How do I work round this threading issue?

Question

I am using a Thread.Join to launch some functionality in a new thread.

Unfortunately I'm getting a runtime error in part of the code, here it is:

    IBuyerRequest NewRequest(string className)
    {
        className.ThrowNullOrEmpty("className");

        var type = Type.GetType(string.Format("MyApplication.BLL.PingtreeEngine.Requests.{0}Request", className));
        object[] args = { _appForm };

        type.ThrowNull("type");

        var instance = Activator.CreateInstance(type, args) as IBuyerRequest;

        return instance;
    }

Activator.CreateInstance is causing the error which is "Exception has been thrown by the target of an invocation".

Anyone got any idea as to how I could possible work round this?

Answer 1

The problem is that the class you are trying to activate is probably attempting to use HttpContext.Current, which doesn't exist on the background thread as it is kept in thread-local storage.

I assume you are doing something like this:

    public void StartBackgroundRequest()
    {
        var thread = new Thread(StartMethod);
        thread.Start();
    }

    private void StartMethod()
    {
        //bunch of stuff
        var request = NewRequest(className); // exception gets throw here
    }

You need to change this so that it captures the HttpContext on the request thread and sets it on your background thread before activating the type:

    public void StartBackgroundRequest()
    {
        var thread = new Thread(StartMethod);
        thread.Start(HttpContext.Current);
    }

    private void StartMethod(object state)
    {
        HttpContext.Current = (HttpContext)state;

        //bunch of stuff
        var request = NewRequest(className); // exception gets throw here
    }

I should note that there may be additional nuances to the implementation of the component you are activating that may mean it doesn't play nice in a multi-threaded environment, I have no idea.

It's also noteworthy that the IIS/ASP.NET threading/process model can be a bit complex, so you may or may not want to actually do this on a background thread depending on your requirements. For example, IIS can recycle the process when there are no more outstanding requests, but your background thread might still be running. If I have a need to run background tasks that need to run to completion, I will usually split that functionality into a separate windows service or similar and proxy those tasks to it so that I have better control over the process and threads.