LINQ Set Operator Distinct

The following table lists all the Set operators available in LINQ.

Distinct

The Distinct extension method returns a new collection of unique elements from the given collection.

IList<string> strList = new List<string>(){ "One", "Two", "Three", "Two", "Three" };
IList<int> intList = new List<int>(){　1,　2,　3,　2,　4,　4,　3,　5　};
var distinctList1　= strList.Distinct();
foreach (var str in distinctList1)
　　　　Console.WriteLine(str);
var distinctList2　= intList.Distinct();
foreach (var i in distinctList2)
　　　　Console.WriteLine(i);

One
Two
Three
1
2
3
4
5

The Distinct extension method does not compare the values of complex type objects. To compare the values of complex types, you need to implement the IEqualityComparer<T> interface. In the following example, the StudentComparer class implements IEqualityComparer<Student> to compare Student objects.

public class Student　
{
　　　　public int StudentID { get; set; }
　　　　public string StudentName { get; set; }
　　　　public int Age { get; set; }
}
class StudentComparer : IEqualityComparer<Student>
{
　　　　public bool Equals(Student x, Student y)
　　　　{
　　　　　　　　if (x.StudentID == y.StudentID　
　　　　　　　　　　　　　　　　&& x.StudentName.ToLower() == y.StudentName.ToLower())
　　　　　　　　　　　　return true;
　　　　　　　　return false;
　　　　}
　　　　public int GetHashCode(Student obj)
　　　　{
　　　　　　　　return obj.StudentID.GetHashCode();
　　　　}
}

Now, you can pass an instance of the StudentComparer class as a parameter to the Distinct() method to compare Student objects, as shown below. Example: Distinct comparison object in C#

IList<Student> studentList = new List<Student>() {　
　　　　　　　　new Student() { StudentID =　1, StudentName = "John", Age =　18　},
　　　　　　　　new Student() { StudentID =　2, StudentName = "Steve", Age =　15　},
　　　　　　　　new Student() { StudentID =　3, StudentName = "Bill", Age =　25　},
　　　　　　　　new Student() { StudentID =　3, StudentName = "Bill", Age =　25　},
　　　　　　　　new Student() { StudentID =　3, StudentName = "Bill", Age =　25　},
　　　　　　　　new Student() { StudentID =　3, StudentName = "Bill", Age =　25　},
　　　　　　　　new Student() { StudentID =　5, StudentName = "Ron", Age =　19　}　
　　　　};
var distinctStudents = studentList.Distinct(new StudentComparer());　
foreach(Student std in distinctStudents)
　　　　Console.WriteLine(std.StudentName);

John
Steve
Bill
Ron

Distinct Operator in Query Syntax

C# query syntax does not support the Distinct operator. However, you can use the Distinct method to query a variable or wrap the entire query in parentheses and then call Distinct().

Using the Distinct keyword in VB.Net query syntax:

Dim strList = New List(Of string) From {"One", "Three", "Two", "Two", "One"}
Dim distinctStr = From s In strList _
　　　　　　　　　　　　　　　　　　Select s Distinct