Question

Is it possible to specify some kind of flag or modifier on a string format param to make it lower case or upper case?

Example of what I want:

String.Format("Hi {0:touppercase}, you have {1} {2:tolowercase}.", "John", 6, "Apples");

Wanted output:

Hi JOHN, you have 6 apples.

PS: Yes I know that I can change the case of the param before using it in the string format, but I don’t want this.

Answer 1

There’s only padding and allignment formating… So the easy way is like you said, use "John".ToUpper() or "John".ToLower().

Another solution could be create a custom IFormatProvider, to provide the string format you want.

This is how will look the IFormatProvider and the string.Format call.

public class CustomStringFormat : IFormatProvider, ICustomFormatter
{
    public object GetFormat(Type formatType)
    {
        if (formatType == typeof(ICustomFormatter))
            return this;
        else
            return null;
    }

    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        string result = arg.ToString();
        switch (format?.ToUpperInvariant()) // culture independent
        {
            case "U": return result.ToUpper();
            case "L": return result.ToLower();
            //more custom formats
            default: return result;
        }
    }
}

And the call will look like:

String.Format(new CustomStringFormat(), "Hi {0:U}", "John");