[Mono-dev] A bug? On Mono we can't use delegate as the dictionary key

[Zoltan Varga]

Hi,

  This is now fixed in SVN HEAD/2.6/2.4 branches, altough the fix will not
make mono 2.4.3.

                 Zoltan

On Wed, Nov 25, 2009 at 2:24 PM, Matt Guo <matt at mattguo.com> wrote:

> Hey, all,
>
> Today I was stucked by a strange problem for several hours, in a word,
> we can't use delegate (including delegates form
> normal methods, annonymous methods and lambda) as the dictionary key.
> It works on .Net, but not on Mono. The reason is that on Mono,
> delegate.GetHashCode () can change: initially the hash code is 0, then
> after the delegate's first invokation the hash code will become
> non-zero (and shall be the address of the method).
> So is that a bug of mono? (I used the latest Mono 2.4 release)
>
> Following example explains everything.
>
> [Code]
>
>                       Dictionary <EventHandler, int> dict = new
> Dictionary<EventHandler, int> ();
>                       List <EventHandler> list = new List<EventHandler> ();
>
>                       EventHandler handler = (s, e) => Console.WriteLine
> (s);
>                       dict.Add (handler, 0);
>                       list.Add (handler);
>                       Console.WriteLine ("{0}, {1}, {2}",
> handler.GetHashCode(),
> dict.ContainsKey (handler), list.Contains (handler));
>                       //Call delegate
>                       handler (null, EventArgs.Empty);
>                       Console.WriteLine ("{0}, {1}, {2}",
> handler.GetHashCode(),
> dict.ContainsKey (handler), list.Contains (handler));
>
> [Mono Output]
> 0, True, True
> -1215724864, False, True
>
> [.Net Output]
> 1881641292, True, True
> 1881641292, True, True
>
>
> --
> B.R.
> GUO Rui (Matt)
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>
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