vb.net - Child Form not refreshing when i try to start the app from MDI form? -

have mdi form , child forms in app.
here situation...

main form : mdi form
register form : child of mdi form
desig form : form open when click 1 button on register form.

now if try refresh items of combo box of register form desig form, not refreshing. i.e. can't see new items in combo box on register form.

now when try start app directly register form same code works same code not running when try start app mdi form.

here codes.
mdi form :

dim regform new register statuslabel.text = "opening workman registration" regform.mdiparent = me regform.show() statuslabel.text = "workman registration" regform.concombo.focus() 

register form:

 public sub refreshcombo()     desigcombo.items.clear()     sitecombo.items.clear()      adddescombo() ' method loads new data database       desigcombo.text = designame ' string variable (designame)   end sub 

desig form

register.refreshcombo() ' run when click on 1 button. 

so thing need remember when using mdi form - child form?

when refresh register desig, works when start app register because there 1 register form , vb automatically considers instance of form.

when register form child form, need use instance of form rather class name, mainform.activemdichild.refreshcombo()