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

Returns distinct elements from a sequence by using a specified IEqualityComparer(Of T) to compare values.

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

'Declaration
<ExtensionAttribute> _
Public Shared Function Distinct(Of TSource) ( _
	source As IEnumerable(Of TSource), _
	comparer As IEqualityComparer(Of TSource) _
) As IEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

Parameters

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

The sequence to remove duplicate elements from.

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

An IEqualityComparer(Of T) to compare values.

Return Value

Type: System.Collections.Generic.IEnumerable(Of TSource)
An IEnumerable(Of T) that contains distinct elements from the source sequence.

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

source 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 Distinct(Of TSource)(IEnumerable(Of TSource), IEqualityComparer(Of TSource)) method returns an unordered sequence that contains no duplicate values. 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 Distinct 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 a sequence of Product objects in the Distinct method, as shown in the following example.

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

' Exclude duplicates. 

Dim noduplicates = products.Distinct(New ProductComparer())

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

' This code produces the following output: 

' apple 9 
' orange 4 
' lemon 12 
' 

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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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