C# Online Compiler | .NET Fiddle

using System;
using System.Threading;
using System.Threading.Tasks;	
using System.Collections;
using System.Collections.Generic;
using System.Collections.Concurrent;
					
public class Program
{
	public static async Task Main()
	{
		OnCancelThrowException();
		OnCancelReturn();
		await CancelTaskAndChildren();
	}
	
	private static void OnCancelThrowException()
	{
		var tokenSource = new CancellationTokenSource();
		var token = tokenSource.Token;
		
		var task = Task.Run(() => {
			token.ThrowIfCancellationRequested();
			
			while(true) {
				// if no clean up needed
				token.ThrowIfCancellationRequested();
				
				/*
				if clean up needed
				if(token.IsCancellationRequested)
				{
					// cleanup code	
					token.ThrowIfCancellationRequested();
				}
				*/
			}
		}, token);
		
		tokenSource.Cancel();
		
		try 
		{
			task.Wait();
		}
		catch(AggregateException e) 
		{
			Console.WriteLine("Task Status: {0}, Error Messaage: {1}", task.Status, e.InnerException.Message);
		}
		finally 
		{
			tokenSource.Dispose();
		}
	}
	
	private static void OnCancelReturn() {
		var tasks = new List<Task<int>>();
		var tokenSource = new CancellationTokenSource();
		var token = tokenSource.Token;
		int completedIterations = 0;
		
		for(int i=0; i<10; i++) {
			tasks.Add(Task.Run(() => {
				int x=0;
				for(x =0; x<200000; x++)
				{
					if(token.IsCancellationRequested)
						return x;
				}
				Interlocked.Increment(ref completedIterations);
				
				if(completedIterations > 5 && !tokenSource.IsCancellationRequested) {
					tokenSource.Cancel();
				}
				return x;
			}, token));
		}
		
		try
		{
			Task.WaitAll(tasks.ToArray());
		}
		catch(AggregateException)
		{
			Console.WriteLine("{0, 10} {1,20} {2, 14}", "Task Id", "Status", "Iterations");
			foreach(var t in tasks)
			{
				Console.WriteLine("{0, 10} {1,20} {2, 14:N0}", t.Id, t.Status, t.Status != TaskStatus.Canceled ? t.Result.ToString(): "Not Started");
			}
		}
	}
	
	private static async Task CancelTaskAndChildren() 
	{
		var tasks = new ConcurrentBag<Task>();
		var tokenSource = new CancellationTokenSource();
		var token = tokenSource.Token;
		Task t1 = Task.Run(() => DoSomeWork(1, token), token);
		Console.WriteLine($"Task {t1.Id} executing");
		tasks.Add(t1);
		
		Task t2 = Task.Run(() => {
			Task t;
			for(int i=3; i<=10; i++) {
				t = Task.Factory.StartNew((obj) => DoSomeWork((int)obj, token), i, token);
				Console.WriteLine($"Task {t.Id} executing");
				tasks.Add(t);
			}
			DoSomeWork(2, token);
		}, token);
		Console.WriteLine($"Task {t2.Id} executing");
		tasks.Add(t2);
		
		Task.Delay(100).Wait();
		tokenSource.Cancel();
		Console.WriteLine($"Tasks cancellation requested");
		
		try
		{
			await Task.WhenAll(tasks.ToArray());
		}
		catch(OperationCanceledException e) 
		{
			Console.WriteLine(nameof(OperationCanceledException));
		}
		finally 
		{
			tokenSource.Dispose();
		}
		
		foreach(var task in tasks)
			Console.WriteLine($"Task {task.Id} {task.Status}");
	}
	
	private static void DoSomeWork(int taskNum, CancellationToken ct) 
	{
		if(ct.IsCancellationRequested) 
		{
			Console.WriteLine($"Task {taskNum} was cancelled before it got started");
			ct.ThrowIfCancellationRequested();
		}
		
		var sw = new SpinWait();
		for(int i=0; i<100; i++)
		{
			sw.SpinOnce();
			if(ct.IsCancellationRequested) 
			{
				Console.WriteLine($"Task {taskNum} cancelled");
				ct.ThrowIfCancellationRequested();
			}
		}
		Console.WriteLine($"Task {taskNum} completed");
	}
}

                Console.WriteLine("{0, 10} {1,20} {2, 14:N0}", t.Id, t.Status, t.Status != TaskStatus.Canceled ? t.Result.ToString(): "Not Started");
 
using System;
using System.Threading;
using System.Threading.Tasks;   
using System.Collections;
using System.Collections.Generic;
using System.Collections.Concurrent;
                    
public class Program
{
    public static async Task Main()
    {
        OnCancelThrowException();
        OnCancelReturn();
        await CancelTaskAndChildren();
    }
    
    private static void OnCancelThrowException()
    {
        var tokenSource = new CancellationTokenSource();
        var token = tokenSource.Token;
        
        var task = Task.Run(() => {
            token.ThrowIfCancellationRequested();
            