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

Updated: October 2008

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)

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

first
Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> whose elements that are not also in second will be returned.

second
Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> whose elements that also occur in the first sequence will cause those elements to be removed from the returned sequence.

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

first or second 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 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.

The following code example demonstrates how to use the Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) method to compare two sequences of integers and return elements that appear only in the first sequence.

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)
    Console.WriteLine(number);

/*
 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 : 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, 
    // GetHashCode must return the same value for these objects. 

    public override int GetHashCode()
    {

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

        // Get the 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.

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 that appear in the second array.

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

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

/*
  This code produces the following output:

  orange 4
  lemon 12
*/

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

Date

History

Reason

October 2008

Added more examples.

Customer feedback.

Community Additions

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