Why use System.Threading.Interlocked.Decrement instead of minus?

Question

I converted some c# code to vb.net and the converter.telerik.com turned this:

i--;

into this:

System.Math.Max(System.Threading.Interlocked.Decrement(i), i + 1)

Whats up with all the fancy-ness?

Answer 1

Michał Piaskowski's comment triggered the following explanation:

The semantics of i-- in C# are to return the current value of i (i.e., the value before the decrement occurs) and then decrement i by one.

So, we need to convert that to VB. We can not use i -= 1 because this does not return the current value of i before the decrement. So, we need an operation that will decrement i but return the value of i before the decrement, something like:

Function DoPostDecrement(ByRef i As Integer) As Integer
    i -= 1
    Return i + 1
End Function

But this suggests using the following to avoid having to write a method to perform the above:

System.Math.Max(
    someValueThatIsEqualToiMinusOne,
    someValueThatIsEqualtoiBeforeTheDecrement
)

But VB.NET won't let you use i -= 1 or i = i - 1 in place of someValueThatIsEqualToiMinusOne. However, System.Threading.Interlocked.Decrement(i) is legit and equal to the value of i - 1. Once you do that, because parameters are evaluated left to right, someValueThatIsEqualtoiBeforeTheDecrement should be i + 1 (at that point the decrement has been performed to i + 1 is the pre-decrement value.

Note that the above method DoPostDecrement and the System.Math.Max, System.Threading.Interlocked.Decrement construct could have different semantics in a multithreaded context.

Answer 2

The Interlocked operation is atomic; in multithreaded contexts you can safely use it without holding a lock, if you're careful.