winforms - Multiple child and parent form in C# -

i have 6 forms (let f1,f2,f2a,f2b,f2c,f2d)

i'm trying make f2a - f2d childs of f2 while f2 parentform f1 , f1 child form f2

what i've tried

f1

private void button1_click(object sender, eventargs e) {      f2 nx = new f2(this);       this.visible = false;       nx.visible = true; } 

f2

public f2(f1 parentform) {     initializecomponent();      this.of = parentform;      f2a na = new f2a(this); //it give me error describe down there. }  public f1 of; 

f2a - f2d

    public f2* (f2 parentform) //well lets * stand letter of each form     {         initializecomponent();          this.of = parentform;     }     public f2 of; 

on f2 gives me 2 error

1.the best overloaded method match 'gui_x.f2a.f2a(system.windows.forms.f2)' has invalid arguments

2.argument 1: cannot convert 'gui_x.f2' 'system.windows.forms.mainmenu'

so why isn't work f2 f2a - f2d while it's work f1 f2 ? wrong put ?how can resolve ?

i'm new c# , grateful if me , explain easy possible.

and if i'm using windows form project.

thankyou in advance.

edit

i'm sorry,i'm forgot initialize of in f2*'s,thanks @s_i_r answer. still won't resolve problem

would please verify type of this.of f2 in f2* classes

public f2* (f2 parentform)  {     initializecomponent();      this.of = parentform; } private f2 of; //make sure f2 not f1 or other 

edit:

just declare f2a-f2d private object in f2 class when needed show them

public class f2 {   private f2a f2a;   private f2b f2b;   public f1 of;    public f2(f1 parent)   {     initializecomponent();     this.of = parent;     f2a = new f2a(this);     f2b = new f2b(this);   }  // call f2a - f2d method public void showf2a() {    f2a.showdialog(); }