有些code去与Ed的正确答案：

Some code to go with Ed's correct answer:

    Point startPos;      // mouse-down position
    Point currentPos;    // current mouse position
    bool drawing;        // busy drawing
    List<Rectangle> rectangles = new List<Rectangle>();  // previous rectangles

    private Rectangle getRectangle() {
        return new Rectangle(
            Math.Min(startPos.X, currentPos.X),
            Math.Min(startPos.Y, currentPos.Y),
            Math.Abs(startPos.X - currentPos.X),
            Math.Abs(startPos.Y - currentPos.Y));
    }

    private void canevas_MouseDown(object sender, MouseEventArgs e) {
        currentPos = startPos = e.Location;
        drawing = true;
    }

    private void canevas_MouseMove(object sender, MouseEventArgs e) {
        currentPos = e.Location;
        if (drawing) canevas.Invalidate();
    }

    private void canevas_MouseUp(object sender, MouseEventArgs e) {
        if (drawing) {
            drawing = false;
            var rc = getRectangle();
            if (rc.Width > 0 && rc.Height > 0) rectangles.Add(rc);
            canevas.Invalidate();
        }
    }

    private void canevas_Paint(object sender, PaintEventArgs e) {
        if (rectangles.Count > 0) e.Graphics.DrawRectangles(Pens.Black, rectangles.ToArray());
        if (drawing) e.Graphics.DrawRectangle(Pens.Red, getRectangle());
    }

要获得canevas具有双缓冲开启，所以它画不闪烁，使用Project +添加新项，选择类并粘贴此code：

To get a 'canevas' that has double-buffering turned on, so it painting doesn't flicker, use Project + Add New Item, select "Class" and paste this code:

using System;
using System.Windows.Forms;

class Canvas : Panel {
    public Canvas() {
        this.DoubleBuffered = true;
        this.SetStyle(ControlStyles.ResizeRedraw, true);
    }
}

编译。从工具箱顶部的新控件拖到窗体，取代了原有的canevas。更新相应的事件处理程序。

Compile. Drag the new control from the top of the toolbox onto your form, replacing the original 'canevas'. Update the event handlers accordingly.