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

Updated: October 2008

Produces the set union of two sequences by using a specified IEqualityComparer<T>.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IEnumerable<TSource> Union<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 distinct elements form the first set for the union.

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

An IEnumerable<T> whose distinct elements form the second set for the union.

comparer
Type: System.Collections.Generic.IEqualityComparer<TSource>

The IEqualityComparer<T> to compare values.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> that contains the elements from both input sequences, excluding duplicates.

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.

If comparer is null, the default equality comparer, Default, is used to compare values.

When the object returned by this method is enumerated, Union enumerates first and second in that order and yields each element that has not already been yielded.

The Concat<TSource> method differs from the Union method because the Concat<TSource> method returns all the elements in the input sequences including duplicates, whereas Union returns only unique values.

The following example shows how to implement an equality comparer that can be used in the Union 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, 
    // 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 the hash code for the Name field if it is not null. 
        int hashProductName = product.Name == null ? 0 : product.Name.GetHashCode();

        // Get the 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 Union method, as shown in the following example.

Product[] store1 = { new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "orange", Code = 4 } };

Product[] store2 = { new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "lemon", Code = 12 } };


...


// Get the products from both arrays,
// excluding duplicates.

IEnumerable<Product> union =
  store1.Union(store2, new ProductComparer());

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

/*
    This code produces the following output:

    apple 9
    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 an example.

Customer feedback.

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