Common pitfall with async in C#

Just a quick rant today... :)

I love how easy it is to write asynchronous code with Tasks and the async and await keywords in C#. One problem that I keep running into is this kind of thing in code examples on line.

public static T Value<T>(this HttpContent content)
{
    var task = content.ReadAsAsync<T>();
    task.Wait();
    return task.Result;
}
Now there's a couple of problems with the above code. Whoever wrote it obviously thought it was going to do something magic, after all - they've chosen to use the ReadAsAsync<T>() method! Unfortunately, the very next line calls Wait() on the returned Task. So the Thread that could have been doing other work while the content was being read is just blocking anyway. I.e. there is no benefit at all to using the Async code.
There's also the fact that accessing task.Result would have blocked anyway meaning the Wait() was pointless.

This code was checked in with the comment "Repopulate Async so no one has to wait!". I'm not intentionally tearing someone down - it was just a good example of misunderstanding asynchronous code. The intention was laudable, a long running non-cpu bound web api action? Prime candidate for asynchronous and credit to the guy for noticing that.

So what was the solution? 

Remove the pointless extension method (only used once) and await ReadAsync in the asynchronous action that was using it.

Don't just use async methods for the sake of it, use them when something higher up the call chain can actually take advantage of it!

Comments

Post a Comment

Popular posts from this blog

Trimming strings in action parameters in ASP.Net Web API

In standard MVC it's very easy to create a model binder to trim all of the strings passed into action methods. The approach I used for that was from here:  http://blogs.taiga.nl/martijn/2011/09/29/custom-model-binders-and-request-validation/ . It works well and it trims the properties of models too. The problem came when I tried to do the same thing for Web API. I added a similar StringTrimmingBinder and registered it as a service using the very non-discoverable API: var provider = new SimpleModelBinderProvider( typeof ( string ),  new StringTrimmingModelBinder()); GlobalConfiguration.Configuration.Services.Insert( typeof (ModelBinderProvider),  0,  provider); As a side note, I have no idea how you're supposed to find out how to register a model binder without opening up Bing and Googling it. Anyway, this worked great when the parameters to the action were actually strings but doesn't work with complex types as parameters. Now, I'v...
Read more

Full text search in Entity Framework 6 using command interception

Image
Background One of the main problems we encountered using Entity Framework was its lack of support for full text search. Our search function is based around a series of processors that attempt to match the incoming phrase against a set of patterns that we're expecting (10+). For example, there's one for dates and times, recent financial quarters as well as more domain specific ones. Everything is cached and so is really fast, except when it isn't: Here's the monitoring chart for our SQL DB in Azure That's a spike to 100% resource utilisation because we fall through to a phrase search as a last resort (which cannot be cached) - it's exceedingly rare but it still hurts. Before EF, the app used full text search and it was one of the casualties when the app was migrated to EF4 several years ago. The lack of full text search has been getting more and more painful as the amount of data has grown. We really needed full text search back... Command intercept...
Read more

Composing Expressions in C#

This post follows on from  Reusing predicates in Entity Framework . In my previous post I introduced the "and" extension method which allowed me to create a new entity framework safe expression from two existing expressions. Here's how we make that happen: The code is from a blogpost from Colin Meek from 2008:  InvocationExpression and LINQ to Entities . I'll just dump my version of the code here. The idea is exactly the same (even using the visitor as an immutable stack): public class InvocationExpander : ExpressionVisitor { private readonly ParameterExpression _parameter; private readonly Expression _expansion; private readonly InvocationExpander _previous; public static T Expand<T>(T expr) where T : LambdaExpression { return (T) new InvocationExpander().Visit(expr); } public InvocationExpander() { } private InvocationExpander(ParameterExpression parameter, Expression expansion, Invocati...
Read more