.NET陷阱 三 “正确”使用控件也会造成内存泄露

在我们的代码中，有时会在控件中添加对数据对象的引用。比如使用树节点的Tag属性保存相应的对象，以便在界面操作中能简单的进行访问。因为其它地方不会引用这些数据，所以我们期望在控件被销毁时，垃圾回收机制能回收相应的内存。但当软件运行了一段时间后，内存使用量会变得非常大。下面是简化后的示例代码：

using System;
 using System.Windows.Forms;   

 namespace MemoryLeak
 {
     public class MainForm : Form
     {
         private Button holderButton;
         private Button controlButton;
         private FlowLayoutPanel panel;
         private object checkGc;   

         public MainForm()
         {
             DumpMemoryUsage("before allocate checkGc.");
             checkGc = MakeLargeObject();
             DumpMemoryUsage("after allocate checkGc.");   

             holderButton = new Button();
             holderButton.Enabled = false;
             holderButton.AutoSize = true;
             holderButton.Text = "The button holds large object.";
             DumpMemoryUsage("before allocate holderButton.Tag.");
             holderButton.Tag = MakeLargeObject();
             DumpMemoryUsage("after allocate holderButton.Tag.");   

             controlButton = new Button();
             controlButton.AutoSize = true;
             controlButton.Text = "The button controls holderButton.";
             controlButton.Click += (sender, e) =>
             {
                 DumpMemoryUsage("before release checkGc and holderButton.Tag.");
                 panel.Controls.Remove(holderButton);
                 holderButton.Dispose();
                 holderButton = null;   

                 checkGc = null;
                 DumpMemoryUsage("after release checkGc and holderButton.Tag.");
             };   

             panel = new FlowLayoutPanel();
             panel.AutoSize = true;
             panel.FlowDirection = FlowDirection.TopDown;
             panel.Controls.Add(controlButton);
             panel.Controls.Add(holderButton);   

             Controls.Add(panel);
         }       

         private void DumpMemoryUsage(string msg)
         {
             GC.Collect();
             Console.WriteLine(msg);
             Console.WriteLine(GC.GetTotalMemory(true));
         }   

         private object MakeLargeObject()
         {
             var largeObject = new object[100];
             for (int i = 0; i < largeObject.Length; ++i)
             {
                 var array = new int[100][];
                 largeObject[i] = array;
                 for (int j = 0; j < array.Length; ++j)
                 {
                     array[j] = new int[100];
                 }
             }   

             return largeObject;
         }
     }   

     static class Program
     {
         static void Main()
         {
             Application.Run(new MainForm());
         }
     }
 }

代码中的checkGc变量是为了在输出中确认垃圾回收已经进行了。下面是输出结果：

before allocate checkGc.
 281576
 after allocate checkGc.
 4605632
 before allocate holderButton.Tag.
 4606384
 after allocate holderButton.Tag.
 8930480
 before release checkGc and holderButton.Tag.
 8940016
 after release checkGc and holderButton.Tag.
 4616824

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