C# string format flag or modifier to lowercase param

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");