Question

Assume I have the following code:

let a = ref 4.

printfn "1) a = %g" !a

let t1 = System.Threading.Thread (fun () ->
  lock a (fun () ->
    printfn "locked"
    System.Threading.Thread.Sleep 1000
    printfn "unlocked") )
t1.Start()

System.Threading.Thread.Sleep 100

a := 8.
printfn "2) a = %g" !a

Which gives the following result:

1) a = 4
locked
2) a = 8

val a : float ref = {contents = 8.0;}
val t1 : System.Threading.Thread

unlocked

Why does a equals 8. when I locked it? Is it possible to lock a record with mutable values and refs?

PS: I need to lock an object which is both accessed by me and at the same time by WCF.

Answer 1

I agree with @Dmitry that using lock k (fun () -> ...) doesn’t mean that you prevent mutation on k; it does mean that you acquire a key k to access an object. Because the key is unique, you have mutual exclusive access to the object to avoid getting wrong results.

Based on your example, running this below code in Debug mode results in results of 1, 3 or 6 for a arbitrarily. These values can be explained by the situation when one thread accessing the old value of a and the other thread trying to update that cell.

let a = ref 4;;

printfn "1) a = %i" !a

let t1 = System.Threading.Thread (fun () ->  
    printfn "locked in thread 1"    
    a:= !a + 2
    printfn "unlocked in thread 1"    
    )

let t2 = System.Threading.Thread (fun () ->  
    printfn "locked in thread 2"    
    a:= !a - 3
    printfn "unlocked in thread 2"    
    )

t1.Start()
t2.Start()

System.Threading.Thread.Sleep 1000 // wait long enough to get the correct value
printfn "2) a = %i" !a;;
System.Console.ReadKey() |> ignore;;

To ensure the correct result (which should be 3), you can introduce an object monitor, and any thread which would like to update a has to acquire monitor first:

let monitor = new Object()
let a = ref 4;;

printfn "1) a = %i" !a

let t1 = System.Threading.Thread (fun () ->  
    printfn "locked in thread 1"    
    lock monitor (fun () -> a:= !a + 2)
    printfn "unlocked in thread 1"    
    )

let t2 = System.Threading.Thread (fun () ->  
    printfn "locked in thread 2"    
    lock monitor (fun () -> a:= !a - 3)
    printfn "unlocked in thread 2"    
    )

t1.Start()
t2.Start()

System.Threading.Thread.Sleep 1000 // wait long enough to get the correct value
printfn "2) a = %i" !a;;
System.Console.ReadKey() |> ignore;;