Quick and ugly example of the benefits of using Task.WhenAll for running many concurrent Tasks

concurrentTasksDemo.c

using System.Diagnostics;

var numTasks = 10;
var taskTimeMs = 1000;
Console.WriteLine($"Running {numTasks} tasks at {taskTimeMs} ms each.");

Stopwatch stopWatch = new Stopwatch();

await DoSlow();
await DoFast();

async Task DoFast()
{
    Console.WriteLine("Using Task.WhenAll");
    List<Task> tasks = new List<Task>();
    for (int i = 0; i < numTasks; i++)
    {
        tasks.Add(Task.Run(() => Thread.Sleep(taskTimeMs)));
    }
    StartStopwatch();
    await Task.WhenAll(tasks);
    StopStopwatch();
    PrintElapsedTime();
}

async Task DoSlow()
{
    Console.WriteLine("Using sequential tasks");
    StartStopwatch();
    for (int i = 0; i < numTasks; i++)
    {
        await Task.Run(() => Thread.Sleep(taskTimeMs));
    }
    StopStopwatch();
    PrintElapsedTime();
}

void StartStopwatch()
{
    stopWatch.Restart();
}

void StopStopwatch()
{
    stopWatch.Stop();
}

void PrintElapsedTime()
{
    // Get the elapsed time as a TimeSpan value.
    TimeSpan ts = stopWatch.Elapsed;

    // Format and display the TimeSpan value.
    string elapsedTime = String.Format("{0:00}:{1:00}:{2:00}.{3:00}",
        ts.Hours, ts.Minutes, ts.Seconds,
        ts.Milliseconds / 10);
    Console.WriteLine("RunTime " + elapsedTime);
}