using System;
using System.Threading;
using System.Threading.Tasks;

using CancellationTokenSource cancellationTokenSource = new();

Console.CancelKeyPress += (_, e) => {
    cancellationTokenSource.Cancel();
    e.Cancel = true;
};

try
{
    Console.WriteLine("Press Ctl-C to terminate.");
    await VeryLongTask(cancellationTokenSource.Token);
}
catch (OperationCanceledException)
{
    Console.WriteLine();
    Console.WriteLine("Boo! Operation cancelled!");
}

async Task VeryLongTask(CancellationToken cancellationToken)
{
    Console.WriteLine("Starting a very long operation.");

    while (true)
    {
        cancellationToken.ThrowIfCancellationRequested();
        Console.Write(".");
        await Task.Delay(TimeSpan.FromSeconds(1));
    }
}

using System;
using System.Threading;
using System.Threading.Tasks;

using CancellationTokenSource cancellationTokenSource = new();
Console.WriteLine("Press any key to terminate.");

// "Start" tasks
var readAnyKey = ReadAnyKey();
var longOperation = VeryLongTask(cancellationTokenSource.Token);

// Wait for any task to finish
var finished = await Task.WhenAny(readAnyKey, longOperation);

// A key's been pressed
if (finished == readAnyKey)
{
    // causes cancelling the other task
    cancellationTokenSource.Cancel();

    // just wait for another task to be cancelled
    try
    {
        await longOperation;
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine();
        Console.WriteLine("Boo! Operation cancelled!");
    }
}

// This task is completed as soon as any key is pressed
Task ReadAnyKey() => Task.Run(() => Console.ReadKey(true));

// Some long operation
async Task VeryLongTask(CancellationToken cancellationToken)
{
    Console.WriteLine("Starting a very long operation.");

    while (true)
    {
        cancellationToken.ThrowIfCancellationRequested();
        Console.Write(".");
        await Task.Delay(TimeSpan.FromSeconds(1), cancellationToken);
    }
}