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LINQ Union Method in C# with Examples
In this article, I am going to discuss the LINQ Union Method in C# with Examples. Please read our previous article where we discussed the LINQ Intersect Method in C# with Examples. As part of this article, we are going to discuss the following pointers related to the LINQ Union method in C#.
- What is LINQ Union Method?
- Examples to Understand LINQ Union Method with Value Type in C#
- What happens if any of the sequences is null while performing the Union Operation?
- LINQ Union Method Example with String Array using C#
- LINQ Union Method with Complex Type in C#
- Using IEqualityComparer Comparer with LINQ Union Method in C#
- Using Anonymous Type with Union Method in C#
- Overriding Equals() and GetHashCode() Methods of Object Class
- Implementing IEquatble<T> Interface in C#
What is LINQ Union Method?
The LINQ Union Method in C# is used to combine multiple data sources into one data source by removing duplicate elements. There are two overloaded versions available for the LINQ Union Method as shown below.
The one and only difference between the above two LINQ Union methods is that the second overloaded version takes IEqualityComparer as an argument. That means when we are working with Complex Data Types, in order to work as expected, we can use the overloaded method which takes the IEqualityComparer parameter.
Examples to Understand LINQ Union Method with Value Type in C#
Let us understand LINQ Union Method with an example. Please have a look at the below image. As you can see in the below image, here we have two integer data sources i.e. DataSource 1 and Data Source 2. DataSource 1 contains elements such as 1, 2, 3, 4, 5, and 6, and DataSource 2 contains elements such as 1, 3, 5, 8, 9, and 10. If we want to retrieve all the elements from both collections by removing the duplicate element i.e. if we want the result as 1, 2, 3, 4, 5, 6, 8, 9, and 10, then we need to use the LINQ Union method.
LINQ Union Method Example using Method and Query Syntax:
The following example shows the use of the LINQ Union Method using both Method and Query Syntax to fetch all elements from both collections by removing the duplicate elements. In query syntax, there is no such operator called Union, so here we need to use the mixed syntax i.e. both the query and method syntax to achieve the same.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<int> dataSource1 = new List<int>() { 1, 2, 3, 4, 5, 6 };
List<int> dataSource2 = new List<int>() { 1, 3, 5, 8, 9, 10 };
//Method Syntax
var MS = dataSource1.Union(dataSource2).ToList();
//Query Syntax
var QS = (from num in dataSource1
select num)
.Union(dataSource2).ToList();
foreach (var item in MS)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
}
} Run the application and you will see the output as expected i.e. 1 2 3 4 5 6 8 9 10
What happens if any of the sequences is null while performing the Union Operation?
The Union Method will throw an exception if any of the sequences is null. In the below example, the second sequence is null and while performing the Union operation using the Union Method it will throw an exception.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<int> dataSource1 = new List<int>() { 1, 2, 3, 4, 5, 6 };
List<int> dataSource2 = null;
//Method Syntax
var MS = dataSource1.Union(dataSource2).ToList();
foreach (var item in MS)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
}
} Now run the application and you will get the following exception
LINQ Union Method Example with String Array using C#:
Let us see an example to understand how LINQ Union Method works with string collection in C#. Here, in the below example, we have two arrays of countries that also contain the same country name and our requirement is to return all countries from both collections by removing the duplicate country names.
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
string[] dataSource1 = { "India", "USA", "UK", "Canada", "Srilanka" };
string[] dataSource2 = { "India", "uk", "Canada", "France", "Japan" };
//Method Syntax
var MS = dataSource1.Union(dataSource2).ToList();
//Query Syntax
var QS = (from country in dataSource1
select country)
.Union(dataSource2).ToList();
foreach (var item in MS)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
}
} Now run the application and it will give us the following output.
As you can see it displays the country UK twice. This is because the default comparer that is being used by the LINQ Union method is case-insensitive. So if you want to ignore the case-sensitive then you need to use the other overloaded version of the Union method which takes IEqualityComparer as an argument. Let us modify the Main method of the program class as follows to use the StringComparer as an argument that implements the IEqualityComparer interface.
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
string[] dataSource1 = { "India", "USA", "UK", "Canada", "Srilanka" };
string[] dataSource2 = { "India", "uk", "Canada", "France", "Japan" };
//Method Syntax
var MS = dataSource1.Union(dataSource2, StringComparer.OrdinalIgnoreCase).ToList();
//Query Syntax
var QS = (from country in dataSource1
select country)
.Union(dataSource2, StringComparer.OrdinalIgnoreCase).ToList();
foreach (var item in MS)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
}
}
Now run the application and it should display the data as expected as shown in the below image.
LINQ Union Method with Complex Type in C#:
The LINQ Union() Method like other Set Methods (such as Distinct, Expect, and Intersect) also works in a different manner when working with complex data types such as Product, Employee, Student, etc. Let us understand this with an example. Create a class file with the name Student.cs and then copy and paste the following code into it.
namespace LINQDemo
{
public class Student
{
public int ID { get; set; }
public string Name { get; set; }
}
}
The above Student class is a very simple class with just two properties i.e. ID and Name. Let’s say, we have the following two data sources which contain some duplicate data.
As you can see in the above image, we have two collections of student data. And if you notice we have two students Hina and Anurag appeared in both collections. Our requirement is to fetch all the student names from both collections by removing the duplicate name. That means we want the names Hina and Anurag to appear only once in the result. To achieve the same we need to use the LINQ Union Method. So, modify the Main method of the Program class as follows. In the below example, from the first data source, we are fetching the student names by using the select method and then we use the Union method and then we fetching the student names from the second data source.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<Student> StudentCollection1 = new List<Student>()
{
new Student {ID = 101, Name = "Preety" },
new Student {ID = 102, Name = "Sambit" },
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
};
List<Student> StudentCollection2 = new List<Student>()
{
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
new Student {ID = 107, Name = "Pranaya"},
new Student {ID = 108, Name = "Santosh"},
};
//Method Syntax
var MS = StudentCollection1.Select(x => x.Name)
.Union(StudentCollection2.Select(y => y.Name)).ToList();
//Query Syntax
var QS = (from std in StudentCollection1
select std.Name)
.Union(StudentCollection2.Select(y => y.Name)).ToList();
foreach (var name in MS)
{
Console.WriteLine(name);
}
Console.ReadKey();
}
}
} Output:
When you run the above code, you will get the output as expected as shown in the below image. Here, you can see, the names Hina and Anurag appeared only once.
Now our requirement has changed and Now we need to select all the information of all the students from both collections by removing the duplicate students. In order to do this, let us modify the Main Method of the Program class as shown below.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<Student> StudentCollection1 = new List<Student>()
{
new Student {ID = 101, Name = "Preety" },
new Student {ID = 102, Name = "Sambit" },
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
};
List<Student> StudentCollection2 = new List<Student>()
{
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
new Student {ID = 107, Name = "Pranaya"},
new Student {ID = 108, Name = "Santosh"},
};
//Method Syntax
var MS = StudentCollection1.Union(StudentCollection2).ToList();
//Query Syntax
var QS = (from std in StudentCollection1
select std).Union(StudentCollection2).ToList();
foreach (var student in MS)
{
Console.WriteLine($" ID : {student.ID} Name : {student.Name}");
}
Console.ReadKey();
}
}
} Once you run the application, then you will see that it displays all the students without removing the duplicate students as shown in the below image.
As you can see in the above output, the students Hina and Anurag appeared twice. This is because the default comparer which is used for comparison by the LINQ Union Method is only checked whether two object references are equal and not the individual property values of the complex object. As we already discussed in our previous three articles, we can overcome this problem in many different ways. Let us understand each way one by one.
Using IEqualityComparer Comparer with LINQ Union Method in C#:
In this approach, we need to create a class and then we need to implement the IEqualityComparer interface and provide the implementation for the Equals and GetHashCode method. So, create a class file with the name StudentComparer.cs and then copy and paste the following code into it.
using System.Collections.Generic;
namespace LINQDemo
{
public class StudentComparer : IEqualityComparer<Student>
{
public bool Equals(Student x, Student y)
{
return x.ID == y.ID && x.Name == y.Name;
}
public int GetHashCode(Student obj)
{
return obj.ID.GetHashCode() ^ obj.Name.GetHashCode();
}
}
}
Next, we need to create an instance of StudentComparer class and then we need to pass that instance to the Union method which takes IEqualityComparer as a parameter. So, modify the Main method of the Program class as follows.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<Student> StudentCollection1 = new List<Student>()
{
new Student {ID = 101, Name = "Preety" },
new Student {ID = 102, Name = "Sambit" },
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
};
List<Student> StudentCollection2 = new List<Student>()
{
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
new Student {ID = 107, Name = "Pranaya"},
new Student {ID = 108, Name = "Santosh"},
};
StudentComparer studentComparer = new StudentComparer();
//Method Syntax
var MS = StudentCollection1
.Union(StudentCollection2, studentComparer).ToList();
//Query Syntax
var QS = (from std in StudentCollection1
select std)
.Union(StudentCollection2, studentComparer).ToList();
foreach (var student in MS)
{
Console.WriteLine($" ID : {student.ID} Name : {student.Name}");
}
Console.ReadKey();
}
}
}
With the above changes in place, run the application and you will get the output as expected as shown in the below image. Here, you can see Hina and Anurag appeared only once.
Using Anonymous Type with Union Method in C#:
In this approach, we need to select all the individual properties to an anonymous type using the LINQ select operator or Select Extension Method. The following program does exactly the same thing.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<Student> StudentCollection1 = new List<Student>()
{
new Student {ID = 101, Name = "Preety" },
new Student {ID = 102, Name = "Sambit" },
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
};
List<Student> StudentCollection2 = new List<Student>()
{
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
new Student {ID = 107, Name = "Pranaya"},
new Student {ID = 108, Name = "Santosh"},
};
//Method Syntax
var MS = StudentCollection1.Select(x => new { x.ID, x.Name })
.Union(StudentCollection2.Select(x => new { x.ID, x.Name })).ToList();
//Query Syntax
var QS = (from std in StudentCollection1
select new { std.ID, std.Name })
.Union(StudentCollection2.Select(x => new { x.ID, x.Name })).ToList();
foreach (var student in MS)
{
Console.WriteLine($" ID : {student.ID} Name : {student.Name}");
}
Console.ReadKey();
}
}
}
With the above changes in place, now run the application and you will get the output as expected as shown in the below image.
Overriding Equals() and GetHashCode() Methods of Object Class
This is the third approach and in this approach, we need to override the Equals and GetHashCode() in the Student class. As we know, by default, any type (predefined or user-defined) .NET Framework is inherited from the Object class. That means the Student class is also inherited from the Object class. So, we can override the Equals() and GetHashCode() methods of the Object class within the Student class. So, modify the Student class as shown below. Here, we are overriding the Equals() and GetHashCode() methods.
namespace LINQDemo
{
public class Student
{
public int ID { get; set; }
public string Name { get; set; }
public override bool Equals(object obj)
{
//As the obj parameter type is object, so we need to
//cast it to Student Type
return this.ID == ((Student)obj).ID && this.Name == ((Student)obj).Name;
}
public override int GetHashCode()
{
//Get the ID hash code value
int IDHashCode = this.ID.GetHashCode();
//Get the string HashCode Value
//Check for null refernece exception
int NameHashCode = this.Name == null ? 0 : this.Name.GetHashCode();
return IDHashCode ^ NameHashCode;
}
}
}
Next, we need to modify the Main method of the Program class as shown below. Here, we need to use the overloaded version of the Union method which does not take any parameter.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<Student> StudentCollection1 = new List<Student>()
{
new Student {ID = 101, Name = "Preety" },
new Student {ID = 102, Name = "Sambit" },
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
};
List<Student> StudentCollection2 = new List<Student>()
{
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
new Student {ID = 107, Name = "Pranaya"},
new Student {ID = 108, Name = "Santosh"},
};
//Method Syntax
var MS = StudentCollection1.Union(StudentCollection2).ToList();
//Query Syntax
var QS = (from std in StudentCollection1
select std).Union(StudentCollection2).ToList();
foreach (var student in MS)
{
Console.WriteLine($" ID : {student.ID} Name : {student.Name}");
}
Console.ReadKey();
}
}
}
Now run the application code and, you will get the same output as the previous two examples as expected as shown in the below image.
Implementing IEquatble<T> Interface in C#
In this approach, we need to implement IEquatble<T> Interface in Student Class and we need to implement the Equals Method of the IEquatble<T> Interface and we also need to override the GetHashCode method of the Object class. So, modify the Student class as shown below.
using System;
namespace LINQDemo
{
public class Student : IEquatable<Student>
{
public int ID { get; set; }
public string Name { get; set; }
public bool Equals(Student other)
{
return this.ID.Equals(other.ID) && this.Name.Equals(other.Name);
}
public override int GetHashCode()
{
int IDHashCode = this.ID.GetHashCode();
int NameHashCode = this.Name == null ? 0 : this.Name.GetHashCode();
return IDHashCode ^ NameHashCode;
}
}
}
Here, first, we implement the Equals method of the IEquatable interface and then we override the GetHashCode method of the Object class. With the above changes in place, now modify the Main Method of the Program class as shown below.
using System.Collections.Generic;
using System;
using System.Linq;
namespace LINQDemo
{
class Program
{
static void Main(string[] args)
{
List<Student> StudentCollection1 = new List<Student>()
{
new Student {ID = 101, Name = "Preety" },
new Student {ID = 102, Name = "Sambit" },
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
};
List<Student> StudentCollection2 = new List<Student>()
{
new Student {ID = 105, Name = "Hina"},
new Student {ID = 106, Name = "Anurag"},
new Student {ID = 107, Name = "Pranaya"},
new Student {ID = 108, Name = "Santosh"},
};
//Method Syntax
var MS = StudentCollection1.Union(StudentCollection2).ToList();
//Query Syntax
var QS = (from std in StudentCollection1
select std).Union(StudentCollection2).ToList();
foreach (var student in MS)
{
Console.WriteLine($" ID : {student.ID} Name : {student.Name}");
}
Console.ReadKey();
}
}
}
Now, run the application and you will also get the same output as expected as shown in the below image.
In the next article, I am going to discuss the LINQ Concat Method using C# with Examples. Here, in this article, I try to explain LINQ Union Method using C# with Examples. I hope you enjoy this article and understand the concept of the Linq Union Method in C# with Examples.
