using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class DepartmentStudentCount

    {

        public int Count { get; set; }

        public string DepartmentName { get; set; }

        public int DepartmentId { get; set; }

    }

    public class Student

    {

        public int StudentId { get; set; }

        public string StudentName { get; set; }

        public int DepartmentId { get; set; }

    }

    public class Department

    {

        public int DepartmentId { get; set; }

        public string DepartmentName { get; set; }

    }

​

    public static void Main()

    {

        List<Student> studentDetails = new List<Student>()

        {( new Student() { StudentId=100, StudentName="Jake", DepartmentId=1 }),

        ( new Student() { StudentId=101, StudentName="Ram", DepartmentId=1 }),

        ( new Student() { StudentId=103, StudentName="Kev", DepartmentId=2 }),

        ( new Student() { StudentId=104, StudentName="John", DepartmentId=3 }),

        ( new Student() { StudentId=105, StudentName="Ak", DepartmentId=0 })

        };

​

        List<Department> departments = new List<Department>()

        {

        new Department(){DepartmentId=1, DepartmentName="Physics" },

        new Department(){DepartmentId=2, DepartmentName="Chemistry" },

        new Department(){DepartmentId=3, DepartmentName="Maths" },

        new Department(){DepartmentId=4, DepartmentName="Biology" },

        new Department(){DepartmentId=5, DepartmentName="Social Sience" },

        new Department(){DepartmentId=6, DepartmentName="Zoology" }

        };

// Get the Department name and sutudent registered count

        var countList = departments.Select(x => new DepartmentStudentCount() {

                DepartmentId = x.DepartmentId,

                DepartmentName = x.DepartmentName,

                Count = studentDetails.Where(a => a.DepartmentId == x.DepartmentId).Count()

        }).ToList();

        foreach (var c in countList) { Console.WriteLine("Department: " + c.DepartmentName + " Count: " + c.Count); }

​

        // Get the Department names which doesn't have any students registered

        // Get the Student with No department assigned

    }

}

​