Interesting interview exercise result: return, post increment and ref behavior [duplicate]

i += Increment(ref i); is equivalent to

i = i + Increment(ref i);

The expression on the right hand side of the assignment is evaluated from left to right, so the next step is

i = 0 + Increment(ref i);

return i++ returns the current value of i (which is 0), then increments i

i = 0 + 0;

Before the assignment the value of i is 1 (incremented in the Increment method), but the assignment makes it 0 again.