MSDN例子之一：Control.Invalidate

 

这个例子主要是实现了拖动一个图像到窗体，图像便显示在窗体上你所拖动的位置。
主要是练习了以下的方法。

Control.Invalidate使控件的整个图面无效并导致重绘控件。

如果没有这个方法的话，窗体只是在最大化、最小化或者被覆盖过才进行重画。

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

namespace Control.Invalidate
{
    /// <summary>
    /// 这个例子主要是实现了拖动一个图像到窗体，图像便显示在窗体上你所拖动的位置。
    /// 主要是练习了以下的方法。
    /// Control.Invalidate使控件的整个图面无效并导致重绘控件。
    /// </summary>
    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(Form1_DragDrop);
            this.DragEnter += new DragEventHandler(Form1_DragEnter);
        }

        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 ))
            {
                e.Effect = DragDropEffects.Copy;

            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
            //throw new Exception("The method or operation is not implemented.");
        }

        void Form1_DragDrop(object sender, DragEventArgs e)
        {
            //handle filedrop data
            if (e.Data .GetDataPresent (DataFormats .FileDrop)  )
            {
                //assign the file names to a string array,
                //case the user has selected mutiple files
                string[] files = (string [])e.Data.GetData(DataFormats.FileDrop);
                try
                {
                    //assign the first image to the picture variable
                    this.picture = Image.FromFile(files[0]);
                    this.pictureLocation = this.PointToClient(new Point(e.X, e.Y));


                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                    throw;
                }
            }
            //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.FileDrop);
                    //set the picture location equal to the drop position
                    this.pictureLocation = this.PointToClient(new Point(e.X, e.Y));

                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                    throw;
                }
            }

            //Force the form to be redraw with the image 
            this.Invalidate();

            //throw new Exception("The method or operation is not implemented.");
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            
            base.OnPaint(e);
            if (this.pictureLocation != null && this .picture!= null)

            {
                e.Graphics.DrawImage(this.picture, this.pictureLocation);
            }
        }
    }
}