Multiple Parallel.ForEach calls, MemoryBarrier?

The work started by a Parallel.ForEach() will be done before it returns. Internally, ForEach() spawns a Task for each iteration, and calls Wait() on each one. As a result, you do not need to synchronize access between ForEach() calls.

You do need to keep that in mind for individual tasks with ForEach() overloads that allow you access to loop state, aggregating results from tasks, etc. For example in this trivial example which sums up 1 ≤ x ≤ 100, the Action passed to localFinally of Parallel.For() has to be concerned about synchronization issues,

var total = 0;Parallel.For(0, 101, () => 0,  // <-- localInit(i, state, localTotal) => { // <-- body  localTotal += i;  return localTotal;}, localTotal => { <-- localFinally  Interlocked.Add(ref total, localTotal); // Note the use of an `Interlocked` static method});// Work of previous `For()` call is guaranteed to be done hereConsole.WriteLine(total);

In your example, it is not necessary to insert a memory barrier between the ForEach() calls. Specifically, loop IV can depend on the results of II being completed, and Parallel.ForEach() already inserted III for you.

Snippet sourced from: Parallel Framework and avoiding false sharing

Since more than one thread will be accessing the same variable "dr.B", you will need to make sure your C# code is thread-safe.

Try using "lock" round each operationhttps://msdn.microsoft.com/en-us/library/c5kehkcz.aspx

e.g.

private Object thisLock1 = new Object();...lock(thisLock1){    dr.C = 2.0 * dr.B;}...lock(thisLock1){    dr.B = 2.0*dr.A;}

However, doing this will defeat the parallel processing. since each thread has to wait until the next one is done.

Make sure to read the potential pitfall with parallel processing:https://msdn.microsoft.com/en-us/library/dd997403%28v=vs.110%29.aspx