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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)

Public Shared Function Distinct(Of TSource) (
	source As IEnumerable(Of TSource),
	comparer As IEqualityComparer(Of TSource)
) As IEnumerable(Of TSource)


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

The sequence to remove duplicate elements from.

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.

Type Parameters


The type of the elements of source.

Exception Condition

source is null.

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 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 Distinct(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 a sequence of Product objects in the Distinct(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(New ProductComparer())

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

' This code produces the following output:
' apple 9
' orange 4
' lemon 12

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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