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Enumerable.Except<TSource> Method (IEnumerable<TSource>, IEnumerable<TSource>)

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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)

Syntax

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

public static IEnumerable<TSource> Except<TSource>(
    this IEnumerable<TSource> first,
    IEnumerable<TSource> second
)

Type Parameters

  • TSource
    The type of the elements of the input sequences.

Parameters

Return Value

Type: System.Collections.Generic.IEnumerable<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<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

first or second is nulla null reference (Nothing in Visual Basic).

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 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.

The default equality comparer, Default, is used to compare values of the types that implement the IEqualityComparer<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.

Examples

The following code example demonstrates how to use the Except<TSource>(IEnumerable<TSource>, IEnumerable<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.
      outputBlock.Text &= output.ToString() & vbCrLf

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


      double[] numbers1 = { 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 };
      double[] numbers2 = { 2.2 };

      IEnumerable<double> onlyInFirstSet = numbers1.Except(numbers2);

      foreach (double number in onlyInFirstSet)
         outputBlock.Text += number + "\n";

      /*
       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, you have to implement the IEqualityComparer<T> generic interface in your 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


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;
    }
}

After you implement this interface, you can use sequences of Product objects in the Except<TSource>(IEnumerable<TSource>, IEnumerable<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
            outputBlock.Text &= product.Name & " " & product.Code.ToString() & vbCrLf
        Next
        ' This code produces the following output:
        '
        ' orange 4
        ' lemon 12


        Product[] fruits1 = { new Product { Name = "apple", Code = 9 }, 
                               new Product { Name = "orange", Code = 4 },
                                new Product { Name = "lemon", Code = 12 } };

        Product[] fruits2 = { new Product { Name = "apple", Code = 9 } };

        //Get all the elements from the first array
        //except for the elements from the second array.

        IEnumerable<Product> except =
            fruits1.Except(fruits2);

        foreach (var product in except)
            outputBlock.Text += product.Name + " " + product.Code + "\n";

        /*
          This code produces the following output:

          orange 4
          lemon 12
        */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.