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.
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 is less than zero or greater than or equal to the length of the current instance.
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 the first character position has been examined. For example, if startIndex is string.Length - 1, the method searches every character from the last character in the string to the beginning.
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) method always returns startIndex, which is the character position at which the search begins. In the following example, the string.LastIndexOf(string, int) method is used to find a substring that includes a soft hyphen (U+00AD) and that precedes or includes the final "m" in a string. Because the soft hyphen in the search string is ignored, calling the method to find a substring that consists of the soft hyphen and "m" returns the position of the "m" in the string, whereas calling it to find a substring that consists of the soft hyphen and "n" returns the position of the "n". When the search string contains only the soft hyphen, the method returns the index of the "m", which represents the value of startIndex.
code reference: System.String.LastIndexOf#22