System.FormatException error when multithreading - multithreading

Question

I need to create two threads for a little F# project that implements locking to avoid a race condition. (I have scanned StackOverflow for answers to no avail hence this post)
Here is the code:
let bookFood _ =
Console.WriteLine("Enter amount of grocery bags: ")
let groceryBags = int(Console.ReadLine())
Console.WriteLine("Enter customer name: ")
let name = string(Console.ReadLine().ToString())
let book bags name orders =
lock(bags,name) (fun()-> orders |> List.map (fun order ->
if order.bags = bags then { order with customer = name }
else order ))
orders <- book groceryBags name orders
ThreadPool.QueueUserWorkItem(WaitCallback(bookFood)) |> ignore
ThreadPool.QueueUserWorkItem(WaitCallback(bookFood)) |> ignore
Thread.Sleep(5000)
The problem that when I run this code the following happens:
I'm prompted to enter amount of grocery bags twice (as should be I suppose?, because I'm invoking the bookFood function twice, right?)
Then I'm prompted to enter customer name, as soon as I enter this the application crashes with the following error: System.FormatException has been thrown, the input string is not in the correct format.
I have searched everywhere for a solution, but I'm reaching out for help now.
Clarification:
How do I solve this problem? Is it because I have coded the locking incorrectly or am I not creating two threads correctly? It works perfect when I only invoke the bookFood method once, but the task requires me to create two threads and use locking to avoid race condition , and that's when I get this error.
A big thank you to the active F# community here that is helping out, you know who you are.

Answer 1

If you're using the debugger, you should find that the point it breaks at is:
let groceryBags = int(Console.ReadLine())
Remember how you're "prompted to enter amount of grocery bags twice" - that's because you're running a call to ReadLine() on two threads.
When you enter the number of bags, the next ReadLine call will be the other book method's "number of bags", and you enter a name there, which can't be converted to an int, so it throws an exception saying it's not in the correct format.
If you had entered only numbers, including for the name, you shouldn't get this error.
Console functions are not thread-safe, and you really shouldn't be using it that way.
Use Int32.TryParse to see if your string does match the format instead of throwing an exception when it doesn't.
The lock needs a shared object between all your threads. You are creating a new tuple object in every lock - which won't do anything. Try locking on a specifically created lock object, just let gate = new Object() will do.