How to convert an object to a string with a format string? [duplicate]

Question

I have an object which may be boxed from int, float, decimal, DateTime, TimeSpan etc. Now I want to convert it to a string with a format string, such as "D4", "P2", or "yyyyMMdd". But the object class doesn't have a ToString method which receives the format string. So the following code doesn't work.

string Format(object value, string format)
{
    return value.ToString(format); // error
}

Below is my ugly workaround.

string Format(object value, string format)
{
    if (value is int) return ((int)value).ToString(format);
    if (value is float) return ((float)value).ToString(format);
    if (value is double) return ((double)value).ToString(format);
    if (value is decimal) return ((decimal)value).ToString(format);
    if (value is DateTime) return ((DateTime)value).ToString(format);
    if (value is TimeSpan) return ((TimeSpan)value).ToString(format);
    ...
}

Is there a smarter way?

Answer 1

There is an interface for that, it is IFormattable:

public static string Format(object value, string format, IFormatProvider formatProvider = null)
{
    if (value == null)
    {
        return string.Empty;
    }

    IFormattable formattable = value as IFormattable;

    if (formattable != null)
    {
        return formattable.ToString(format, formatProvider);
    }

    throw new ArgumentException("value");
}

(this interface is the one used by the various string.Format, Console.Write, ...)