Singleton Per Call Context (Web Request) in Unity

user_141101 0
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A few days ago, I had an issue with ASP.Net threading. I wanted to have a singleton object per web request. I actually need this for my unit of work. I wanted to instantiate a unit of work per web request so that identity map is valid through out the request. This way I could use an IoC to inject my own IUnitOfWork to my repository classes transparently, and I could use the same instance to query and then update my entities.

Since I am using Unity, I mistakenly used PerThreadLifeTimeManager. I soon realised that ASP.Net threading model does not support what I want to achieve. Basically it uses a threadpool and recycles threads, and that means that I get one UnitOfWork per thread!! However, what I wanted was one unit of work per web request.

A bit of googling gave me this great post. That was exactly what I wanted; except for the unity part which was quite easy to achieve.

This is my implementation for PerCallContextLifeTimeManager for unity:

public class PerCallContextLifeTimeManager : LifetimeManager
{
    private const string Key = "SingletonPerCallContext";

    public override object GetValue()
    {
        return CallContext.GetData(Key);
    }

    public override void SetValue(object newValue)
    {
        CallContext.SetData(Key, newValue);
    }

    public override void RemoveValue()
    {
    }
}

And of course I use this to register my unit of work with a code similar to this:

unityContainer
            .RegisterType<IUnitOfWork, MyDataContext>(
            new PerCallContextLifeTimeManager(),
            new InjectionConstructor());

Hope it saves someone a bit of time.

lifetime unit-of-work unity-container asp.net singleton
3 Answers

While this is fine calling this PerCallContextLifeTimeManager, I'm pretty sure this is not "safe" to be considered an ASP.Net Per-request LifeTimeManager.

If ASP.Net does its thread-swap then the only thing taken across to the new thread through CallContext is the current HttpContext - anything else you store in CallContext will be gone. This means under heavy load the code above could have unintended results - and I imagine it would be a real pain to track down why!

The only "safe" way to do this is with HttpContext.Current.Items, or doing something like:

public class PerCallContextOrRequestLifeTimeManager : LifetimeManager
{
    private string _key = string.Format("PerCallContextOrRequestLifeTimeManager_{0}", Guid.NewGuid());

    public override object GetValue()
    {
      if(HttpContext.Current != null)
        return GetFromHttpContext();
      else
        return GetFromCallContext();
    }

    public override void SetValue(object newValue)
    {
      if(HttpContext.Current != null)
        return SetInHttpContext();
      else
        return SetInCallContext();
    }

    public override void RemoveValue()
    {
    }
}

This obviously means taking dependencies on System.Web :-(

Much more information on this available at:

http://piers7.blogspot.com/2005/11/threadstatic-callcontext-and_02.html

Beepcake_265070
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