C#画图的一个问题

Question

我想用C#在picturebox内画图，发现失败。就是直线画在picturebox外的部分能正常显示，而在picturebox内的部分被遮住。我的picturebox背景色必须是白色，不能改。请问如何解决？

Answer 1

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 draw1
{
    public partial class Form1 : Form
    {
        bool bDraw;
        public Form1()
        {
            InitializeComponent();
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            PointF startPointF=new PointF(20,50);
            PointF endPointF=new PointF(300,50);
            if (bDraw)
            {
                Graphics g = e.Graphics;
                Pen p = new Pen(Color.Blue, 2);
                g.DrawLine(p, startPointF, endPointF);
                g.Dispose();
            }
        } 
        private void btnDraw_Click(object sender, EventArgs e)
        {
            this.Invalidate();
            bDraw = true;
        }
        private void Form1_Load(object sender, EventArgs e)
        {        }
    }
}
namespace draw1
{
    partial class Form1
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }        #region Windows Form Designer generated code        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.btnDraw = new System.Windows.Forms.Button();
            this.pbImg = new System.Windows.Forms.PictureBox();
            ((System.ComponentModel.ISupportInitialize)(this.pbImg)).BeginInit();
            this.SuspendLayout();
            // 
            // btnDraw
            // 
            this.btnDraw.Location = new System.Drawing.Point(192, 260);
            this.btnDraw.Name = "btnDraw";
            this.btnDraw.Size = new System.Drawing.Size(79, 26);
            this.btnDraw.TabIndex = 0;
            this.btnDraw.Text = "Draw";
            this.btnDraw.UseVisualStyleBackColor = true;
            this.btnDraw.Click += new System.EventHandler(this.btnDraw_Click);
            // 
            // pbImg
            // 
            this.pbImg.BackColor = System.Drawing.Color.White;
            this.pbImg.BorderStyle = System.Windows.Forms.BorderStyle.FixedSingle;
            this.pbImg.Location = new System.Drawing.Point(112, 12);
            this.pbImg.Name = "pbImg";
            this.pbImg.Size = new System.Drawing.Size(363, 215);
            this.pbImg.TabIndex = 1;
            this.pbImg.TabStop = false;
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(511, 316);
            this.Controls.Add(this.pbImg);
            this.Controls.Add(this.btnDraw);
            this.Name = "Form1";
            this.Text = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            ((System.ComponentModel.ISupportInitialize)(this.pbImg)).EndInit();
            this.ResumeLayout(false);        }        #endregion        private System.Windows.Forms.Button btnDraw;
        private System.Windows.Forms.PictureBox pbImg;
    }
}