How to find the index of an item in a C# list in a single step?

To find the index of an item in a C# list in a single step, you can use several built-in methods. The most common approaches are IndexOf() for exact matches and FindIndex() for condition-based searches.

Syntax

For exact item matching −

int index = list.IndexOf(item);

For condition-based searching −

int index = list.FindIndex(predicate);

Using IndexOf() for Exact Matches

The IndexOf() method returns the zero-based index of the first occurrence of the specified item −

using System;
using System.Collections.Generic;

public class Program {
   public static void Main() {
      List<string> sports = new List<string> {
         "Football",
         "Soccer", 
         "Tennis",
         "Basketball"
      };

      // Find exact match
      int index = sports.IndexOf("Tennis");
      Console.WriteLine("Tennis found at index: " + index);

      // Item not found returns -1
      int notFound = sports.IndexOf("Cricket");
      Console.WriteLine("Cricket found at index: " + notFound);
   }
}

The output of the above code is −

Tennis found at index: 2
Cricket found at index: -1

Using FindIndex() for Conditional Searches

The FindIndex() method uses a predicate to find items based on custom conditions −

using System;
using System.Collections.Generic;

public class Program {
   public static void Main() {
      List<string> sports = new List<string> {
         "American Football",
         "Association Soccer",
         "Lawn Tennis",
         "Street Basketball"
      };

      // Find item containing specific text
      int index1 = sports.FindIndex(s => s.Contains("Tennis"));
      Console.WriteLine("Item containing 'Tennis' found at index: " + index1);

      // Find item starting with specific text
      int index2 = sports.FindIndex(s => s.StartsWith("Street"));
      Console.WriteLine("Item starting with 'Street' found at index: " + index2);

      // Find item with specific length
      int index3 = sports.FindIndex(s => s.Length > 15);
      Console.WriteLine("First item with length > 15 found at index: " + index3);
   }
}

The output of the above code is −

Item containing 'Tennis' found at index: 2
Item starting with 'Street' found at index: 3
First item with length > 15 found at index: 0

Using FindIndex() with Custom Objects

You can also use FindIndex() to search within custom objects −

using System;
using System.Collections.Generic;

public class Player {
   public string Name { get; set; }
   public int Score { get; set; }
}

public class Program {
   public static void Main() {
      List<Player> players = new List<Player> {
         new Player { Name = "Alice", Score = 95 },
         new Player { Name = "Bob", Score = 87 },
         new Player { Name = "Charlie", Score = 92 }
      };

      // Find player by name
      int index1 = players.FindIndex(p => p.Name == "Bob");
      Console.WriteLine("Bob found at index: " + index1);

      // Find player by score condition
      int index2 = players.FindIndex(p => p.Score > 90);
      Console.WriteLine("First player with score > 90 found at index: " + index2);
   }
}

The output of the above code is −

Bob found at index: 1
First player with score > 90 found at index: 0

Comparison

Conclusion

Use IndexOf() for finding exact item matches and FindIndex() for condition-based searches in C# lists. Both methods return -1 when no match is found, making them reliable for single-step index operations.