写一个 IDisposable 的实际例子

Question

我一直在阅读 Dispose 模式，我有点理解它的用途（清理资源，这样我的应用程序就不会泄漏内存），但我想在一个实际的例子中看到它。

我的想法是编写一个简单的应用程序，首先使用一些资源但不释放它们，然后在更改一些代码后适当地释放这些资源。我想看到的是内存使用情况 before/after 代码更改以可视化处理的帮助。

问：我可以使用哪些对象？我尝试用一些大图像（JPEG 图像 15+ MB 大小）来做，但我不能用它构建一个实际的例子。当然，我对其他想法持开放态度。

Answer 1

以下示例展示了实现 IDisposable 接口的一般最佳做法。创建一些 MyResouce 实例并在不调用 Dispose 方法的情况下对其进行测试，存在内存泄漏。然后每次完成 MyResouce 实例后调用 Dispose，没有内存泄漏。 Reference

public class DisposeExample
{
    // A base class that implements IDisposable. 
    // By implementing IDisposable, you are announcing that 
    // instances of this type allocate scarce resources. 
    public class MyResource: IDisposable
    {
        // Pointer to an external unmanaged resource. 
        private IntPtr handle;
        // Other managed resource this class uses. 
        private Component component = new Component();
        // Track whether Dispose has been called. 
        private bool disposed = false;

        // The class constructor. 
        public MyResource(IntPtr handle)
        {
            this.handle = handle;
        }

        // Implement IDisposable. 
        // Do not make this method virtual. 
        // A derived class should not be able to override this method. 
        public void Dispose()
        {
            Dispose(true);
            // This object will be cleaned up by the Dispose method. 
            // Therefore, you should call GC.SupressFinalize to 
            // take this object off the finalization queue 
            // and prevent finalization code for this object 
            // from executing a second time.
            GC.SuppressFinalize(this);
        }

        // Dispose(bool disposing) executes in two distinct scenarios. 
        // If disposing equals true, the method has been called directly 
        // or indirectly by a user's code. Managed and unmanaged resources 
        // can be disposed. 
        // If disposing equals false, the method has been called by the 
        // runtime from inside the finalizer and you should not reference 
        // other objects. Only unmanaged resources can be disposed. 
        protected virtual void Dispose(bool disposing)
        {
            // Check to see if Dispose has already been called. 
            if(!this.disposed)
            {
                // If disposing equals true, dispose all managed 
                // and unmanaged resources. 
                if(disposing)
                {
                    // Dispose managed resources.
                    component.Dispose();
                }

                // Call the appropriate methods to clean up 
                // unmanaged resources here. 
                // If disposing is false, 
                // only the following code is executed.
                CloseHandle(handle);
                handle = IntPtr.Zero;

                // Note disposing has been done.
                disposed = true;

            }
        }

        // Use interop to call the method necessary 
        // to clean up the unmanaged resource.
        [System.Runtime.InteropServices.DllImport("Kernel32")]
        private extern static Boolean CloseHandle(IntPtr handle);

        // Use C# destructor syntax for finalization code. 
        // This destructor will run only if the Dispose method 
        // does not get called. 
        // It gives your base class the opportunity to finalize. 
        // Do not provide destructors in types derived from this class.
        ~MyResource()
        {
            // Do not re-create Dispose clean-up code here. 
            // Calling Dispose(false) is optimal in terms of 
            // readability and maintainability.
            Dispose(false);
        }
    }
    public static void Main()
    {
        // Insert code here to create 
        // and use the MyResource object.
    }
}

Answer 2

此单元测试执行您所描述的内容。它在调用 Dispose 和不调用 Dispose 的情况下都做同样的事情，以显示不调用 Dispose.

会发生什么。

两种方法都创建一个文件，打开文件，写入，然后再次打开并再次写入。

第一个方法抛出 IOException 因为 StreamWriter 没有被释放。文件已经打开，无法再次打开。

第二个在尝试重新打开文件之前处理，并且无一例外地工作。

[TestClass]
public class DisposableTests
{
    [TestMethod]
    [ExpectedException(typeof(IOException))]
    public void DoesntDisposeStreamWriter()
    {
        var filename = CreateFile();
        var fs = new StreamWriter(filename);
        fs.WriteLine("World");
        var fs2 = new StreamWriter(filename);
        fs2.WriteLine("Doesn't work - the file is already opened.");
    }

    [TestMethod]
    public void DisposesStreamWriter()
    {
        var filename = CreateFile();
        var fs = new StreamWriter(filename);
        fs.WriteLine("World");
        fs.Dispose();
        var fs2 = new StreamWriter(filename);
        fs2.WriteLine("This works");
        fs2.Dispose();
    }

    private string CreateFile()
    {
        var filename = Guid.NewGuid() + ".txt";
        using (var fs = new StreamWriter(filename))
        {
            fs.WriteLine("Hello");
        }
        return filename;
    }
}

您可能看不到内存使用问题，因为这不是 IDisposable 专门解决的问题。所有对象都使用内存，但大多数都不是一次性的。垃圾收集器从不再引用的对象中回收内存（例如在方法中创建但在方法结束时超出范围的对象。）

IDisposable 适用于 class 保留某些未被垃圾收集的资源的情况。另一个例子是SqlConnection。这与内存无关，与 SQL 服务器的连接有关。只有这么多可用。（它最终会被释放，但不是以可预测的方式——这是题外话。）

class 可能 IDisposable 的确切原因各不相同。他们通常没有共同点。 IDisposable不关心是什么原因。这只是意味着 class 中有些东西需要清理。

写一个 IDisposable 的实际例子

Writing a practical example of IDisposable

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