.NET Parallel Processing

A guide for APIs and patterns for parallel processing of CPU-bound tasks.

Quick Reference: See QUICKREF.md for essential patterns at a glance.

1. Core APIs

API	Purpose
Parallel.For, Parallel.ForEach	CPU-bound parallel processing
PLINQ (.AsParallel())	LINQ query parallelization
Partitioner<T>	Large data partitioning
ConcurrentDictionary<K,V>	Thread-safe dictionary

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2. Parallel Class

2.1 Parallel.ForEach

public sealed class ImageProcessor
{
    public void ProcessImages(IEnumerable<string> imagePaths)
    {
        var options = new ParallelOptions
        {
            MaxDegreeOfParallelism = Environment.ProcessorCount
        };

        Parallel.ForEach(imagePaths, options, path =>
        {
            ProcessImage(path);
        });
    }
}

2.2 Early Termination

Parallel.For(0, data.Length, (i, state) =>
{
    if (data[i] == target)
    {
        state.Break();
    }
});

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3. PLINQ

var results = data
    .AsParallel()
    .WithDegreeOfParallelism(Environment.ProcessorCount)
    .Where(d => d.IsValid)
    .Select(d => Transform(d))
    .ToList();

// When order preservation is needed
var results = data
    .AsParallel()
    .AsOrdered()
    .Select(d => Process(d))
    .ToList();

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4. Thread-Safe Collections

Collection	Purpose
ConcurrentDictionary<K,V>	Thread-safe dictionary
ConcurrentQueue<T>	Thread-safe FIFO queue
ConcurrentBag<T>	Thread-safe unordered collection

// Collecting results during parallel processing
var results = new ConcurrentBag<Result>();

Parallel.ForEach(data, item =>
{
    var result = Process(item);
    results.Add(result);
});

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5. ThreadLocal

Prevents contention with thread-local variables

private readonly ThreadLocal<StringBuilder> _localBuilder =
    new(() => new StringBuilder());

public void ProcessInParallel()
{
    Parallel.For(0, 1000, i =>
    {
        var sb = _localBuilder.Value!;
        sb.Clear();
        sb.Append(i);
    });
}

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6. Important Notes

Parallel Processing vs Async

• CPU-bound: Use Parallel
• I/O-bound: Use async-await

// ❌ Using Parallel for I/O operations
Parallel.ForEach(urls, url => httpClient.GetAsync(url).Result);

// ✅ Using async-await for I/O operations
await Task.WhenAll(urls.Select(url => httpClient.GetAsync(url)));

Shared State Synchronization

// ❌ Parallel writes to regular collection
var list = new List<int>();
Parallel.For(0, 1000, i => list.Add(i)); // Race condition!

// ✅ Using thread-safe collection
var bag = new ConcurrentBag<int>();
Parallel.For(0, 1000, i => bag.Add(i));

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7. References

• Parallel Class
• PLINQ