Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.
The zero-based starting index position of value if that string is found, or -1 if it is not found or if the current instance equals string.Empty. If value is string.Empty, the return value is the smaller of startIndex and the last index position in this instance.
Type Reason ArgumentNullException value is a null reference. ArgumentOutOfRangeException
startIndex or count is less than zero.
startIndex - count is smaller than -1.
Index numbering starts from zero. That is, the first character in the string is at index zero and the last is at string.Length - 1.
The search begins at the startIndex character position of this instance and proceeds backward toward the beginning until either value is found or count character positions have been examined. For example, if startIndex is string.Length - 1, the method searches backward count characters from the last character in the string.
This method performs a word (case-sensitive and culture-sensitive) search using the current culture.
Character sets include ignorable characters, which are characters that are not considered when performing a linguistic or culture-sensitive comparison. In a culture-sensitive search, if value contains an ignorable character, the result is equivalent to searching with that character removed. If value consists only of one or more ignorable characters, the string.LastIndexOf(string, int, int) method always returns startIndex, which is the character position at which the search begins. In the following example, the string.LastIndexOf(string, int, int) method is used to find the position of a soft hyphen (U+00AD) in the two characters that precede the final "m" of two strings. Only one of the strings contains the required substring. In both cases, because the soft hyphen is an ignorable character, the method returns the index of "m" in the string when it performs a culture-sensitive comparison. Note that in the case of the first string, which includes the soft hyphen followed by an "m", the method fails to return the index of the soft hyphen but instead returns the index of the "m".
code reference: System.String.LastIndexOf#23