Contains

This C# method scans a List. It searches for a specific element to see if that element occurs at least once in the collection.

Contains is a useful method that declaratively searches. To use Contains, no for-loop is required. You can type (and maintain) less code.

Contains example

We add System.Collections.Generic at the top. The example in the program shows the Contains extension being used on the List type with a case-insensitive search.

using System;
using System.Collections.Generic;
using System.Linq;

// Part 1: create List with three elements.
var list = new List<string>();
list.Add("cat");
list.Add("dog");
list.Add("moth");

// Part 2: search for this element.
if (list.Contains("dog"))
{
    Console.WriteLine("dog was found");
}

// Part 3: search for this element in any string case.
// ... This is the LINQ method with the same name.
if (list.Contains("MOTH", StringComparer.OrdinalIgnoreCase))
{
    Console.WriteLine("MOTH was found (insensitive)");
}

// Part 4: this element is not found.
Console.WriteLine(list.Contains("fish"));
dog was found
MOTH was found (insensitive)
False

Benchmark

We benchmark the performance of List Contains and a custom for-loop on a small List. Only a few elements are searched by each method.

using System;
using System.Collections.Generic;
using System.Diagnostics;

class Program
{
    /// <summary>
    /// Custom implementation.
    /// </summary>
    static bool ContainsLoop(List<string> list, string value)
    {
        for (int i = 0; i < list.Count; i++)
        {
            if (list[i] == value)
            {
                return true;
            }
        }
        return false;
    }

    const int _max = 100000000;
    static void Main()
    {
        var list = new List<string>();
        list.Add("one");
        list.Add("two");
        list.Add("three");
        list.Add("four");
        list.Add("five");

        var s1 = Stopwatch.StartNew();
        // Version 1: use loop to search a list.
        for (int i = 0; i < _max; i++)
        {
            bool f = ContainsLoop(list, "four");
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        // Version 2: use Contains method.
        for (int i = 0; i < _max; i++)
        {
            bool f = list.Contains("four");
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }
}
19.22 ns For-loop, string comparisons [ContainsLoop]
54.60 ns Contains method              [Contains]

Discussion

System.Linq is included by default in new Console applications in Visual Studio. It will allow you to call a separate Contains generic method on all IEnumerable types.

Internals

Contains performs a linear search through the elements, starting with the first element, and uses Equals to check each element value.

We used the Contains method on List. We saw how this method differs from the LINQ Contains method. The methods are implemented with a forward linear search.