var options = new ParallelOptions
{
    MaxDegreeOfParallelism = Environment.ProcessorCount
};

Parallel.ForEach(items, options, item =>
{
    ProcessItem(item);
});

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

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

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

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

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