dispatch work from many threads to one synchronous thread

Question

Say I have 10 threads busily doing something and they sometimes call a method

public HandleWidgets(Widget w) {  HeavyLifting(w) }

However, I don't want my 10 threads to wait on HeavyLifting(w), but instead, dispatch the HeavyLifting(w) work to an 11th thread, the HeavyLifter thread and continue asynchronously. The HeavyLifter thread dispatched to should always be the same thread, and I don't want to make multiple threads (hence, I can't do something quite like this: C# Asynchronous call without EndInvoke?).

HeavyLifting(w) is "fire and forget" in that the threads that call HandleWidgets() don't need a callback or anything like that.

What's a healthy practice for this?

Answer 1

I'm surprised none of the other answers here mention TPL DataFlow. You connect a bunch of blocks together and post data through the chain. You can control the concurrency of each block explicitly, so you could do the following:

var transformBlk =
    new TransformBlock<int,int>(async i => {
        await Task.Delay(i);
        return i * 10;
    }, new ExecutionDataflowBlockOptions{MaxDegreeOfParallelism = 10});

var processorBlk=
    new ActionBlock<int>(async i => {
        Console.WriteLine(i);
    },new ExecutionDataflowBlockOptions{MaxDegreeOfParallelism = 1});

transformBlk.LinkTo(processorBlk);
var data = Enumerable.Range(1, 20).Select(x => x * 1000);
foreach(var x in data)
{
    transformBlk.Post(x);
}

Answer 2

Basically you have threads that are producers of work and one thread that is a consumer of it.

Create a thread and have it Take from a BlockingCollection in a loop. This is your consumer thread which will call HeavyLifting. It will simply wait until an item is available and then proces it:

A call to Take may block until an item is available to be removed.

The other threads can simply add items to the collection.

Note that BlockingCollection doesn't guarantee ordering of items added/removed by itself:

The order in which an item is removed depends on the type of collection used to create the BlockingCollection instance. When you create a BlockingCollection object, you can specify the type of collection to use