How to manage Thread Local Storage (TLS) when using TPL?

I want to store logging context information in TLS so that I can set a value at the entry point, and have that value available in all resulting stacks. This work well, but I also using TPL and the ThreadPool. The problem then becomes how to migrate TLS data to the other threads. I can do it all myself, but then I lose nice methods like Parallel.For.

Is there some way to have TLS copied when using TPL? This will also apply to C# when it gets the await feature.

Thanks, Erick

Typically, this is handled via using an overload of Parallel.For that already provides for thread local data.

This overload allows you to provide an initialization and a finalization delegate, which effectively becomes an initialization per thread for your thread local data, and a reduction function at the end to "merge" the results together (which is run once per thread). I wrote about this in detail here.

The basic form is to do something like:

object sync = new object();
double result = 0;

Parallel.For(0, collection.Count, 
    // Initialize thread local data:
    () => new MyThreadSpecificData(),
    // Process each item
    (i, pls, currentThreadLocalData) => 
    {
        // Generate a NEW version of your local state data
        MyThreadSpecificData newResults = ProcessItem(collection, i, currentThreadLocalData);
        return newResults;
    },
    // Aggregate results
    threadLocalData =>
    {
       // This requires synchronization, as it happens once per thread, 
       // but potentially simultaneously
       lock(sync)
          result += threadLocalData.Results;
    });