How to write to an object after TransformBlock?

Question

I have a list of objects that I need to iterate over parallely. This is what I need to do:

foreach (var r in results)
{
    r.SomeList = await apiHelper.Get(r.Id);
}

Since I want to parellelize it, I tried to use Parallel.ForEach() but it doesn't wait until everything is really complete since apiHelper.Get() is doing an await inside of its own.

Parallel.ForEach(
                results,
                async (r) =>
                {
                    r.SomeList = await apiHelper.Get(r.Id);
                });

So I searched online and found this: Nesting await in Parallel.ForEach

Now I am very new to TPL (20 mins old) and I may be missing something obvious. How do I go ahead?

        var getBlock = new TransformBlock<string, List<Something>>(
            async i =>
            {
                var c = await apiHelper.Get(i);
                return c;
            }, new ExecutionDataflowBlockOptions
            {
                MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
            });

        foreach (var r in results)
        {
            r.SomeList = getBlock.Post(r.Id);  // ERROR: Can't convert boolean to list.
        }

        getBlock.Complete();

Answer 1

Here is an a example using ActionBlock Class in the TPL dataflow library.

It basically give you parallel, and async and its fairly easy to understand

Dataflow example

public static async Task DoWorkLoads(List<Something> results)
{
   var options = new ExecutionDataflowBlockOptions
                     {
                        MaxDegreeOfParallelism = 50
                     };

   var block = new ActionBlock<Something>(MyMethodAsync, options);

   foreach (var result in results)
      block.Post(result );

   block.Complete();
   await block.Completion;

}

...

public async Task MyMethodAsync(Something result)
{       
   result.SomeList = await apiHelper.Get(result.Id);
}

Obviously, you will need error checking, and add pepper and salt to taste

Also, its assuming apiHelper is thread safe