formatting decimals using string format

Question

public static string PadZero(this double number, int decimalPlaces)
    {
        var requiredFormat = "0." + "".PadRight(decimalPlaces, '0');
        var something = $"{number:requiredFormat}";

        return number.IsNotZero() ? something: string.Empty;
    }

This is a helper function to pad zeros to a double number, user can pass the number of zeros that is required to be padded through decimalPlaces. Above function fails my unit tests, output received is {requiredFormat} in all test cases. I have just replaced: var something = $"{number:0.00}"; with a generic variable requiredFormat that can handle any number of zero padding.

Answer 1

There are two problems with your example. The first is that the value of something is not going to produce a string that can be used to format a number. The second is that you are not using something to perform a number format by using string.format.

So first off, the statement:

var something = $"{number:requiredFormat}";

is not going to give you the result that you want, which would be a string that looks something like:

{0:0.0000}

Try changing the code to read:

var something = $"{{0:{requiredFormat}}}";

If you do Console.WriteLine(something) after that statement executes you can inspect the value of something to make sure it is what you are looking for.

After that, change this line:

return number.IsNotZero() ? something: string.Empty;

to read:

return number.IsNotZero() ? string.Format(something, number) : string.Empty;

Even with Interpolated Strings, you have to build the variable format and apply it in two separate steps.

Hope that helps.