c# - Type or namespace could not be found error -

right now, linking 2 projects use of button.
have employee registration form , list of employees.
did "add existing project" list of employees. did "add reference" (list of employees).

i inserted code button on employee registration in order see list of employees form

private void button_view_emp_click(object sender, eventargs e) {    list_employees.list_emp list = new list_employees.list_emp();     this.hide();    list.show(); } 

it displays list of employees form.

the list of employees form has button. , if click it, should return again employee registration form.

i inserted code on button:

private void button_back_click(object sender, eventargs e) {    frm_employee_registration.formemployee_reg ff = new frm_employee_registration.formemployee_reg();    this.hide();    } 

but gives me error of

error 1 type or namespace name 'frm_employee_registration' not found (are missing using directive or assembly reference?) c:\users\name\documents\visual studio 2013\projects\list_employees\list_employees\list_emp.cs 159 13 list_employees

i double- checked spelling of namespaces , matches.
why connects smoothly employee registration list of employees? if other way around, gets error?