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.
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.