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Enumerable.Except(Of TSource) Method (IEnumerable(Of TSource), IEnumerable(Of TSource), IEqualityComparer(Of TSource))

 

Produces the set difference of two sequences by using the specified IEqualityComparer(Of T) to compare values.

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

<ExtensionAttribute>
Public Shared Function Except(Of TSource) (
	first As IEnumerable(Of TSource),
	second As IEnumerable(Of TSource),
	comparer As IEqualityComparer(Of TSource)
) As IEnumerable(Of TSource)

Parameters

first
Type: System.Collections.Generic.IEnumerable(Of TSource)

An IEnumerable(Of T) whose elements that are not also in second will be returned.

second
Type: System.Collections.Generic.IEnumerable(Of TSource)

An IEnumerable(Of T) whose elements that also occur in the first sequence will cause those elements to be removed from the returned sequence.

comparer
Type: System.Collections.Generic.IEqualityComparer(Of TSource)

An IEqualityComparer(Of T) to compare values.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TSource)

A sequence that contains the set difference of the elements of two sequences.

Type Parameters

TSource

The type of the elements of the input sequences.

Exception Condition
ArgumentNullException

first or second is null.

If comparer is null, the default equality comparer, Default, is used to compare values.

The following example shows how to implement an equality comparer that can be used in the Except(Of TSource) 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

After you implement this comparer, you can use sequences of Product objects in the Except(Of TSource) method, as shown in the following example.

Dim fruits1() As Product = 
    {New Product With {.Name = "apple", .Code = 9}, 
     New Product With {.Name = "orange", .Code = 4}, 
     New Product With {.Name = "lemon", .Code = 12}}

Dim fruits2() As Product = 
    {New Product With {.Name = "apple", .Code = 9}}

' Get all the elements from the first array
' except for the elements from the second array.

Dim except = fruits1.Except(fruits2, New ProductComparer())

For Each product In except
    Console.WriteLine(product.Name & " " & product.Code)
Next

' This code produces the following output:
'
' orange 4
' lemon 12

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
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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