Invalidate 重绘控件

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

namespace tuxing
{
    public partial class Form1 : Form
    {
        private Image picture;
        private Point pictureLocation;

        public Form1()
        {
            InitializeComponent();
            // Enable drag-and-drop operations and 
            // add handlers for DragEnter and DragDrop.
            this.AllowDrop = true;
            this.DragDrop += new DragEventHandler(this.Form1_DragDrop);
            this.DragEnter += new DragEventHandler(this.Form1_DragEnter);
        }

        private void Form1_DragDrop(object sender, DragEventArgs e)
        {
             //Handle FileDrop data.
            if (e.Data.GetDataPresent(DataFormats.FileDrop))
            {
                // Assign the file names to a string array, in 
                // case the user has selected multiple files.
                string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
                //这里一定要是字符串数组。为什么？我不知道啊。。。求解
                try
                {
                    // Assign the first image to the picture variable.
                    this.picture = Image.FromFile(files[0]);
                    // Set the picture location equal to the drop point.
                    this.pictureLocation = this.PointToClient(new Point(e.X, e.Y));
                    //PointToClient()将指定屏幕点的位置计算成工作区坐标
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                    return;
                }
            }
            
            // Handle Bitmap data.
            if (e.Data.GetDataPresent(DataFormats.Bitmap))
            {
                try
                {
                    // Create an Image and assign it to the picture variable.
                    this.picture = (Image)e.Data.GetData(DataFormats.Bitmap);
                    //是不是这样，可以用GetData中数据中分别抽出地址，位图数据……
                    // Set the picture location equal to the drop point.
                    this.pictureLocation = this.PointToClient(new Point(e.X, e.Y));
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                    return;
                }
            }
            // Force the form to be redrawn with the image.
            this.Invalidate();
        }

        private void Form1_DragEnter(object sender, DragEventArgs e)
        {
            // If the data is a file or a bitmap, display the copy cursor.
            if (e.Data.GetDataPresent(DataFormats.Bitmap) ||
               e.Data.GetDataPresent(DataFormats.FileDrop))
                //GetDataPresent(),确定此实例中存储的数据是否与指定的格式关联，或是否可以转换成指定格式
            {
                e.Effect = DragDropEffects.Copy;
            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            // If there is an image and it has a location, 
            // paint it when the Form is repainted.
            base.OnPaint(e);
            if (this.picture != null && this.pictureLocation != Point.Empty)
            {
                e.Graphics.DrawImage(this.picture, this.pictureLocation);
            }
        }
    }
}

 

Onpaint

DataFormats

GetData

DragDropEffects

原话：下面的代码示例使用户能够将图像或图像文件拖到窗体上，并使它在放置点显示。 每次绘制窗体时，都重写OnPaint 方法以重新绘制图像；否则图像将保持到下一次重新绘制。 DragEnter 事件处理方法决定拖到窗体中的数据的类型，并提供适当的反馈。 如果 Image 可以从该数据中创建，则 DragDrop 事件处理方法就会在该窗体上显示此图像。 因为 DragEventArgs.X 和 DragEventArgs.Y 值为屏幕坐标，所以示例使用PointToClient 方法将它们转换成工作区坐标。