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

Type Parameters

• TSource
  The type of the elements of source.

Parameters

• source
  Type: System.Collections.Generic.IEnumerable<TSource>
  A sequence in which to locate a value.

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

• comparer
  Type: System.Collections.Generic.IEqualityComparer<TSource>
  An equality comparer to compare values.

Return Value

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

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;
    }

}

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
        */