為什么不能在pictureBox中用此代碼繪制矩形？

Question

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace UTUResultWithCoordinates
{
    public partial class GetCoordinates : Form
    {
        private string sem;
        private string branch;
        private int mouseisdown = 0;
        private int recx = 0;
        private int recy = 0;
        private int mousemovingwhilepressed = 0;

        public GetCoordinates()
        {
            InitializeComponent();

        }

        public GetCoordinates(string p, string p_2)
        {
            // TODO: Complete member initialization
            InitializeComponent();
            branch = p;
            sem = p_2;
            pictureBox1.Controls.Add(pictureBox2);
            pictureBox2.Location = new Point(0, 0);
            pictureBox2.BackColor = Color.Transparent;
            pictureBox2.Width = 1191;
            pictureBox2.Height = 842;

        }

        private void GetCoordinates_Load(object sender, EventArgs e)
        {

            pictureBox1.ImageLocation =        @"D:\DotNet\UTUResultWithCoordinates\UTUResultWithCoordinates\bin\Debug\ComputerScience6.jpg";
        }






        private void pictureBox2_Paint(object sender, PaintEventArgs e)
        {
            if (mouseisdown == 1 && mousemovingwhilepressed==1)
            {
            System.Drawing.Graphics graphicsObj;
            graphicsObj = this.CreateGraphics();
            Pen myPen = new Pen(System.Drawing.Color.Blue, 100);
            Rectangle myRectangle = new Rectangle(recx, recy, 20, 20);
            e.Graphics.DrawRectangle(myPen, myRectangle);
         }

    }

    private void pictureBox2_MouseDown(object sender, MouseEventArgs e)
    {
        mouseisdown = 1;
        recx = e.X;
        recy = e.Y;
        pictureBox2.CreateGraphics();

    }

    private void pictureBox2_MouseMove(object sender, MouseEventArgs e)
    {
        label1.Text = e.X + "," + e.Y;
        mousemovingwhilepressed = 1;
        recx = e.X;
        recy = e.Y;
        pictureBox2.CreateGraphics();
    }

    private void pictureBox2_MouseUp(object sender, MouseEventArgs e)
    {
        mousemovingwhilepressed = 0;
        mouseisdown = 0;
        pictureBox2.CreateGraphics();
    }
}

}

我已經創建了其中顯示圖像的pictureBox1。 然后，我在其中創建了一個pictureBox2，以便可以通過拖動鼠標在該圖像上繪制一個矩形。 但是單擊鼠標沒有任何反應。 有什么錯誤？

Answer 1

調用CreateGraphics不會觸發PictureBox的繪制。

使用Invalidate導致重畫。

有關完整示例，請參見： 如何在C＃中用鼠標選擇PictureBox.Image上的區域。

旁注：

• 在構造函數以外的方法中調用InitializeControl並不是一個好主意。
• 當您需要布爾值時，請使用布爾值，而不是整數。
• 實現IDisposable的對象（例如Pen）應盡可能少地創建，並在不再需要/不再使用時丟棄。