c# - DateTime difference in ticks does not match

Question

I have the following code:

var a = (DateTime.Now.Subtract(DateTime.Now.AddTicks(8))).Ticks;
var b = (DateTime.Now - DateTime.Now.AddTicks(8)).Ticks;

When I check the values I see that:

a = -78
b = -20

How come? Shouldn't both be -8?

Answer 1

You are depending on the system to do everything at the same time, which it cannot. Each time you get DateTime.Now, it has a different value.

A quick experiment reveals that capturing the value of DateTime.Now in the beginning, and then performing operations on that:

var d = DateTime.Now;
var a = (d.Subtract(d.AddTicks(8))).Ticks;
var b = (d - d.AddTicks(8)).Ticks;

Yields the result you were expecting. a and b have the same value, -8.