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List<T>.Contains Method

.NET Framework 4

Determines whether an element is in the List<T>.

Namespace:  System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)
C#
C++
F#
VB
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public bool Contains(
	T item
)

Parameters

item
Type: T
The object to locate in the List<T>. The value can be null for reference types.

Return Value

Type: System.Boolean
true if item is found in the List<T>; otherwise, false.

Implements

ICollection<T>.Contains(T)

This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable<T>.Equals method for T (the type of values in the list).

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

The following example contains a list of complex objects of type Box. The Box class implements the IEquatable<T>.Equals method so that two boxes are considered equal if their dimensions are the same. In this example, the Contains method returns true, because a box that has the specified dimensions is already in the collection.

C#
VB
Copy 

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        List<Box> boxes = new List<Box>();

        boxes.Add(new Box(8, 8, 4));
        boxes.Add(new Box(8, 4, 8));
        boxes.Add(new Box(8, 6, 4));

        if (boxes.Contains(new Box(8, 6, 4)))
        {
            Console.WriteLine("An equal box is already in the collection.");
        }
        else
        {
            Console.WriteLine("Box can be added.");
        }
    }
}

public class Box : IEquatable<Box>
{

    public Box(int h, int l, int w)
    {
        this.Height = h;
        this.Length = l;
        this.Width = w;
    }
    public int Height { get; set; }
    public int Length { get; set; }
    public int Width { get; set; }

    public bool Equals(Box other)
    {
        if (this.Height == other.Height && this.Length == other.Length
            && this.Width == other.Width)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
}

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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Annotations FAQ
Exact Match Curiosity
This is an  exact match.  e.g.
 
String[] Names = { "Andrew Black", "Bob Brown", "Pete Green" };
List<string> Contacts = new List<string>();
Contacts.AddRange(Names);
MessageBox.Show(Contacts.Contains("Bob Brown").ToString());        // True
MessageBox.Show(Contacts.Contains("Bob").ToString());                   // False
 
but MessageBox.Show(Contacts.Find(itm => itm.Contains("Bob"));   // Bob Brown.
 
I find this confusing.  Am I missing something?  Or is this just a finer point with lambdas?
History
Poor perfomance for looking null in List<Nullable<T>>.

IndexOf is much faster then Contains:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
internal static class Test{
    private static void Main(){
        List<int?> a=Enumerable.Repeat((int?)0,1000000).ToList();
        Stopwatch sw=new Stopwatch();
        for(int i=0;i<10;++i){
            sw.Restart();
            a.Contains(null);
            sw.Stop();
            Console.WriteLine(sw.ElapsedTicks);
            sw.Restart();
            a.Contains(1);
            sw.Stop();
            Console.WriteLine(sw.ElapsedTicks);
            sw.Restart();
            a.IndexOf(null);
            sw.Stop();
            Console.WriteLine(sw.ElapsedTicks);
            sw.Restart();
            a.IndexOf(1);
            sw.Stop();
            Console.WriteLine(sw.ElapsedTicks);
            Console.WriteLine();
        }
        Console.ReadKey(true);
    }
}
History