Hybrid Parallelism

Not every part of your query should be parallel. Use AsSequential to drop back into single-threaded mode for specific operations.

1. Thread-Safe Sections

Imagine you are doing heavy math in parallel, but then you need to update a shared resource that isn't thread-safe (like a legacy COM object or a non-concurrent dictionary). You do the math with AsParallel and then switch to AsSequential for the final update.

var query = data.AsParallel()
                .Select(x => HeavyMath(x)) // Parallel
                .AsSequential()
                .Select(x => UpdateSingleThreadedResource(x)); // Sequential
    

2. Managing Concurrency

You can limit the number of threads PLINQ uses with WithDegreeOfParallelism(n). This is critical if your app is running on a server with 64 cores, but you don't want to starve other applications of CPU during a query.

3. Architect Insight

Q: "Can I use PLINQ with EF Core?"

Architect Answer: "NO. AsParallel() is for in-memory collections. Databases already handle parallelism internally using their own execution plans. If you try to use PLINQ on a DB context, you will likely cause thread-safety errors as multiple threads try to use the same database connection simultaneously."