Search for an element matching the conditions and return the zero-based index of the last occurrence within the entire List in C#

The FindLastIndex() method in C# searches for an element that matches the conditions defined by a specified predicate and returns the zero-based index of the last occurrence within the entire List<T>. This method is particularly useful when you need to find the rightmost element that satisfies certain criteria.

Syntax

Following is the syntax for the FindLastIndex() method −

public int FindLastIndex(Predicate<T> match)

Parameters

• match − The Predicate<T> delegate that defines the conditions of the element to search for.

Return Value

Returns the zero-based index of the last occurrence of an element that matches the conditions. If no element is found, it returns -1.

Using FindLastIndex() with Divisibility Condition

This example demonstrates finding the last index of an element divisible by 10 −

using System;
using System.Collections.Generic;

public class Demo {
   private static bool IsMultipleOfTen(int i) {
      return ((i % 10) == 0);
   }
   
   public static void Main(String[] args) {
      List<int> list = new List<int>();
      list.Add(200);
      list.Add(215);
      list.Add(310);
      list.Add(500);
      list.Add(600);
      
      Console.WriteLine("List elements...");
      foreach (int i in list) {
         Console.WriteLine(i);
      }
      
      Console.WriteLine("Last Index of the satisfying condition = " + list.FindLastIndex(IsMultipleOfTen));
   }
}

The output of the above code is −

List elements...
200
215
310
500
600
Last Index of the satisfying condition = 4

Using FindLastIndex() with Even Number Condition

This example finds the last index of an even number in the list −

using System;
using System.Collections.Generic;

public class Demo {
   private static bool IsEven(int i) {
      return ((i % 2) == 0);
   }
   
   public static void Main(String[] args) {
      List<int> list = new List<int>();
      list.Add(200);
      list.Add(215);
      list.Add(310);
      list.Add(500);
      list.Add(655);
      
      Console.WriteLine("List elements...");
      foreach (int i in list) {
         Console.WriteLine(i);
      }
      
      Console.WriteLine("Last Index of the satisfying condition = " + list.FindLastIndex(IsEven));
   }
}

The output of the above code is −

List elements...
200
215
310
500
655
Last Index of the satisfying condition = 3

Using Lambda Expression with FindLastIndex()

You can also use lambda expressions for more concise code −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(String[] args) {
      List<int> numbers = new List<int> { 15, 30, 45, 60, 75, 90, 105 };
      
      Console.WriteLine("List elements...");
      foreach (int num in numbers) {
         Console.WriteLine(num);
      }
      
      int lastEvenIndex = numbers.FindLastIndex(x => x % 2 == 0);
      int lastDivisibleBy15Index = numbers.FindLastIndex(x => x % 15 == 0);
      
      Console.WriteLine("Last even number index: " + lastEvenIndex);
      Console.WriteLine("Last number divisible by 15 index: " + lastDivisibleBy15Index);
   }
}

The output of the above code is −

List elements...
15
30
45
60
75
90
105
Last even number index: 5
Last number divisible by 15 index: 6

Conclusion

The FindLastIndex() method provides an efficient way to locate the last occurrence of an element matching specific conditions in a List<T>. It returns -1 when no matching element is found, making it essential to handle this case in production code.