What does ?? mean in C#? [duplicate]

Question

Possible Duplicate:
What do two question marks together mean in C#?

What does the ?? mean in this C# statement?

int availableUnits = unitsInStock ?? 0;

Answer 1

if (unitsInStock != null)
    availableUnits = unitsInStock;
else
    availableUnits = 0;

Answer 2

This is the null coalescing operator. It translates to: availableUnits equals unitsInStock unless unitsInStock equals null, in which case availableUnits equals 0.

It is used to change nullable types into value types.