Why call dispose(false) in the destructor?

Question

What follows is a typical dispose pattern example:

 public bool IsDisposed { get; private set; }

  #region IDisposable Members

  public void Dispose()
  {
    Dispose(true);
    GC.SuppressFinalize(this);
  }

  protected virtual void Dispose(bool disposing)
  {
    if (!IsDisposed)
    {
      if (disposing)
      {
        //perform cleanup here
      }

      IsDisposed = true;
    }
  }

  ~MyObject()
  {
    Dispose(false);
  }

I understand what dispose does, but what I don't understand is why you would want to call dispose(false) in the destructor? If you look at the definition it would do absolutely nothing, so why would anyone write code like this? Wouldn't it make sense to just not call dispose from the destructor at all?

Answer 1

The finalizer is used as a fall-back if the object is not disposed properly for some reason. Normally the Dispose() method would be called which removes the finalizer hookup and turns the object into a regular managed object that the garbage collector easily can remove.

Here is an example from MSDN of a class that has managed and unmanaged resources to clean up.

Notice that the managed resources are only cleaned up if disposing is true, but unmanaged resources is always cleaned up.

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.
    private 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);
    }
}