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

Returns distinct elements from a sequence by using the default equality comparer 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) _
) 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.

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)) method returns an unordered sequence that contains no duplicate values. It uses the default equality comparer, Default, to compare values.

In Visual Basic query expression syntax, a Distinct clause translates to an invocation of Distinct.

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

For an example that uses IEqualityComparer(Of T) to define a custom comparer, see Distinct(Of TSource)(IEnumerable(Of TSource), IEqualityComparer(Of TSource)).

The following code example demonstrates how to use Distinct(Of TSource)(IEnumerable(Of TSource)) to return distinct elements from a sequence of integers.

        ' Create a list of integers. 
        Dim ages As New List(Of Integer)(New Integer() _
                                         {21, 46, 46, 55, 17, 21, 55, 55})

        ' Select the unique numbers in the List. 
        Dim distinctAges As IEnumerable(Of Integer) = ages.Distinct()

        Dim output As New System.Text.StringBuilder("Distinct ages:" & vbCrLf)
        For Each age As Integer In distinctAges
            output.AppendLine(age)
        Next 

        ' Display the output.
        MsgBox(output.ToString)

        ' This code produces the following output: 
        
        ' Distinct ages: 
        ' 21 
        ' 46 
        ' 55 
        ' 17

If you want to return distinct elements from sequences of objects of some custom data type, you have to implement the IEquatable(Of T) generic interface in the class. The following code example shows how to implement this interface in a custom data type and 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 a sequence of Product objects in the Distinct(Of TSource)(IEnumerable(Of TSource)) 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()

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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