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List<T>.Contains Method (T)

 

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

Namespace:   System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

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.

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 demonstrates the Contains and Existsmethods on a List<T> that contains a simple business object that implements Equals.

using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify a part 
// but the part name can change. 
public class Part : IEquatable<Part>
{
    public string PartName { get; set; }
    public int PartId { get; set; }

    public override string ToString()
    {
        return "ID: " + PartId + "   Name: " + PartName;
    }
    public override bool Equals(object obj)
    {
        if (obj == null) return false;
        Part objAsPart = obj as Part;
        if (objAsPart == null) return false;
        else return Equals(objAsPart);
    }
    public override int GetHashCode()
    {
        return PartId;
    }
    public bool Equals(Part other)
    {
        if (other == null) return false;
        return (this.PartId.Equals(other.PartId));
    }
    // Should also override == and != operators.
}
public class Example
{
    public static void Main()
    {
        // Create a list of parts.
        List<Part> parts = new List<Part>();

        // Add parts to the list.
        parts.Add(new Part() { PartName = "crank arm", PartId = 1234 });
        parts.Add(new Part() { PartName = "chain ring", PartId = 1334 });
        parts.Add(new Part() { PartName = "regular seat", PartId = 1434 });
        parts.Add(new Part() { PartName = "banana seat", PartId = 1444 });
        parts.Add(new Part() { PartName = "cassette", PartId = 1534 });
        parts.Add(new Part() { PartName = "shift lever", PartId = 1634 }); ;

        // Write out the parts in the list. This will call the overridden ToString method
        // in the Part class.
        Console.WriteLine();
        foreach (Part aPart in parts)
        {
            Console.WriteLine(aPart);
        }


        // Check the list for part #1734. This calls the IEquitable.Equals method
        // of the Part class, which checks the PartId for equality.
        Console.WriteLine("\nContains: Part with Id=1734: {0}",
            parts.Contains(new Part { PartId = 1734, PartName = "" }));

        // Find items where name contains "seat".
        Console.WriteLine("\nFind: Part where name contains \"seat\": {0}", 
            parts.Find(x => x.PartName.Contains("seat")));

        // Check if an item with Id 1444 exists.
        Console.WriteLine("\nExists: Part with Id=1444: {0}", 
            parts.Exists(x => x.PartId == 1444));

        /*This code example produces the following output:

        ID: 1234   Name: crank arm
        ID: 1334   Name: chain ring
        ID: 1434   Name: regular seat
        ID: 1444   Name: banana seat
        ID: 1534   Name: cassette
        ID: 1634   Name: shift lever

        Contains: Part with Id=1734: False

        Find: Part where name contains "seat": ID: 1434   Name: regular seat

        Exists: Part with Id=1444: True 
         */
    } 
}

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

using System;
using System.Collections.Generic;

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

        cubes.Add(new Cube(8, 8, 4));
        cubes.Add(new Cube(8, 4, 8));
        cubes.Add(new Cube(8, 6, 4));

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

        //Outputs "An equal cube is already in the collection."
    }
}

public class Cube : IEquatable<Cube>
{

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

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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