ILoggable

A place to keep my thoughts on programming

December 30, 2007 .net , ,

The dangers of deferred execution

I recently wrote about Action & Func, which along with Lambda expression let you do easy inline callbacks like this:

Utility.ActionDownloader.Download(
  Configuration.GetAssetUri(dto.Url),
  (Downloader d) =>
  {
    FloatContainer c = (FloatContainer)XamlReader.Load(d.ResponseText);
    c.Initialize(dto);
  });

i.e. I can call a downloader and inline pass it a bit of code to execute once the download completes. But the catch of course is that looking at the code, and following the usual visual tracing of flow hides the fact that c.Initialize(dto) doesn’t get called until some asynchronous time in the future. Now, that’s always been a side-effect of delegates, but until they became anonymous and inline, the visual deception of code that looks like it’s in the current flow scope but isn’t wasn’t there.

What happened was that I needed my main routine to execute some code after FloatContainer was initialized, and by habit i created an Initialized event on FloatContainer. Of course this was superfluous, since my lambda expression called the synchronous Initialize, i.e my action could be placed inline after that call to c.Initialize(dto) and be guaranteed to be called after initialization had completed.

This scenario just meant I created some superfluous code. However, I’m sure as I use lambda expression more, there will be more pitfalls of writing code that doesn’t consider that its execution time is unknown, as is the state of the objects tied to the scope of the expression.

This last bit about objects tied to the expression scope is especially tricky and I think we will see some help in terms of Immutable concepts weaving their way into C# 3.x or 4.0, as the whole functional aspect of lambda expressions really work best when dealing with objects that cannot change state. Eric Lippert’s been laying the groundwork in a number of posts on the subject and while he constantly disclaims that his ponderings are not a roadmap for C#, I am still going to assume that his interest and recognition of the subject of Immutables will have some impact in a future revision of the language. Well, I at least hope it does.

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