using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ZoomImage.Utils
{
 /// <summary>
 /// 圖片縮放
 /// </summary>
 public class ZoomImageUtil
 {
  /// <summary>
  /// 圖片縮放
  /// </summary>
  /// <param name="bmp">圖片</param>
  /// <param name="width">目標寬度，若為0，表示寬度按比例縮放</param>
  /// <param name="height">目標長度，若為0，表示長度按比例縮放</param>
  public static Bitmap GetThumbnail(Bitmap bmp, int width, int height)
  {
   if (width == 0)
   {
    width = height * bmp.Width / bmp.Height;
   }
   if (height == 0)
   {
    height = width * bmp.Height / bmp.Width;
   }

   Image imgSource = bmp;
   Bitmap outBmp = new Bitmap(width, height);
   Graphics g = Graphics.FromImage(outBmp);
   g.Clear(Color.Transparent);
   // 設(shè)置畫布的描繪質(zhì)量   
   g.CompositingQuality = CompositingQuality.HighQuality;
   g.SmoothingMode = SmoothingMode.HighQuality;
   g.InterpolationMode = InterpolationMode.HighQualityBicubic;
   g.DrawImage(imgSource, new Rectangle(0, 0, width, height + 1), 0, 0, imgSource.Width, imgSource.Height, GraphicsUnit.Pixel);
   g.Dispose();
   imgSource.Dispose();
   bmp.Dispose();
   return outBmp;
  }
 }
}

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

namespace ZoomImage
{
 public partial class Form1 : Form
 {
  public Form1()
  {
   InitializeComponent();
  }

  private void Form1_Load(object sender, EventArgs e)
  {
   openFileDialog1.Multiselect = true;
  }

  private void txtWidth_KeyPress(object sender, KeyPressEventArgs e)
  {
   if (e.KeyChar != 8 && !Char.IsDigit(e.KeyChar))
   {
    e.Handled = true;
   }
  }

  private void txtHeight_KeyPress(object sender, KeyPressEventArgs e)
  {
   if (e.KeyChar != 8 && !Char.IsDigit(e.KeyChar))
   {
    e.Handled = true;
   }
  }

  private void btnSelectImage_Click(object sender, EventArgs e)
  {
   try
   {
    if (txtWidth.Text == "" && txtHeight.Text == "")
    {
     return;
    }

    if (openFileDialog1.ShowDialog() == DialogResult.OK)
    {
     Task.Factory.StartNew(() =>
     {
      string path = Path.GetDirectoryName(openFileDialog1.FileNames[0]) + "\\NewImage\\";

      int i = 0;
      foreach (string fileName in openFileDialog1.FileNames)
      {
       Bitmap bmp = ZoomImageUtil.GetThumbnail(new Bitmap(fileName), Convert.ToInt32(txtWidth.Text == "" ? "0" : txtWidth.Text), Convert.ToInt32(txtHeight.Text == "" ? "0" : txtHeight.Text));
       if (!Directory.Exists(path))
       {
        Directory.CreateDirectory(path);
       }
       File.Delete(path + Path.GetFileName(fileName));
       bmp.Save(path + Path.GetFileName(fileName));
       this.Invoke(new InvokeDelegate(() =>
       {
        lblProgress.Text = string.Format("進度：{1}/{0}", openFileDialog1.FileNames.Length, ++i);
       }));
       Thread.Sleep(1);
      }

      MessageBox.Show("成功！");
     });
    }
   }
   catch (Exception ex)
   {
    MessageBox.Show(ex.Message);
   }
  }

 }

 /// <summary>
 /// 跨線程訪問控件的委托
 /// </summary>
 public delegate void InvokeDelegate();
}