Student.cs

class Student
{
    public string Name { get; set; }
    public int Number { get; set; }
    public int SerialNo { get; set; }
    public int Grade { get; set; }

    public static IEqualityComparer<Student> SerialNoComparer { get => new SerialNoEqualityComparer(); }
    public static IEqualityComparer<Student> NumberComparer { get => new NumberEqualityComparer(); }

    private sealed class SerialNoEqualityComparer : IEqualityComparer<Student>
    {
        public bool Equals(Student s1, Student s2)
        {
            if (s1 == null && s2 == null)
                return true;
            else if (s1 == null || s2 == null)
                return false;
            else if (s1.SerialNo == s2.SerialNo)
                return true;
            else
                return false;
        }

        public int GetHashCode(Student student)
        {
            return student.SerialNo.GetHashCode();
        }
    }

    private sealed class NumberEqualityComparer : IEqualityComparer<Student>
    {
        public bool Equals(Student s1, Student s2)
        {
            if (s1 == null && s2 == null)
                return true;
            else if (s1 == null || s2 == null)
                return false;
            else if (s1.Number == s2.Number)
                return true;
            else
                return false;
        }

        public int GetHashCode(Student student)
        {
            return student.Number.GetHashCode();
        }
    }
}