i = i++ doesn’t increment i. Why? [duplicate]

The behavior is well defined in C# and the evaluation order is:

1. Left side i is evaluated to the variable i
2. Right side is evaluated to 0, and i is incremented (now i==1)
3. The assignment is executed, it sets i to 0. (now i==0)

The end result is i==0.

In general you first create an expression tree. To evaluate it you evaluate first the left side, then the right side and finally the operation at the root. Do that recursively.