What is C# Linq Distinct? Explained With Examples [Updated]

What is LINQ Distinct Method in C#?

C# Linq Distinct() method removes the duplicate elements from a sequence (list) and returns the distinct elements from a single data source. It comes under the Set operators’ category in LINQ query operators, and the method works the same way as the DISTINCT directive in Structured Query Language (SQL).

Syntax of Linq Distinct Method in C#:

IEnumerable<data type> result = numbers.Distinct();

Here is a sample code showing the implementation of C# LINQ Distinct().

using System;
using System.Linq;
using System.Collections.Generic;
namespace Ctutorials
{
    class Linqprogram
    {
        static void Main(string[ ] args)
        {
            String[ ] fruits = { "Apple", "Banana", "Apple", "Plum", "Grapes", "Plum" };
           IEnumerable<string> result= fruits.Distinct();  //LINQ distinct function
           foreach (var item in result)
            {
                Console.WriteLine(item);
            }
            Console.ReadLine();
        }
    }
}

Output

APPLE

BANANA

PLUM

GRAPES

Explanation

The array read multiple strings, and the LINQ distinct removed the repeated elements in the list.

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Examples of LINQ Distinct Method Using Both Method and Query Syntax

Here’s an example code that will read students’ marks with C# LINQ Distinct Method using both Method and Query Syntax.

using System;
using System.Collections.Generic;
using System.Linq;
namespace LINQCode
{
    class GradeMarks
    {
        static void Main(string[] args)
        {
            List<int> intCollection = new List<int>()
            {
                90,67,78,89,98,89,90,87,78,82,90,78
            };           
            //Method Syntax
            var Marks = intCollection.Distinct();            
            //Query Syntax
            var Grades = (from num in intCollection select num).Distinct();
            foreach (var item in Marks)
            {
                Console.WriteLine(item);
            }
            Console.ReadKey();
        }
    }
}

Output

90

67

78

89

98

87

82

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How to Implement IEqualityComparer?

By default, LINQ distinct() in C# is case-sensitive. So, the default equality comparer might work well for numbers, but you might not get the exact distinct list in strings.

To solve this, we use the  IEqualityComparer.

Syntax: public abstract class StringComparer: IComparer, IEqualityComparer, IComparer <string>, 

What if we change the case of the first fruit program we discussed? Doing so will display the entire list since the C# LINQ distinct function is case-sensitive. Let’s see how we can make it work by using the IEqualityComparer.

using System;
using System.Linq;
using System.Collections.Generic;
namespace Ctutorials
{
    class Linqprogram
    {
        static void Main(string[] args)
        {
            string[] fruits = { "Apple", "Banana", "apple", "Plum", "Grapes", "plum" };
            //Using IEqualityComparer
            IEnumerable<string> result= fruits.Distinct(StringComparer.OrdinalIgnoreCase);
            foreach (var item in result)
            {
                Console.WriteLine(item);      
            }
            Console.ReadLine();           
        }
    }
}

Output

APPLE

BANANA

PLUM

GRAPES

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LINQ Distinct Operation With Complex Type

Now, what if we add the name of students to the marks obtained by them above?

Lets understand it through a code sample.

create a file named StudentMarks.cs to store the complex list.

using System;
using System.Linq;
using System.Collections.Generic;
namespace LINQTutorial
{
    public class StudentMarks
    {
        public int Marks { get; set; }
        public string Name { get; set; }
        public static List<Student> GetStudents()
        {
            List<Student> students = new List<Student>()
            {
                new Student {Marks = 90, Name = "Vaibhav" },
                new Student {Marks = 67, Name = "Akash" },
                new Student {Marks = 78, Name = "Anjali" },
                new Student {Marks = 89, Name = "Rithik" },
                new Student {Marks = 98, Name = "Harsh" },
                new Student {Marks = 89, Name = "Anjali" },
                new Student {Marks = 90, Name = "Akash" },
                new Student {Marks = 87, Name = "Sameera" },
                new Student {Marks = 78, Name = "Priya" },
                new Student {Marks = 82, Name = "Harsh" },
                new Student {Marks = 90, Name = "Vaibhav" },
                new Student {Marks = 78, Name = "Anjali" },
            };
            return students;
        }
    }
}

We will use this code in the main method.

using System;
using System.Linq;
namespace LINQTutorial
{
    class MainStudent
    {
        public static void Main(string[] args)
        {
           //Using Method Syntax
            var MS = Student.GetStudents()
                    .Select(std => std.Name)
                    .Distinct().ToList();
            //Using Query Syntax
            var QS = (from std in Student.GetStudents(select std.Name)
                      .Distinct().ToList();
            foreach(var item in MS)
            {
                Console.WriteLine(item);
            }
            Console.ReadKey();
        }
    }
}

By default, The C# LINQ Distinct function will only be able to sort by either removing repeating student names or repeating marks.

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How to Solve the Problem?

This problem can be solved by two different approaches.

1. Overriding the Equals() and GetHashCode() methods within the Student class
2. Using IEquatable<T> interface

Method 1

Create a file named StudentMarks.cs to type this code.

using System.Collections.Generic;
namespace LINQTutorial
{
    public class StudentMarks
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public static List<Student> GetStudents()
        {
            List<Student> students = new List<Student>()
            {
                new Student {Marks = 90, Name = "Vaibhav" },
                new Student {Marks = 67, Name = "Akash" },
                new Student {Marks = 78, Name = "Anjali" },
                new Student {Marks = 89, Name = "Rithik" },
                new Student {Marks = 98, Name = "Harsh" },
                new Student {Marks = 89, Name = "Anjali" },
                new Student {Marks = 90, Name = "Akash" },
                new Student {Marks = 87, Name = "Sameera" },
                new Student {Marks = 78, Name = "Priya" },
                new Student {Marks = 82, Name = "Harsh" },
                new Student {Marks = 90, Name = "Vaibhav" },
                new Student {Marks = 78, Name = "Anjali" },
            };
            return students;
        }
        public override bool Equals(object obj)
        {
            //typecasting obj to Student Type
            return this.ID == ((Student)obj).ID && this.Name == ((Student)obj).Name;
        }
        public override int GetHashCode()
        {
            //Getting the ID hash code value
            int IDHashCode = this.ID.GetHashCode();
            //Getting the string HashCode Value
            int NameHashCode = this.Name == null ? 0 : this.Name.GetHashCode();
            return IDHashCode ^ NameHashCode;
        }
    }
}

Now, we will call the method in the main function through this code:

using System;
using System.Linq;
namespace LINQTutorial
{
    class MainStudent
    {
        public static void Main(string[] args)
        {
            // Method Syntax
            var MS = Student.GetStudents()
                    .Distinct().ToList();
            // Query Syntax
            var QS = (from std in Student.GetStudents()
                      select std)
                      .Distinct().ToList();
            foreach (var item in MS)
            {
                Console.WriteLine($"ID : {item.ID} , Name : {item.Name} ");
            }
            Console.ReadKey();
        }
    }
}

Output

Marks = 90, Name = Vaibhav

Marks = 67, Name = Akash

Marks = 78, Name = Anjali

Marks = 89, Name = Rithik

Marks = 98, Name = Harsh

Marks = 87, Name = Sameera

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Method 2

Write this code sample in a new file named StudentMarks.cs.

using System.Collections.Generic;
namespace LINQTutorial
{
    public class StudentMarks : IEquatable<StudentMarks>
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public static List<StudentMarks> GetStudentsMarks()
        {
            List<StudentMarks> StudentMarkstudents = new List<StudentMarks>()
            {
                new Student {Marks = 90, Name = "Vaibhav" },
                new Student {Marks = 67, Name = "Akash" },
                new Student {Marks = 78, Name = "Anjali" },
                new Student {Marks = 89, Name = "Rithik" },
                new Student {Marks = 98, Name = "Harsh" },
                new Student {Marks = 89, Name = "Anjali" },
                new Student {Marks = 90, Name = "Akash" },
                new Student {Marks = 87, Name = "Sameera" },
                new Student {Marks = 78, Name = "Priya" },
                new Student {Marks = 82, Name = "Harsh" },
                new Student {Marks = 90, Name = "Vaibhav" },
                new Student {Marks = 78, Name = "Anjali" },
            };
            return students;
        }
        public bool Equals(StudentMarks other)
        {
            if (object.ReferenceEquals(other, null))
            {
                return false;
            }
            if (object.ReferenceEquals(this, other))
            {
                return true;
            }
            return this.ID.Equals(other.ID) && this.Name.Equals(other.Name);
        }
        //creating the hashcode function
        public override int GetHashCode()
        {
            int IDHashCode = this.ID.GetHashCode();
            int NameHashCode = this.Name == null ? 0 : this.Name.GetHashCode();
            return IDHashCode ^ NameHashCode;
        }
    }
}

Now, paste this code in the main method.

using System;
using System.Linq;
namespace LINQTutorial
{
    class MainStudent
    {
        static void Main(string[] args)
        {
            //Method Syntax
            var MS = StudentMarks.GetStudentsMarks()
                    .Distinct().ToList();
            //Query Syntax
            var QS = (from std in StudentMarks.GetStudents()
                      select std)
                      .Distinct().ToList();
            foreach (var item in MS)
            {
                Console.WriteLine($"ID : {item.ID} , Name : {item.Name} ");
            }
            Console.ReadKey();
        }
    }
}

Output

Marks = 90, Name = Vaibhav

Marks = 67, Name = Akash

Marks = 78, Name = Anjali

Marks = 89, Name = Rithik

Marks = 98, Name = Harsh

Marks = 87, Name = Sameera

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Conclusion

Now that you have a strong understanding of C# linq distinct, you should learn the other important aspects and concepts of the language with this tutorial. However, in order to become a proficient full stack developer, knowledge of just one language may not be enough. You can enroll in our world-class Full Stack Developer - MERN Stack and scale your programming and development career to new heights. Explore and enrol now!