To reach your objective you need to change the #
character with a 0
0 means, place a number in this position also if it is a zero value
doubleVariable.ToString("0.00", cultureInfo);
You can read about Custom Numeric Format here
"0" - Zero placeholder Replaces the zero with the corresponding digit if one is present; otherwise, zero appears in the result string.
Instead for the #
character you could read
"#" - Digit placeholder Replaces the "#" symbol with the corresponding digit if one is present; otherwise, no digit appears in the result string.