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

Produces the set difference of two sequences by using the specified IEqualityComparer<T> 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,
	IEqualityComparer<TSource> comparer
)

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.
comparer
Type: System.Collections.Generic.IEqualityComparer<TSource>
An IEqualityComparer<T> to compare values.

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.

ExceptionCondition
ArgumentNullException

first or second is null.

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 Except method.


public class Product
{
    public string Name { get; set; }
    public int Code { get; set; }
}

// Custom comparer for the Product class
class ProductComparer : IEqualityComparer<Product>
{
    // Products are equal if their names and product numbers are equal.
    public bool Equals(Product x, Product y)
    {

        //Check whether the compared objects reference the same data.
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether any of the compared objects is null.
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;

        //Check whether the products' properties are equal.
        return x.Code == y.Code && x.Name == y.Name;
    }

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

    public int GetHashCode(Product product)
    {
        //Check whether the object is null
        if (Object.ReferenceEquals(product, null)) return 0;

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

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

        //Calculate the hash code for the product.
        return hashProductName ^ hashProductCode;
    }

}


After you implement this comparer, you can use sequences of Product objects in the Except 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 from the second array.

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

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

        /*
          This code produces the following output:

          orange 4
          lemon 12
        */



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

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

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