The index of the first character the returned font controls.
The number of characters after the start index controlled by the returned font.
A FormattedStringFont instance which controls the formatting of a portion of the string.
is less than zero.
is less than one. A zero length string cannot be controlled by a formatting font.
If the start index is greater than or equal to the length of the unformatted string, no exception will be thrown. It will be thrown later when one of the formatting properties of the returned FormattedStringFont is set.