Implementing a Dispose method

【Implementing a Dispose method】

0、Dispose() and Dispose(Boolean)

　　The IDisposable interface requires the implementation of a single parameterless method, Dispose. However, the dispose pattern requires two Dispose methods to be implemented:

• A public non-virtual (NonInheritable in Visual Basic) IDisposable.Dispose implementation that has no parameters.

• A protected virtual (Overridable in Visual Basic) Dispose method whose signature is:

　　　　

1、Dispose() has a standard implementation, The Dispose method performs all object cleanup, so the garbage collector no longer needs to call the objects' Object.Finalizeoverride.

　　实现System.IDsiposable.Dispose()方法。不能将此方法设置为virtual，子类不能override此方法。此方法的实现必须为如下代码。

　　　　 // 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);
        }

2、实现析构函数（Finalize）。析构函数必须实现为以下代码。

　　　　 // 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);
        }

3、实现virtual Dispose(bool disposing)方法，必须为protected。如果dispoing为true，说明从Dipose()方法调用而来；如果为false，说明从析构函数（Finalize）调用而来。在析构函数中，不能再使用引用类型（Ref）成员类型，所以只处理unmanaged resource的释放即可；而在Dipose()调用过程中，managed res & unmanaged res都要释放。

　　最后，添加一个成员变量disposed来标记是否已经释放过。

　　　　 // 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;

            }
        }

3.1、Here's the general pattern for implementing the dispose pattern for a derived class that overrides Object.Finalize:

　　A class derived from a class that implements the IDisposable interface shouldn't implement IDisposable, because the base class implementation of IDisposable.Dispose is inherited by its derived classes.

using System;

class DerivedClass : BaseClass
{
   // Flag: Has Dispose already been called?
   bool disposed = false;
   
   // Protected implementation of Dispose pattern.
   protected override void Dispose(bool disposing)
   {
      if (disposed)
         return; 
      
      if (disposing) {
         // Free any other managed objects here.
         //
      }
      
      // Free any unmanaged objects here.
      //
      disposed = true;
      
      // Call the base class implementation.
      base.Dispose(disposing);
   }

   ~DerivedClass()
   {
      Dispose(false);
   }
}

 4、An object must also call the Dispose method of its base class if the base class implements IDisposable. 

　　如果基类有Dispose()方法，子类必须的Dispose()必须调用基类的Dispose()。

 5、什么时候需要Dispose()？

　　1）包含Disposable对象的容器类需要实现Dipose，而不需要实现Finalize()。因为清空一个容器，并不表明需要释放unmanged resource。

　　2）自身包含unmanged resource的类需实现Dispose、Finalize。

 6、DO throw an ObjectDisposedException from any member that cannot be used after the object has been disposed of.

　　当调用一个已经Disposed对象时，抛出 ObjectDisposedException。

public class DisposableResourceHolder : IDisposable {  
    bool disposed = false;  
    SafeHandle resource; // handle to a resource  

    public void DoSomething(){  
           if(disposed) throw new ObjectDisposedException(...);  
        // now call some native methods using the resource   
            ...  
    }  
    protected virtual void Dispose(bool disposing){  
        if(disposed) return;  
        // cleanup  
        ...  
        disposed = true;  
    }  
}  

7、一些C#自带类库的Close()语言与Dispose()语义是一样的。

public class Stream : IDisposable {  
    IDisposable.Dispose(){  
        Close();  
    }  
    public void Close(){  
        Dispose(true);  
        GC.SuppressFinalize(this);  
    }  
}  

参考：

1、https://docs.microsoft.com/en-us/dotnet/api/system.idisposable.dispose?view=netframework-4.7.1

2、https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/dispose-pattern