A Unicode character array whose elements are the length number of characters in this instance starting from character position startIndex.
Type Reason ArgumentOutOfRangeException startIndex or length is less than zero.
-or-
startIndex plus length is greater than the length of the current instance.
The startIndex parameter is zero-based. That is, the index of the first character in the string instance is zero.
If length is zero, the returned array is empty and has a zero length. If this instance is null or an empty string (""), the returned array is empty and has a zero length.