Set operator: Except

The Except() method requires two collections. It returns a new collection with elements from the first collection which do not exist in the second collection (parameter collection).

Example: Except in method syntax C#
            
IList<string> strList1 = new List<string>(){"One", "Two", "Three", "Four", "Five" };
IList<string> strList2 = new List<string>(){"Four", "Five", "Six", "Seven", "Eight"};

var result = strList1.Except(strList2);

foreach(string str in result)
        Console.WriteLine(str);

Output:
One
Two
Three

Except extension method doesn't return the correct result for the collection of complex types. You need to implement IEqualityComparer interface in order to get the correct result from Except method.

Implement IEqualityComparer interface for Student class as shown below:

Example: IEqualityComparer with Except method C#

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 above StudentComparer class in Except extension method in order to get the correct result:

Example: Except() with object type C#
    
IList<Student> studentList1 = 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 = 5, StudentName = "Ron" , Age = 19 } 
    };

IList<Student> studentList2 = new List<Student>() { 
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
    };

var resultedCol = studentList1.Except(studentList2,new StudentComparer()); 

foreach(Student std in resultedCol)
    Console.WriteLine(std.StudentName);
    
      
Output:
John
Steve

The Except operator is Not Supported in C# & VB.Net Query syntax. However, you can use Distinct method on query variable or wrap whole query into brackets and then call Except().

The following figure shows how each set operators works on the collections:

LINQ Set operators