How does local initialization with Parallel ForEach work?

Question

I am unsure about the use of the local init function in Parallel.ForEach, as described in the msdn article: http://msdn.microsoft.com/en-us/library/dd997393.aspx

Answer 1

as an extension to @Honza Brestan's answer. The way Parallel foreach splits the work into tasks can also be important, it will group several loop iterations into a single task so in practice localInit() is called once for every n iterations of the loop and multiple groups can be started simultaneously.

The point of a localInit and localFinally is to ensure that a parallel foreach loop can combine results from each itteration into a single result without you needing to specify lock statements in the body, to do this you must provide an initialisation for the value you want to create (localInit) then each body itteration can process the local value, then you provide a method to combine values from each group (localFinally) in a thread-safe way.

If you don't need localInit for synchronising tasks, you can use lambda methods to reference values from the surrounding context as normal without any problems. See Threading in C# (Parallel.For and Parallel.ForEach) for a more in depth tutorial on using localInit/Finally and scroll down to Optimization with local values, Joseph Albahari is really my goto source for all things threading.