Enumerable.Union Method

Definition

Produces the set union of two sequences.

Overloads

Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>)

Produces the set union of two sequences by using the default equality comparer.

Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>)

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

Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>)

Produces the set union of two sequences by using the default equality comparer.

public static System.Collections.Generic.IEnumerable<TSource> Union<TSource> (this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second);
Type Parameters
TSource

The type of the elements of the input sequences.

Parameters
first
IEnumerable<TSource>

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

second
IEnumerable<TSource>

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

Returns
IEnumerable<TSource>

An IEnumerable<T> that contains the elements from both input sequences, excluding duplicates.

Exceptions

first or second is null.

Examples

The following code example demonstrates how to use Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) to obtain the union of two sequences of integers.

int[] ints1 = { 5, 3, 9, 7, 5, 9, 3, 7 };
int[] ints2 = { 8, 3, 6, 4, 4, 9, 1, 0 };

IEnumerable<int> union = ints1.Union(ints2);

foreach (int num in union)
{
    Console.Write("{0} ", num);
}

/*
 This code produces the following output:

 5 3 9 7 8 6 4 1 0
*/

' Create two arrays of integer values.
Dim ints1() As Integer = {5, 3, 9, 7, 5, 9, 3, 7}
Dim ints2() As Integer = {8, 3, 6, 4, 4, 9, 1, 0}

' Get the set union of the two arrays.
Dim union As IEnumerable(Of Integer) = ints1.Union(ints2)

' Display the resulting set's values.
Dim output As New System.Text.StringBuilder
For Each num As Integer In union
    output.AppendLine(num & " ")
Next
MsgBox(output.ToString())

' This code produces the following output:
'
' 5 
' 3 
' 9 
' 7 
' 8 
' 6 
' 4 
' 1 
' 0 

If you want to compare sequences of objects of a custom data type, you have to implement the IEqualityComparer<T> generic interface in a helperclass. The following code example shows how to implement this interface in a custom data type and provide GetHashCode and Equals methods.

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

public class ProductComparer : IEqualityComparer<ProductA>
{

    public bool Equals(ProductA x, ProductA y)
    {
        //Check whether the objects are the same object. 
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether the products' properties are equal. 
        return x != null && y != null && x.Code.Equals(y.Code) && x.Name.Equals(y.Name);
    }

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

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

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

After you implement this interface, you can use sequences of ProductA objects in the Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) method, as shown in the following example.

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

ProductA[] store2 = { new ProductA { Name = "apple", Code = 9 }, 
                       new ProductA { Name = "lemon", Code = 12 } };
//Get the products from the both arrays
//excluding duplicates.

IEnumerable<ProductA> union =
  store1.Union(store2);

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

/*
    This code produces the following output:
 
    apple 9
    orange 4
    lemon 12
*/
' Get the products from the both arrays
' excluding duplicates.

Dim union = store1.Union(store2)

For Each product In union
    Console.WriteLine(product.Name & " " & product.Code)
Next

' This code produces the following output:
'
' apple 9
' orange 4
' lemon 12
' 

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.

This method excludes duplicates from the return set. This is different behavior to the Concat method, which returns all the elements in the input sequences including duplicates.

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.

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.

Union<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>)

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

public static System.Collections.Generic.IEnumerable<TSource> Union<TSource> (this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource> comparer);
Type Parameters
TSource

The type of the elements of the input sequences.

Parameters
first
IEnumerable<TSource>

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

second
IEnumerable<TSource>

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

comparer
IEqualityComparer<TSource>

The IEqualityComparer<T> to compare values.

Returns
IEnumerable<TSource>

An IEnumerable<T> that contains the elements from both input sequences, excluding duplicates.

Exceptions

first or second is null.

Examples

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 
    // 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;
    }

}
Public Class Product
    Public Property Name As String
    Public Property Code As Integer
End Class

' Custom comparer for the Product class
Public Class ProductComparer
    Implements IEqualityComparer(Of Product)

    Public Function Equals1(
        ByVal x As Product, 
        ByVal y As Product
        ) As Boolean Implements IEqualityComparer(Of Product).Equals

        ' Check whether the compared objects reference the same data.
        If x Is y Then Return True

        'Check whether any of the compared objects is null.
        If x Is Nothing OrElse y Is Nothing Then Return False

        ' Check whether the products' properties are equal.
        Return (x.Code = y.Code) AndAlso (x.Name = y.Name)
    End Function

    Public Function GetHashCode1(
        ByVal product As Product
        ) As Integer Implements IEqualityComparer(Of Product).GetHashCode

        ' Check whether the object is null.
        If product Is Nothing Then Return 0

        ' Get hash code for the Name field if it is not null.
        Dim hashProductName = 
            If(product.Name Is Nothing, 0, product.Name.GetHashCode())

        ' Get hash code for the Code field.
        Dim hashProductCode = product.Code.GetHashCode()

        ' Calculate the hash code for the product.
        Return hashProductName Xor hashProductCode
    End Function
End Class

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 } };
Dim store1() As Product = 
    {New Product With {.Name = "apple", .Code = 9}, 
     New Product With {.Name = "orange", .Code = 4}}

Dim store2() As Product = 
    {New Product With {.Name = "apple", .Code = 9}, 
     New Product With {.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
*/
' Get the products from the both arrays
' excluding duplicates.

Dim union = store1.Union(store2, New ProductComparer())

For Each product In union
    Console.WriteLine(product.Name & " " & product.Code)
Next

' This code produces the following output:
'
' apple 9
' orange 4
' lemon 12
' 

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.

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

Applies to