Try this code:
double lower_value = -8.3;
double upper_value = 22.8;
int step = 5;
int low = (int)lower_value / step;
int up = (int)upper_value / step;
var tt = Enumerable.Range(low, up - low + 1).Select(i => i * step);
EDIT
This code is intended for all negative values of the lower_value
and for positive values which are divisible by the step
. To make it work for all other positive values as well, the following correction should be applied:
if (lower_value > step * low)
low++;