Addition with preincrement losing 1

Question

Why does TestAddition result in 12 instead o 13? It should be 5 + 1 + 7 = 13, but assert fails with

Expected: 13

But was: 12

int method(int a)
{
    return 7;
}

[Test]
public void TestAddition()
{
    int row = 5;
    row += method(++row);

    Assert.AreEqual(13, row, "Why is it twelve instead of 13?");
}

Answer 1

Because your

row += method(++row);

equals to

row = row + method(++row);

Since + operator is left associative, it calculates first row as a 5 and method always return 7 no matter which parameter it takes.

row = row + method(++row);
       ^          ^
       5          7

That's why result will be 12.

Answer 2

row += method(++row);

is the same as

row = row + method(++row);

The operands are evaluated from left to right, so the value of the left operand (row) is evaluated before row is incremented in method(++row).

The expected result is 12.