C# Post Increment

Question

While I am testing post increment operator in a simple console application, I realized that I did not understand full concept. It seems weird to me:

int i = 0;
bool b = i++ == i;
Console.WriteLine(b);

The output has been false. I have expected that it would be true. AFAIK, at line 2, because of the post increment, compiler does comparison and assigned b to true, after i incremented by one. But obviously I am wrong. After that I modify the code like that:

int i = 0;
bool b = i == i++;
Console.WriteLine(b);

This time output has been true. What did change from first sample?

Answer 1

Suppose i has the value 0 initially, as it does in your examples.

i++ == i reads i (0), increments i, reads i again (1), and compares the two values: 0 == 1.

i == i++ reads i (0), reads i again (0), increments i, and compares the two values: 0 == 0.

The increment happens immediately after reading the old value.

Answer 2

Answering to your first snippet of code:

Here, bool b = i++ == i;is 0 == 1and this is because as you know i++ is a post increment so i remains 0 at i++ but after that part is finished executing and it is being compared to the right hand side which is i , by this time the value has change to 1 due to that previous post increment. This is why you are getting False when doing : bool b = i++ == i;.

Like @hvd said: The increment happens immediately after reading the old value.

Answer 3

The order of evaluation of the postfix and the equality operator is from left to right, so the code behaves as explained in the code comments.

int i = 0;
bool b = i++ == i;
// 1.) i++ returns old value i.e. 0
// 2.) but after that it increments and becomes 1
// 3.) hence, bool b = 0 == 1; --> evaluates to false
Console.WriteLine(b); // prints false

int i = 0;
bool b = i == i++;
// 1.) i returns old value i.e. 0
// 2.) i++ returns old value i.e. 0, as this is the end of the statement after that it would increment
// 3.) hence, bool b = 0 == 0; --> evaluates to true
Console.WriteLine(b); // prints true