Why does the Interlocked.Add() method have to return a value?

Question

public static int Add(ref int location1,int value)

I was trying to use the Interlocked.Add(ref int location1,int value) method to add to a number in an atomic manner in multi-threading scenario. But I got a question to myself: why does the method return the location1 value again? Instead we could directly use the variable which is passed as "ref".

Some pseudo code below:

int a = 6;
int b = 7;

// some thing else

Interlocked.Add(ref a, b);

// Use the variable 'a' here.

Answer 1

Because the variable ref a could change "again" before Interlocked returns (or even after it returns and before you use a). The function instead returns the value it calculated.

Example:

int a = 5;

// on thread 1
int b = Interlocked.Add(ref a, 5); // b = 10

// on thread 2, at the same time
int c = Interlocked.Add(ref a, 5); // c = 15

// on thread 1
Thread.Sleep(1000); // so we are "sure" thread 2 executed 
Thread.MemoryBarrier(); // just to be sure we are really reading a
bool x1 = (b == 10); // true
bool x2 = (a == 15); // true