Parallel.ForEach运行时，MaxDegreeOfParallelism是否会因初始化方法返回值变化而改变？

Question #

Suppose I have a Parallel.ForEach in my code where the MaxDegreeOfParallelism of ParallelOptions is set via Utils.Threads(), like this:

Parallel.ForEach(linesList, new ParallelOptions { MaxDegreeOfParallelism = Utils.Threads() }, line => { /* ... */ });

Utils.Threads() returns a random integer between 1 and 10. Will MaxDegreeOfParallelism change during the execution of Parallel.ForEach, or will it stay the value obtained from the first call to Utils.Threads()?

Answer #

Great question! The short answer is: it will always stay the value returned by the first call to Utils.Threads().

Here's why: When you initialize the ParallelOptions object, the Utils.Threads() method is invoked exactly once—right at the moment you set the MaxDegreeOfParallelism property. The Parallel.ForEach method reads this property value once when it starts executing, and then uses that fixed value for the entire duration of the loop.

Think of it this way: ParallelOptions is just a plain .NET class with a regular property. Once you assign the result of Utils.Threads() to MaxDegreeOfParallelism, that value is stored in the object. Parallel.ForEach doesn't re-check the property or re-invoke Utils.Threads() while processing elements. Even if you manually modified the MaxDegreeOfParallelism property later (which your code doesn't do), the running Parallel.ForEach wouldn't pick up the change—it locks in the value at startup.

For example: If Utils.Threads() returns 5 when the ParallelOptions is created, your Parallel.ForEach will cap concurrent execution at 5 threads for the entire loop. Even if subsequent calls to Utils.Threads() return 2 or 10, it won't affect how this particular loop runs.

内容的提问来源于stack exchange，提问作者user9331278