What class actually does the formatting of a composite format spec?

Question

If I specifiy a FormatString property for some databound value of {0:00000}, where I want a number to always have five digits, padded with '0' where necessary, what method on what class is actually invoked to perform the formatting and return the formatted string?

Answer 1

It is the formatted object itself, its class has to implement IFormattable

public interface IFormattable
{
    string ToString(string format, IFormatProvider formatProvider);
}

And so any type, like struct Int32 can do its own formatting. In absence of the interface Format() defaults to the normal ToString().