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

Produces the set difference of two sequences by using the default equality comparer to compare values.

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

'Declaration
<ExtensionAttribute> _
Public Shared Function Except(Of TSource) ( _
	first As IEnumerable(Of TSource), _
	second As IEnumerable(Of TSource) _
) As IEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of the input sequences.

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.

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).

ExceptionCondition
ArgumentNullException

first or second is Nothing.

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The set difference of two sets is defined as the members of the first set that do not appear in the second set.

NoteNote

This method returns those elements in first that do not appear in second. It does not also return those elements in second that do not appear in first.

To compare custom data types, implement the IEquatable(Of T) generic interface and provide your own GetHashCode and Equals methods for the type. The default equality comparer, Default, is used to compare values of types that implement IEquatable(Of T).

The following code example demonstrates how to use the Except(Of TSource)(IEnumerable(Of TSource), IEnumerable(Of TSource)) method to compare two sequences of numbers and return elements that appear only in the first sequence.


        ' Create two arrays of doubles.
        Dim numbers1() As Double = {2.0, 2.1, 2.2, 2.3, 2.4, 2.5}
        Dim numbers2() As Double = {2.2}

        ' Select the elements from the first array that are not
        ' in the second array.
        Dim onlyInFirstSet As IEnumerable(Of Double) = numbers1.Except(numbers2)

        Dim output As New System.Text.StringBuilder
        For Each number As Double In onlyInFirstSet
            output.AppendLine(number)
        Next

        ' Display the output.
        MsgBox(output.ToString())

        ' This code produces the following output:
        '
        ' 2
        ' 2.1
        ' 2.3
        ' 2.4
        ' 2.5



If you want to compare sequences of objects of some custom data type, implement the IEquatable(Of T) generic interface in your class. The following code example shows how to implement this interface in a custom data type, Product, and to provide GetHashCode and Equals methods.


Public Class Product
    Implements IEquatable(Of Product)

    Public Property Name As String
    Public Property Code As Integer

    Public Function Equals1(
        ByVal other As Product
        ) As Boolean Implements IEquatable(Of Product).Equals

        ' Check whether the compared object is null.
        If other Is Nothing Then Return False

        ' Check whether the compared object references the same data.
        If Me Is Other Then Return True

        ' Check whether the products' properties are equal.
        Return Code.Equals(other.Code) AndAlso Name.Equals(other.Name)
    End Function

    Public Overrides Function GetHashCode() As Integer

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

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

        ' Calculate the hash code for the product.
        Return hashProductName Xor hashProductCode
    End Function
End Class



After you implement this interface, you can use sequences of Product objects to call the Except(Of TSource)(IEnumerable(Of TSource), IEnumerable(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)

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

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



.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

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