meet it

Threads that share a total need a .

Task.Run(() => ...) hands work to a from a shared pool: a separate worker running at the same time as your code. Four tills, four threads, real parallel work. And a new danger: total += 5 is three steps: read, add and write back.

Two threads can read the same old total, both add 5, and both write: one sale is lost. That is a , and the answer changes from run to run. Wrap the update in lock (gate) { ... }. Only one thread at a time may hold the lock on gate. The others wait.

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

var gate = new object();
int total = 0;
var tills = Enumerable.Range(1, 4).Select(till => Task.Run(() =>
{
    for (int sale = 0; sale < 10000; sale++)
    {
        lock (gate)
        {
            total += 5;
        }
    }
}));
await Task.WhenAll(tills);
Console.WriteLine($"Takings: {total}");

Without the lock, the takings would come out short, by a different amount each run.

guess first

Each task has its own slot. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

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

int[] results = new int[3];
var work = Enumerable.Range(0, 3).Select(i => Task.Run(() =>
{
    results[i] = (i + 1) * 100;
}));
await Task.WhenAll(work);
Console.WriteLine(string.Join(" ", results));

your turn

Your turn. Count the visitors safely.

On line 11, lock on the shared gate object, so only one turnstile at a time can add a visitor.

Your code

output

press run to see what happens

fix it

The savings jar will not compile. Can you fix it?

Broken code to fix

output

press run to see what happens

C# says int is not a reference type, as the lock statement needs, on line 10.
A lock needs one object that every thread shares. A number gets copied, so it cannot be one.
Add var gate = new object(); above line 5, change lock (pence) to lock (gate), then run.

nailed it

your threads can share safely now. :D

Task.Run hands work to another thread.
total += 5 is read, add, write, and threads can interleave.
lock on one shared object lets one thread in at a time.

new words in your collection

thread
race condition
lock

This lesson isn’t saved. A free account remembers every lesson you finish.

Threads that share a total need a lock.

Each task has its own slot. What will this show?

Your turn. Count the visitors safely.

The savings jar will not compile. Can you fix it?

your threads can share safely now. :D

Threads that share a total need a .