Task.WhenAll for parallel execution in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Task.WhenAll for parallel execution

Start multiple tasks

↓

Tasks run in parallel

↓

Wait for all tasks to complete

↓

Continue after all tasks finish

Start several tasks at once, let them run at the same time, then wait until all are done before moving on.

Execution Sample

C Sharp (C#)

async Task Example()
{
  var task1 = Task.Delay(1000);
  var task2 = Task.Delay(2000);
  await Task.WhenAll(task1, task2);
  Console.WriteLine("All done");
}

Starts two delay tasks in parallel and waits for both to finish before printing.

Execution Table

Step	Action	Task1 Status	Task2 Status	Output
1	Start task1 and task2	Running	Running
2	Wait for tasks to complete	Running	Running
3	task1 completes after 1s	Completed	Running
4	task2 completes after 2s	Completed	Completed
5	Continue after all tasks done	Completed	Completed	All done

💡 Both tasks completed, so Task.WhenAll finishes and program continues.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
task1	null	Running	Completed	Completed	Completed
task2	null	Running	Running	Completed	Completed

Key Moments - 2 Insights

Why does the program wait before printing "All done"?

Because Task.WhenAll waits until both task1 and task2 are completed, as shown in execution_table steps 2 to 5.

Do task1 and task2 run one after another or at the same time?

They run in parallel, both start immediately at step 1 and run simultaneously until they finish.

Visual Quiz - 3 Questions

Look at the execution table, what is the status of task1 at step 3?

A Running

B Completed

C Not started

D Faulted

💡 Hint

Check the 'Task1 Status' column at step 3 in the execution_table.

At which step do both tasks complete?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for when both 'Task1 Status' and 'Task2 Status' show 'Completed' in the execution_table.

If task2 took only 500ms instead of 2000ms, when would Task.WhenAll complete?

A After 2 seconds

B After 500ms

C After 1 second

D Immediately

💡 Hint

Task.WhenAll waits for all tasks; the longest task duration controls completion time.

Concept Snapshot

Task.WhenAll runs multiple tasks at the same time.
It waits until all tasks finish before continuing.
Use await Task.WhenAll(task1, task2, ...).
This helps do work in parallel efficiently.
The program pauses at await until all tasks complete.

Full Transcript

This example shows how Task.WhenAll runs tasks in parallel and waits for all to finish. First, two tasks start running at the same time. Task1 finishes after 1 second, task2 after 2 seconds. The program waits until both are done, then prints "All done". This lets you do multiple things at once and continue only when all are ready.