MouseMove(object sender,
System.Windows.Forms.MouseEventArgs e)
{
// Find out the ending point of
// the rectangle and calculate the
// difference of starting and ending
// points to find out the height and width
// of the rectangle
            x = e.X;
y = e.Y;
diffX = e.X - curX;
diffY = e.Y - curY;
// If drag mode is true, call refresh
// that forces window to repaint
            if (dragMode)
{
//每次刷新其实都是调用ONPAINT函数
                this.Refresh();
}
}
在UP的时候,也画,并把标志量改为FALSE
private void Form1_MouseUp(object sender,
System.Windows.Forms.MouseEventArgs e)
{
diffX = x - curX;
diffY = y - curY;
//这里下一个留个问题?SWITCH是OO被人痛批的,可这里如果不采用SWITCH,有什么好的办法呢!
            switch (drawIndex)
{
//不管是矩形或者是椭圆形,都是取X,Y,差值就可以了
                case 1:
{
// Draw a line
                        curGraphics.DrawLine(curPen,
curX, curY, x, y);
break;
}
case 2:
{
// Draw an ellipse
                        curGraphics.DrawEllipse(curPen,
curX, curY, diffX, diffY);
break;
}
case 3:
{
// Draw rectangle
                        curGraphics.DrawRectangle(curPen,
curX, curY, diffX, diffY);
break;
}
case 4:
{
// Fill rectangle
                        curGraphics.FillRectangle(curBrush,
curX, curY, diffX, diffY);
break;
}
case 5:
{
// Fill ellipse
                        curGraphics.FillEllipse(curBrush,
curX, curY, diffX, diffY);
break;
}
}
// Refresh 
            RefreshFormBackground();
// Set drag mode to false
            dragMode = false;
}