Using threadpool and func objects

Question

When using a thread pool and its queuecallbackitem, can I not pass in a func object (from the method parameter)?

I don't see a func which takes one parameter but returns nothing. There isfunc<T, TResult> but how can I set TResult to be null (want to indicate "this method returns void")?

Also, how could I use the threadpool for methods which return and take all sorts of paremeters? Could I not store Func objects in a generic collection and also an int to indicate priority, then execute those funcs?

Finally, in a static object (such as collection), what synchronisation in a global application would it need?

Marc Gravell · Accepted Answer · 2008-12-27 20:12:41Z

The only Func<...>/Action<...> delegate that is similar to WaitCallback is Action<object>. It won't be directly usable; however, you can wrap delegates inside eachother:

        Action<object> func = // TODO
        ThreadPool.QueueUserWorkItem(state=>func(state));

To return a result, one option is to update external state. Lambdas / anon-methods are good for this, since they offer closure support:

        Func<int, int> func = x => x * 5;
        int query = 4, result = 0;
        ThreadPool.QueueUserWorkItem(state=> {
            result = func(query);
        });

After execution, result (from the above context) will be updated. However, a callback is more common:

        Func<int, int> func = x => x * 5;
        int query = 4;
        Action<int> callback = x =>
        {
            Console.WriteLine(x);
        };
        ThreadPool.QueueUserWorkItem(state=> {
            int result = func(query);
            callback(result);
        });

Where the callback function does something useful with the result.

Hopefully that also shows how you can execute arbitrary functions in a thread-pool thread.

Re synchronization; you are on a secondary thread, so you definitely would need synchronization if talking to any shared state. However, you might choose to use the UI to synchronize the result (if suitable) - i.e. (from a winform):

ThreadPool.QueueUserWorkItem(state => {
    // now on worker thread
    int result = ... // TODO

    this.Invoke((Action)delegate {
       // now on UI thread
       this.Text = result.ToString();
    });

});

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