I try to validate that an input string is a float. To achieve this I pass the input string like "1.000e-45" which is closer to 0 than the c# float minimum allows (1.401298E-45). I expect float.tryParse to return false. Instead it returns true and the parsed number is rounded up to the float minimum (1.401298E-45). If the string contains an even lower number, float.TryParse() seems to round down to 0.
Do I miss something?
What could I do to validate that low numbers like this are not interpreted as float?
var isFloat = float.TryParse("1.000e-45", NumberStyles.Float, CultureInfo.InvariantCulture, out var number);
Update: the user input MUST be a float in a string. it is not allowed to be of higher precision because of a specification. Just imagine it as some kind of API which gives me a string which should be a float value. I have to validate that it is in the ranges of a float.
Update 2: this is not a question about float precision. I know that you cannot have 100% precision because of the binary aspect and the size of the mantisse. As i stated in the question, i know the most close value to 0 for a float. I just didnt expect TryParse() to do rounding like this when its close to 0. I expected it to fail.
Update 3: I hope this makes it more clear. That is the specification that i have to follow.
- The input is a string. the string represents a number. like "3.45e12"
- the number is not allowed to be outside of the range of "±3.4028235e+38".
- the number is not allowed to be smaller in magnitute than "±1.401298e-45". That is what the specification says."0" is allowed too, but not something like "1e-100" or "1e-46".
- What i want to do is validate that the user input (a string) fullfills this specification. I dont have to convert the string number into a c# number type and calculate with it. I just have to validate the input string number and then i send the string number to an API. This api would have problems when the string does not match the specification.
- I tried to use float.TryParse() for this validation. It works for the min/max ranges (.Net Framework). But it seems to be the wrong approach. float.TryParse() will always return true for number strings that are closer to 0 than it can actually represent and it will round the result to fit into a float: it rounds down to 0 (e.g. for "1e-46") or rounds up to 1.401298E-45 (e.g. for "1e-45". It makes sense to some point, because it does the same when it can not handle the precision of a value like "999999999999999.9" which is not above max and not too close to 0. It rounds it to 1e13.