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

Updated: October 2008

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)
Dim first As IEnumerable(Of TSource)
Dim second As IEnumerable(Of TSource)
Dim comparer As IEqualityComparer(Of TSource)
Dim returnValue As IEnumerable(Of TSource)

returnValue = first.Except(second, _

Type Parameters


The type of the elements of the input sequences.


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

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

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.

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.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(Of TSource). When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).


first or second is Nothing.

If comparer is Nothing, 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 method.

Public Class Product
    Public Name As String 
    Public 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 the hash code for the Name field if it is not null. 
        Dim hashProductName = _
            If(product.Name Is Nothing, 0, product.Name.GetHashCode())

        ' Get the 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 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)

' This code produces the following output: 

' orange 4 
' lemon 12

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0




October 2008

Added an example.

Customer feedback.