Explain the difference between `Task.WhenAll()` and `Task.WhenAny()`

Task.WhenAll() waits for all tasks in a collection to complete before continuing. It returns a task that completes when all input tasks have completed.

Key characteristics of Task.WhenAll():

• Waits for ALL tasks to finish
• Returns an array of results (if tasks return values)
• If any task throws an exception, the returned task will be faulted
• All exceptions are aggregated in an AggregateException
• Useful for parallel execution where you need all results

// Example: Download multiple files concurrently
var task1 = DownloadFileAsync("file1.txt");
var task2 = DownloadFileAsync("file2.txt");
var task3 = DownloadFileAsync("file3.txt");

// Wait for all downloads to complete
await Task.WhenAll(task1, task2, task3);
Console.WriteLine("All files downloaded");

Task.WhenAny() returns as soon as ANY one of the tasks completes. It returns a task that represents the first completed task.

Key characteristics of Task.WhenAny():

• Returns when the FIRST task completes
• Returns the completed task itself (not the result)
• Useful for timeout scenarios or racing multiple operations
• Other tasks continue running in the background

// Example: Implement timeout pattern
var dataTask = FetchDataAsync();
var timeoutTask = Task.Delay(TimeSpan.FromSeconds(5));

var completedTask = await Task.WhenAny(dataTask, timeoutTask);

if (completedTask == timeoutTask)
{
    throw new TimeoutException("Operation timed out");
}

var result = await dataTask;