EnumerableDistinct Method
 

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)

Syntax
public static IEnumerable<TSource> Distinct<TSource>(
	this IEnumerable<TSource> source
)
public:
generic<typename TSource>
[ExtensionAttribute]
static IEnumerable<TSource>^ Distinct(
	IEnumerable<TSource>^ source
)
static member Distinct<'TSource> : 
        source:IEnumerable<'TSource> -> IEnumerable<'TSource>
<ExtensionAttribute>
Public Shared Function Distinct(Of TSource) (
	source As IEnumerable(Of TSource)
) As IEnumerable(Of TSource)

Parameters

source
Type:

The sequence to remove duplicate elements from.

Return Value

Type:

An IEnumerable that contains distinct elements from the source sequence.

Type Parameters

TSource

The type of the elements of source.

Exceptions
Exception Condition
ArgumentNullException

source is null.

Remarks

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

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

The default equality comparer, , is used to compare values of the types that implement the IEquatable 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 IEqualityComparerto define a custom comparer, see Distinct.

Examples

The following code example demonstrates how to use Distinct to return distinct elements from a sequence of integers.

List<int> ages = new List<int> { 21, 46, 46, 55, 17, 21, 55, 55 };

IEnumerable<int> distinctAges = ages.Distinct();

Console.WriteLine("Distinct ages:");

foreach (int age in distinctAges)
{
    Console.WriteLine(age);
}

/*
 This code produces the following output:

 Distinct ages:
 21
 46
 55
 17
*/
' 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 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 : IEquatable<Product>
{
    public string Name { get; set; }
    public int Code { get; set; }

    public bool Equals(Product other)
    {

        //Check whether the compared object is null. 
        if (Object.ReferenceEquals(other, null)) return false;

        //Check whether the compared object references the same data. 
        if (Object.ReferenceEquals(this, other)) return true;

        //Check whether the products' properties are equal. 
        return Code.Equals(other.Code) && Name.Equals(other.Name);
    }

    // If Equals() returns true for a pair of objects  
    // then GetHashCode() must return the same value for these objects. 

    public override int GetHashCode()
    {

        //Get hash code for the Name field if it is not null. 
        int hashProductName = Name == null ? 0 : Name.GetHashCode();

        //Get hash code for the Code field. 
        int hashProductCode = Code.GetHashCode();

        //Calculate the hash code for the product. 
        return hashProductName ^ hashProductCode;
    }
}
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 method, as shown in the following example.

Product[] products = { new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "orange", Code = 4 }, 
                       new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "lemon", Code = 12 } };

//Exclude duplicates.

IEnumerable<Product> noduplicates =
    products.Distinct();

foreach (var product in noduplicates)
    Console.WriteLine(product.Name + " " + product.Code);

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

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
' 
Version Information
Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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