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

 

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
)

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.

Type Parameters

TSource

The type of the elements of the input sequences.

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

The following example shows how to implement an equality comparer that can be used in the Union<TSource> 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 Union<TSource> 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 the 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
*/

Universal Windows Platform
Available since 4.5
.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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