Access controls on a different form.

[taigon]

I've searched for resolutions to not being able to access controls on one form from another and I'm having some issues.

From a separate form, I use this code...

               frmMain.tab_BottomPanel_MDITabs.TabPages(Convert.ToInt32(this.Tag)).Tag = true;

But I get the object reference required error.

I've tried doing this on frmMain...

       public string tab_BottomPanel_MDITabs_Tag
        {
            get { return (string) this.tab_BottomPanel_MDITabs.Tag; }
            set { this.tab_BottomPanel_MDITabs.Tag = value; }
        }

but from the other form, even tab_BottomPanel_MDITabs_Tag doesn't show up in the list.

How do I access the controls on the main form? The reason for this is because the secondary form is loaded into the main form in an MDI tab control so I need to be able to access the controls on the main form.

[motil]

you need to send a reference of the from which you want to use its controls to the target form

let's say you have form1 with this methods:
public void SayHello() {
MessageBox.Show("Hello");
}

now you want to call the "SayHello" Method from form2, so in form2 constructor you you pass form1 instance

from2

public class form2{

private form1 _MainForm
public form2(form1 MainForm) {
this._MainForm = MainForm;
}

// now you can call form1 method
this._MainForm.SayHello();

}

and from form1 you open form2 like so:

form2 f = new form2(this);
f.Show();

I wrote all of the code by hand since i'm not next to my computer sorry errors might be present ;

[jmcilhinney]

I've searched for resolutions to not being able to access controls on one form from another and I'm having some issues.

The resolution is to not even try. No form should ever access controls on another. All communication between forms should involve getting and setting properties, calling methods and raising and handling events.

Generally speaking, when you have two forms, only one of them should be aware of the other. One form will create the other and the one being created shouldn't really have to know that the one that created it exists.

If the creator wants to change something on the created, it should either set a property or call a method. The code inside that property or method will then update the appropriate control(s), so the created form is updating its own controls and the creator doesn't even have to know that the controls exist.

Conversely, if the created wants to change something on the creator, it should raise an event. The creator handles that event and then, again, updates its own controls. This is the one and only "proper" way for forms to pass data around.

[taigon]

Hi jmc,

If you don't mind, could you possibly post a very simple example with 2 forms showing how to properly update say a textbox for example. With a string generated on Form2, how can I update the textbox.text with the string value?