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.This method begins searching at the startIndex character position of this instance and proceeds backward toward the beginning of the current instance 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. The search is case-sensitive.

This method performs an ordinal (culture-insensitive) search, where a character is considered equivalent to another character only if their Unicode scalar values are the same. To perform a culture-sensitive search, use the System.Globalization.CompareInfo.LastIndexOf(string, char) method, where a Unicode scalar value representing a precomposed character, such as the ligature "Æ" (U+00C6), might be considered equivalent to any occurrence of the character's components in the correct sequence, such as "AE" (U+0041, U+0045), depending on the culture.

The following example demonstrates the string.LastIndexOf(char) method.

using System;
public class StringLastIndexOfTest {
   public static void Main() {
      String str = "aa bb cc dd";
      
      Console.WriteLine( str.LastIndexOf('d', 8) );
      Console.WriteLine( str.LastIndexOf('b', 8) );
   }
}

The output is