Originally Posted by Pat Hartman
When using the "Form_" syntax from anywhere, you MUST include the entire path starting with the main form.
Form_formname should never be used to refer to a form. Refer to a form via the Forms Collection. Using Form_ often works but it is inviting trouble.
Always use Forms!formname or Forms("formname") unless you are directly referring to it from its Parent or one of its Members.
Contrary to a unfortunately too common belief, Form_ isn't just another syntactical variation of the Forms Collection but a reference to the form object via its Module. (You can verify this by referring to a form that does not have a Module. It will return an error, "External Name not defined".)
Moreover, if the Form is not already loaded to the Forms Collection, a hidden instance will load. This can lead to unexpected results.
Of course, if the Form referred to is already loaded as a subform, the Form_ reference will not return that subform but instead load a hidden instance completely separate from the subform.
BTW You can check if a Form is loaded by testing its IsLoaded Property via the AllForms Collection.
This will return True if the form is an item in the Forms Collection. It will return False if an instance is not loaded or is loaded as a subform.