Icon still usable after calling Dispose()?

Question

I've been trying to sort out some of our code using Dispose properly in a number of places that things were being left hanging around. Once such instance was Icons, and something I noticed which I thought was odd, if I call Icon.Dispose() I was still able to use the Icon.

So I extracted it out into a little console application that I fully expected to crash (throw an ObjectDisposedException), but it didn't... Am I mis-understanding what dispose should be doing here?

using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;
using System.IO;

namespace DisposeTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Icon icon = new Icon(@"C:\temp\test.ico");
            icon.ToBitmap().Save(@"C:\temp\1.bmp");
            icon.Save(new FileStream(@"C:\temp\1.ico", FileMode.OpenOrCreate, FileAccess.ReadWrite));
            icon.Dispose();
            GC.Collect(); // Probably not needed, but just checking.
            icon.Save(new FileStream(@"C:\temp\2.ico", FileMode.OpenOrCreate, FileAccess.ReadWrite));
            icon.ToBitmap().Save(@"C:\temp\2.bmp");
        }
    }
}

Answer 1

I extracted it out into a little console application that I fully expected to crash, but it didn't. Am I mis-understanding what dispose should be doing here?

Programs that contain bad programming practices are not required to crash and burn. It sure is nice if they do, but they do not have to. To my knowledge the documentation does not state that "program crashes if you use a disposed icon" is a supported feature of the library, so you cannot rely on it.

(If the documentation does say that somewhere then you've found a bug; if you feel like reporting it on Connect, that would be appreciated.)

For a related question that people seem to find entertaining, see this question about taking advantage of non-crashing behaviour in C:

Can a local variable's memory be accessed outside its scope?

Finally, your comment is correct in noting that the garbage collector is probably not relevant. Remember, the point of "Dispose" is to throw away unmanaged resources. The point of the garbage collector is to throw away managed memory and to throw away unmanaged resources via finalizers. Since the icon's memory is still alive (and the disposer will probably have suppressed finalization regardless), forcing a garbage collection here is unlikely to do anything.

Answer 2

It doesn't crash because the Dispose() method doesn't do anything in your case. The Icon class wrapper can wrap two distinct sources of icon data. In your case, the icon data comes from a file and is stored in a byte[]. Which is not a system resource that requires disposing, the garbage collector already takes care of it.

The other icon data source is from a Windows handle. Fairly rare, but you'll get that when using Icon.FromHandle() or one of the System.Drawing.SystemIcons. Now it does wrap an unmanaged resource that makes using Dispose() useful.

This pattern is not unusual. There are also plenty of cases where Dispose() is inherited into a class where it doesn't make much sense. Like MemoryStream or BackgroundWorker. The conventional wisdom is to dispose them anyway.

Answer 3

This behaviour, while a little strange-looking, does make some sense. Usually, Dispose() is used to release unmanaged resources; it's a way to "play nice" with other processes in the system. In this case, you're likely releasing the HICON handle during Dispose(). The rest of the (managed) resources associated with the Icon (including the actual byte[] data) will be release when the managed object is collected.

You try to do that by calling GC.Collect(), but that's not going to do anything, since the Icon is still alive: it's referenced by the local variable called icon, and is not currently eligible for collection.