C# parallelism: Task.WhenAll and Parallel.ForEachAsync with MaxDegreeOfParallelism

// ======================================================================
// Q: How do you run work in parallel and limit concurrency?
//
// Parallel.For / PLINQ for CPU-bound work. Task.WhenAll with no limit over
// 10,000 items = 10,000 concurrent calls.
// ======================================================================

// Independent I/O calls: concurrently
var stockTask = erp.GetWarehouseStockAsync(sku, ct);
var daysTask  = erp.GetDeliveryDaysAsync(sku, ct);
await Task.WhenAll(stockTask, daysTask);          // time = the slowest one, not the sum

// Many items with a limit on concurrent calls (so you don't take down the ERP)
await Parallel.ForEachAsync(skus,
    new ParallelOptions { MaxDegreeOfParallelism = 4, CancellationToken = ct },
    async (sku, token) => await SyncPriceAsync(sku, token));