Enumerable.Contains<TSource> Method (IEnumerable<TSource>, TSource, IEqualityComparer<TSource>)

Determines whether a sequence contains a specified element by using a specified IEqualityComparer<T>.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Contains(Of TSource) ( _
    source As IEnumerable(Of TSource), _
    value As TSource, _
    comparer As IEqualityComparer(Of TSource) _
) As Boolean
public static bool Contains<TSource>(
    this IEnumerable<TSource> source,
    TSource value,
    IEqualityComparer<TSource> comparer
)

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

  • value
    Type: TSource
    The value to locate in the sequence.

Return Value

Type: System.Boolean
true if the source sequence contains an element that has the specified value; otherwise, false.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source is nulla null reference (Nothing in Visual Basic).

Remarks

Enumeration is terminated as soon as a matching element is found.

If comparer is nulla null reference (Nothing in Visual Basic), the default equality comparer, Default, is used to compare elements to the specified value.

Examples

The following example shows how to implement an equality comparer that can be used in the Contains method.

Public Class Product
    Public Property Name As String
    Public Property Code As Integer
End Class

' Custom comparer for the Product class
Public Class ProductComparer
    Implements IEqualityComparer(Of Product)

    Public Function Equals1(
        ByVal x As Product, 
        ByVal y As Product
        ) As Boolean Implements IEqualityComparer(Of Product).Equals

        ' Check whether the compared objects reference the same data.
        If x Is y Then Return True

        'Check whether any of the compared objects is null.
        If x Is Nothing OrElse y Is Nothing Then Return False

        ' Check whether the products' properties are equal.
        Return (x.Code = y.Code) AndAlso (x.Name = y.Name)
    End Function

    Public Function GetHashCode1(
        ByVal product As Product
        ) As Integer Implements IEqualityComparer(Of Product).GetHashCode

        ' Check whether the object is null.
        If product Is Nothing Then Return 0

        ' Get hash code for the Name field if it is not null.
        Dim hashProductName = 
            If(product.Name Is Nothing, 0, product.Name.GetHashCode())

        ' Get hash code for the Code field.
        Dim hashProductCode = product.Code.GetHashCode()

        ' Calculate the hash code for the product.
        Return hashProductName Xor hashProductCode
    End Function
End Class
public class Product
{
    public string Name { get; set; }
    public int Code { get; set; }
}

// Custom comparer for the Product class
class ProductComparer : IEqualityComparer<Product>
{
    // Products are equal if their names and product numbers are equal.
    public bool Equals(Product x, Product y)
    {

        //Check whether the compared objects reference the same data.
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether any of the compared objects is null.
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;

        //Check whether the products' properties are equal.
        return x.Code == y.Code && x.Name == y.Name;
    }

    // If Equals() returns true for a pair of objects 
    // then GetHashCode() must return the same value for these objects.

    public int GetHashCode(Product product)
    {
        //Check whether the object is null
        if (Object.ReferenceEquals(product, null)) return 0;

        //Get hash code for the Name field if it is not null.
        int hashProductName = product.Name == null ? 0 : product.Name.GetHashCode();

        //Get hash code for the Code field.
        int hashProductCode = product.Code.GetHashCode();

        //Calculate the hash code for the product.
        return hashProductName ^ hashProductCode;
    }

}

After you implement this comparer, you can use a sequence of Product objects in the Contains method, as shown in the following example.


Dim fruits() As Product = 
   {New Product With {.Name = "apple", .Code = 9}, 
    New Product With {.Name = "orange", .Code = 4}, 
    New Product With {.Name = "lemon", .Code = 12}}

Dim apple = New Product With {.Name = "apple", .Code = 9}
Dim kiwi = New Product With {.Name = "kiwi", .Code = 8}

Dim prodc As New ProductComparer()

Dim hasApple = fruits.Contains(apple, prodc)
Dim hasKiwi = fruits.Contains(kiwi, prodc)

outputBlock.Text += "Apple? " & hasApple + vbCrLf
outputBlock.Text += "Kiwi? " & hasKiwi + vbCrLf


' This code produces the following output:
'
' Apple? True
' Kiwi? False

        Product[] fruits = { new Product { Name = "apple", Code = 9 }, 
                               new Product { Name = "orange", Code = 4 }, 
                               new Product { Name = "lemon", Code = 12 } };

        Product apple = new Product { Name = "apple", Code = 9 };
        Product kiwi = new Product {Name = "kiwi", Code = 8 };

        ProductComparer prodc = new ProductComparer();

        bool hasApple = fruits.Contains(apple, prodc);
        bool hasKiwi = fruits.Contains(kiwi, prodc);

        outputBlock.Text += "Apple? " + hasApple + "\n";
        outputBlock.Text += "Kiwi? " + hasKiwi + "\n";

        /*
            This code produces the following output:

            Apple? True
            Kiwi? False
        */    

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.