The type of one of the expression in the join clause is incorrect.Type inference failed in the call to 'GroupJoin'.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections;

namespace employee
{
    public class Department
    {
        public int deptId { get; set; }
        public string deptName { get; set; }
    }
    public class Employee
    {
        private employee.Department dept;

        public Employee(employee.Department dept)
        {
            // TODO: Complete member initialization
            this.dept = dept;
        }
        public int empId { get; set; }
        public string empName { get; set; }
        public int empSalary { get; set; }
        public Department Department { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
      {
            ArrayList list = new ArrayList();
            Department dept = new Department();
            dept.deptId = 1;
            dept.deptName = "IT";
            list.Add(dept);
            ArrayList list1 = new ArrayList();
            Employee emp = new Employee(dept);
            emp.empId = 100;
            emp.empName = "anusha";
            emp.empSalary = 10000;
            emp.Department = dept;
            list1.Add(emp);
            dept = new Department();
            dept.deptId = 2;
            dept.deptName = "HR";
            list.Add(dept);
            emp = new Employee(dept);
            emp.empId = 101;
            emp.empName = "usha";
            emp.empSalary = 20000;
            emp.Department = dept;
            list1.Add(emp);
            var query = from Employee employee in list1
                        where employee.empId == 101
                        select employee;
            foreach(Employee e in query)
            {
                Console.WriteLine("EmpID\tEmpName\tEmpSal\tDepartmentID");
                Console.WriteLine(e.empId+"\t"+e.empName +"\t"+e.empSalary+"\t"+e.Department.deptId+"");
            }
            var query1=(from Employee employee1 in list1 
                       join Department department1 in list 
                       on employee1.Department equals department1.deptId into o
                       from department1 in o.DefaultIfEmpty()
                       select new
                       {
                           id=employee1.empId,
                           name=employee1.empName,
                           salary=employee1.empSalary,
                           departmentname=department1.deptName
                       });
            foreach (Employee e1 in query1)
            {
                Console.WriteLine("EmpID\tEmpName\tEmpSal\tDepartmentID");
                Console.WriteLine(e1.empId + "\t" + e1.empName + "\t" + e1.empSalary + "\t" + e1.Department.deptName + "");
            }
            Console.ReadLine();
        }
    }
}