TPL Dataflow, alternative to JoinBlock limitations?

Question

I look for an alternative to JoinBlock which can be linked to by n-TransformBlocks and join/merge messages of all TransformBlock source blocks together in order to pass a co

Answer 1

If you want to perform multiple parallel operations for each item, it makes more sense IMHO to perform these operations inside a single block, instead of splitting them to multiple blocks and then trying to join the independent results into a single object again. So my suggestion is to do something like this:

var block = new TransformBlock(async item =>
{
    Task task1 = Task.Run(() => CalculateProperty1(item.Id));
    Task task2 = Task.Run(() => CalculateProperty2(item.Id));
    await Task.WhenAll(task1, task2).ConfigureAwait(false);
    item.Property1 = task1.Result;
    item.Property2 = task2.Result;
    return item;
}, new ExecutionDataflowBlockOptions()
{
    MaxDegreeOfParallelism = 2
});

In the above example items of type MyClass are passed through a TransformBlock. The properties Property1 and Property2 of each item are calculated in parallel using a separate Task for each property. Then both tasks are awaited, and when both are complete the results are assigned to the properties of the item. Finally the processed item is returned.

The only thing you want to be aware with this approach is that the degree of parallelism will be the product of the internal parallel operations and the MaxDegreeOfParallelism option of the block. So in the above example the degree of parallelism will be 2 x 2 = 4. To be precise this will be the maximum degree of parallelism, because it is possible that one of the two internal calculations will be slower than the other. So at any given moment the actual degree of parallelism could be anything between 2 and 4.