Gridview displaying error

public class cmp:Icmp
    {
       DateTime? _datetime;
       public int Employeeid
       {
           get;
           set;
       }


       public string EmpName
       {
           get;
           set;
       }

       public int jobid
       {
           get;
           set;
       }

       public int managerid
       {
           get;
           set;
       }

       public DateTime? hiredate
       {

           get { return _datetime; }
           set { value = _datetime; }

       }

public List<cmp> GetEmployees()
        {
            List<cmp> listemmployees = new List<cmp>();
            CompanyDataContext db = new CompanyDataContext();
            listemmployees = db.Employees.Select(p => new cmp { Employeeid = p.empid, EmpName = p.empname, hiredate=p.hiredate }).ToList();


            return listemmployees;
        }

<asp:GridView ID="gv1" runat="server" DataSourceID="osd"
            AutoGenerateColumns="False" AutoGenerateEditButton="True" DataKeyNames="Employeeid"
             AutoGenerateDeleteButton="True" AllowPaging="True" PageSize="3" AutoGenerateSelectButton="true" EnablePersistedSelection="true" >
            <Columns>
                <asp:BoundField DataField="Employeeid" HeaderText="Employeeid"
                    SortExpression="Employeeid" />
                <asp:BoundField DataField="EmpName" HeaderText="EmpName"
                    SortExpression="EmpName" />
                <asp:BoundField DataField="jobid" HeaderText="jobid" SortExpression="jobid" />
                <asp:BoundField DataField="managerid" HeaderText="managerid"
                    SortExpression="managerid" />
                <asp:BoundField DataField="hiredate" HeaderText="hiredate"
                    SortExpression="hiredate" />
                <asp:BoundField DataField="salary" HeaderText="salary"
                    SortExpression="salary" />
            </Columns>
        </asp:GridView>