LINQ Distinct Operator

LINQ Distinct operator is used for removing duplicates elements from the sequence (list). Distinct operator comes under Set operators category in LINQ query operators.

For example, we have one collection A = { 1, 2, 3, 3, 4, 4, 5}. In this collection 3 and 4 elements are repeated two times and we have to create a new sequence which takes these duplicates only one time. So we use distinct operator over A collection. C = A.distinct(); C = { 1, 2, 3, 4, 5}.

Distinct operator is only available in Method Syntax and not available in Query Syntax.

Below is the syntax of Distinct operator.

public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source);
public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer);

There are two overload methods. First one takes no parameter and second one takes IEqualityComparer parameter to compare complex types.

C# LINQ Distinct Example

string[] employeesNames = { "Asutosh", "Kapil", "Sumit", "Rajat", "Preeti", "Sumit", "Kanupriya", "Kapil" };

var distinctEmployees = employeesNames.Distinct();

foreach(var name in distinctEmployees)
{
    Console.WriteLine(name);
}

Result
------
Asutosh
Kapil
Sumit
Rajat
Preeti
Kanupriya

In the above example, “Kapil” and “Sumit” names are repeated in employeeNames collection. By using Distinct operator we get a new collection which remove duplicate name.

Distinct with IEqualityComparer

By default Distinct operator can not compare complex types. For this, we need to use second overload which takes an instance of IEqualityComparer. IEqualityComparer is used for comparing complex types. IEqualityCompare has two methods Equals and GetHashCode. Both methods are compulsory to implement.

internal class Student
{
    public int ID { get; set; }
    public string Name { get; set; }
}

internal class StudentNameComparer : IEqualityComparer<Student>
{
    public bool Equals(Student x, Student y)
    {
        if (string.Equals(x.Name, y.Name, StringComparison.OrdinalIgnoreCase))
        {
            return true;
        }
        return false;
    }

    public int GetHashCode(Student obj)
    {
        return obj.Name.GetHashCode();
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        List<Student> students = new List<Student>();

        students.Add(new Student { ID = 1, Name = "Asutosh" });
        students.Add(new Student { ID = 1, Name = "Kapil" });
        students.Add(new Student { ID = 1, Name = "Sumit" });
        students.Add(new Student { ID = 1, Name = "Rajat" });
        students.Add(new Student { ID = 1, Name = "Preeti" });
        students.Add(new Student { ID = 1, Name = "Sumit" });
        students.Add(new Student { ID = 1, Name = "Kanupriya" });
        students.Add(new Student { ID = 1, Name = "Kapil" });

        var distinctStudents = students.Distinct(new StudentNameComparer());

        foreach (var student in distinctStudents)
        {
            Console.WriteLine(student.Name);
        }
    }
}

Result
------
Asutosh
Kapil
Sumit
Rajat
Preeti
Kanupriya