WriteOnceBlockT

(Readonly variable: Memorizes its first data item and passes out copies of it as its output. Ignores all other data items)

If BufferBlock is the most fundamental block in TPL Dataflow, WriteOnceBlock is the simplest.

It stores at most one value, and once that value has been set, it will never be replaced or overwritten.

You can think of WriteOnceBlock in as being similar to a readonly member variable in C#, except instead of only being settable in a constructor and then being immutable, it’s only settable once and is then immutable.

Splitting a Task’s Potential Outputs

public static async void SplitIntoBlocks(this Task<T> task,
    out IPropagatorBlock<T> result, 
    out IPropagatorBlock<Exception> exception)
{
    result = new WriteOnceBlock<T>(i => i);
    exception = new WriteOnceBlock<Exception>(i => i);

    try 
    { 
        result.Post(await task); 
    }
    catch(Exception ex) 
    { 
        exception.Post(ex); 
    }
}

Introduction to TPL Dataflow by Stephen Toub