[C#]Effective C# 條款十八：實現標準Dispose模式

IDisposable接口為.Net程式釋放非託管資源的標準解決方案，可為類別提供即時釋放資源的機制。

其內含有Dispose方法，為資源釋放動作的本體，在該方法的實作上需注意到需內含下面幾個重點任務:

1.判斷是否做過釋放動作  

因為透過Dispose做資源的釋放後，資源仍舊會有短時間會滯留在記憶體中。此時若該物件仍具有強引用參考，垃圾收集器會無法對該物件做回收的動作。因此該物件有可能會因此再次被錯用 ，重覆釋放到已經釋放的資源，故需在該方法實作時做些檢查的動作。

public class MyResource: IDisposable
{
    private bool disposed = false;
    ...
    protected virtual void Dispose(bool disposing)
    {           
        if(this.disposed)
            return;
       ...
            disposed = true;
    }
  }  

2.釋放托管資源

釋放物件中的託管資源。這部分包含釋放有實作IDisposable接口的類別成員，以及斷開有繫結的事件。

3.釋放非托管資源

釋放物件中的非託管資源。

4.取消解構子調用

如我們所了解的，具有解構子的物件其在被垃圾收集器回收處理時，會先被放入解構佇列之中，再交由另一個專門處理解構動作的執行緒去做解構的動作，當解構的動作完成，該物件又會被放回原來的佇列等待垃圾收集器的回收，因此其性能上的耗費會比沒有解構子的物件還來的多。由於IDisposable在實作上會習慣加入解構子做為保險措施，防止類別的使用者忘記叫用Dispose方法，造成資源的洩漏。故在釋放完資源後，我們應該隨即在後呼叫GC.SuppressFinalize，告知垃圾收集器該物件的解構動作跳過不處理。像是:

    ...
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }
    protected virtual void Dispose(bool disposing)
    {
        ...
    }

     ~MyResource()
    {
        Dispose(false);
    }
    ...

完整的實作範例如下(摘至MSDN):

using System; using System.ComponentModel; // The following example demonstrates how to create// a resource class that implements the IDisposable interface// and the IDisposable.Dispose method.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. } }

這邊要再特別注意的是，在建立衍生類別時，若其基底類別實作有IDisposable介面，衍生類別也需隨之實作IDisposable介面。

另外若在實現衍生類別的同時，有對Dispose方法或解構子覆寫的需求，務必要調用對應的基類方法，以確保基類資源能被徹底釋放。

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