为什么后台线程中的图形操作会阻塞主 UI 线程中的图形操作？

我有一个后台线程正在给定文件夹中创建图像的灰度缩略图。我看到的问题是后台线程中的 Graphics.DrawImage() 调用似乎以某种方式阻止了主 UI 线程上的 Graphics 操作。

我可能会误解我在这里看到的内容，并且直到今晚晚些时候才有机会进行任何深入的分析，尽管我不希望能够找到太多东西。

我试图想出尽可能小的重现案例。如果您将默认项目中的表单替换为下面的表单（并在文件夹中有一些图像进行测试），您会注意到动画标签在窗口中来回弹跳时会出现卡顿。然而，如果您取消顶部#define 的注释，以便子控件动画化而不是重绘窗口内容，则它运行得非常流畅。

任何人都可以看到我在这里做错了什么，或者帮助我弄清楚如何在更新循环期间避免这种口吃？

//#define USE_LABEL_CONTROL

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Threading;
using System.Windows.Forms;
using Timer = System.Windows.Forms.Timer;

namespace ThreadTest
{
    public partial class Form1 : Form
    {
        private const string ImageFolder = "c:\\pics";
        private const string ImageType = "*.jpg";

        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnLoad(EventArgs e)
        {
            this.Size = new Size(300, 300);

            string[] ImageFiles = Directory.GetFiles(ImageFolder, 
                                                        ImageType, 
                                                        SearchOption.AllDirectories);

            // kick off a thread to create grayscale thumbnails of all images
            this.thumbnailThread = new Thread(this.thumbnailThreadFunc);
            this.thumbnailThread.Priority = ThreadPriority.Lowest;
            this.thumbnailThread.Start(ImageFiles);

            // set a timer to start us off...
            this.startTimer = new Timer();
            this.startTimer.Interval = 500;
            this.startTimer.Tick += this.startTimer_Tick;
            this.startTimer.Start();

#if USE_LABEL_CONTROL
            this.label.Location = this.labelRect.Location;
            this.label.Size = this.labelRect.Size;
            this.label.Text = "Loaded: 0";
            this.label.BorderStyle = BorderStyle.FixedSingle;
            this.Controls.Add(this.label);
#endif

            base.OnLoad(e);
        }

        void startTimer_Tick(object sender, EventArgs e)
        {
            // kill the timer
            this.startTimer.Stop();

            // update ourself in a loop
            while (this.IsHandleCreated)
            {
                int NextTick = Environment.TickCount + 50;

                // update the label position
                this.labelRect.Offset(this.currentLabelDirection, 0);
                if (this.labelRect.Right == this.ClientRectangle.Right ||
                    this.labelRect.Left == 0)
                {
                    this.currentLabelDirection = -this.currentLabelDirection;
                }

                // update the display
#if USE_LABEL_CONTROL
                this.label.Text = "Loaded: " + this.thumbs.Count;
                this.label.Location = this.labelRect.Location;
#else
                using (Graphics Dest = this.CreateGraphics())
                {
                    this.redrawControl(Dest, this.ClientRectangle);
                }
#endif

                Application.DoEvents();
                Thread.Sleep(Math.Max(0, NextTick - Environment.TickCount));
            }
        }

        private void thumbnailThreadFunc(object ThreadData)
        {
            string[] ImageFiles = (string[]) ThreadData;
            foreach (string ImageFile in ImageFiles)
            {
                if (!this.IsHandleCreated)
                {
                    return;
                }

                using (Image SrcImg = Image.FromFile(ImageFile))
                {
                    Rectangle SrcRect = new Rectangle(Point.Empty, SrcImg.Size);

                    Rectangle DstRect = new Rectangle(Point.Empty, new Size(300, 200));
                    Bitmap DstImg = new Bitmap(DstRect.Width, DstRect.Height);
                    using (Graphics Dst = Graphics.FromImage(DstImg))
                    {
                        using (ImageAttributes Attrib = new ImageAttributes())
                        {
                            Attrib.SetColorMatrix(this.grayScaleMatrix);
                            Dst.DrawImage(SrcImg, 
                                            DstRect, 
                                            0, 0, SrcRect.Width, SrcRect.Height, 
                                            GraphicsUnit.Pixel, 
                                            Attrib);
                        }
                    }

                    lock (this.thumbs)
                    {
                        this.thumbs.Add(DstImg);
                    }
                }
            }
        }

#if !USE_LABEL_CONTROL
        private void redrawControl (Graphics Dest, Rectangle UpdateRect)
        {
            Bitmap OffscreenImg = new Bitmap(this.ClientRectangle.Width, 
                                                this.ClientRectangle.Height);
            using (Graphics Offscreen = Graphics.FromImage(OffscreenImg))
            {
                Offscreen.FillRectangle(Brushes.White, this.ClientRectangle);
                Offscreen.DrawRectangle(Pens.Black, this.labelRect);
                Offscreen.DrawString("Loaded: " + this.thumbs.Count,
                                        SystemFonts.MenuFont,
                                        Brushes.Black,
                                        this.labelRect);
            }
            Dest.DrawImageUnscaled(OffscreenImg, 0, 0);
            OffscreenImg.Dispose();
        }

        protected override void OnPaintBackground(PaintEventArgs e)
        {
            return;
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            this.redrawControl(e.Graphics, e.ClipRectangle);
        }
#endif


        private ColorMatrix grayScaleMatrix = new ColorMatrix(new float[][] 
                                                        {
                                                            new float[] {.3f, .3f, .3f, 0, 0},
                                                            new float[] {.59f, .59f, .59f, 0, 0},
                                                            new float[] {.11f, .11f, .11f, 0, 0},
                                                            new float[] {0, 0, 0, 1, 0},
                                                            new float[] {0, 0, 0, 0, 1}
                                                        });
        private Thread thumbnailThread;
        private Timer startTimer;
        private List<Bitmap> thumbs = new List<Bitmap>();
        private Label label = new Label();
        private int currentLabelDirection = 1;
        private Rectangle labelRect = new Rectangle(0, 125, 75, 20);
    }
}

事实证明，答案是使用多个进程来处理后台 GDI+ 任务。如果您在 VS2010 中的并发分析器下运行上述代码，您将看到前台线程阻塞在由后台线程中的 DrawImage() 调用保护的关键部分上。

该线程还讨论了这个问题，并指出由于它使用关键部分，锁将是每个进程的，并且后台任务可以使用多个进程而不是线程来并行化：

并行化 GDI+ 图像大小调整 .net https://stackoverflow.com/questions/3719748/parallelizing-gdi-image-resizing-net