private void Form1_Paint(
object sender, PaintEventArgs e) { Graphics g = e.Graphics; g.FillRectangle(Brushes.White,
this.ClientRectangle);
//create essensial objects for painting text strings SizeF sizeF = g.MeasureString(
"Test",
this.Font); StringFormat sf =
new StringFormat(); sf.LineAlignment = StringAlignment.Center;
//set properties of the grid int cellHeight = (
int)sizeF.Height +
4;
int cellWidth =
80;
int nbrColumns =
50;
int nbrRows =
50;
//output general info to console Console.WriteLine(
"-------------------------"); Console.WriteLine(
"e.ClipRectangle = " + e.ClipRectangle); Console.WriteLine(
"The following cells need to be redrawn " +
"(in whole or in part):");
//draw the cells and the output to console for (
int row =
0; row < nbrRows; ++row) {
for (
int col =
0; col < nbrColumns; ++col) { Point cellLocation =
new Point(col * cellWidth, row * cellHeight); Rectangle cellRect =
new Rectangle(cellLocation.X, cellLocation.Y, cellWidth, cellHeight);
if (cellRect.IntersectsWith(e.ClipRectangle)) { Console.WriteLine(
"Row:{0} cel:{1}", row, col); g.FillRectangle(Brushes.LightBlue, cellRect); g.DrawRectangle(Pens.Black, cellRect);
string s = String.Format(
"{0},{1}", col, row); g.DrawString(s,
this.Font, Brushes.Black, cellRect, sf); }
else { } }这段代码中的if语句指出，只有矩形与ClipRectangle的交集不为空，事件处理程序才将创建此矩形。
换句话说，如果操作系统不打算向绘图表面提交矩形的至少部分区域，它将不会创建矩形。
如果没有if。。else语句，事件处理程序将需要生成50行和50列，也就是2500个矩形，
但是if。。else语句将每一个矩形的预定位置与clipRectangle进行比较，所以代码生成的唯一矩形是实际绘制到可视的绘图表面的那些矩形。可以通过Graphics。IsVisibleClipEmpty测试代替Rectangle。IntersectsWith测试来实现一个类似的效果，
即将if语句改为if(!g.IsVisibleClipEmpty)区别是IntersectsWith把每一个矩形与裁剪矩形比较，IsVisibleClipEmpty把每一个矩形与客户矩形比较。
转载于:https://www.cnblogs.com/ttssrs/archive/2012/03/14/2396516.html
