How to access form controls from separate class?

paket · December 17th, 2005, 02:23 PM

I am trying to figure out how to change the properties of controls residing on a form from a separate class. I don't want to make the form controls public (I'm assuming the Windows Form Designer is smarter than I am ;). I created a sample project with a label, two buttons and an external class. (See code below)

One button sends a string to a method in an external class (imaginitively named external_class), which calls an accessor method to change the label. The other button calls the accessor method directly.

The problem is that when the accessor method is called from the external class, it fails silently - it doesn't change the text property of the label. When the accessor method is called via the other button, directly, without going through the external class, it works.

Can anyone explain to me what is going on here?
thanks

Code:

form1.cs:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace FormAccessTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btn_change_Click(object sender, EventArgs e)
        {
            external_class obj = new external_class();
            obj.chng_lbl("Change via External class");
        }

        public void accessor_method(string labeltext)
        {
            MessageBox.Show("accessor_method call: "+labeltext);
            lbl_status.Text = labeltext;
        }

        private void btn_change2_Click(object sender, EventArgs e)
        {
            // why does calling accessor_method from here work, 
        //but not from external_class.chng_lbl ?
            accessor_method("Direct Change");
        }

        private void lbl_status_Click(object sender, EventArgs e)
        {

        }
    }
}

external_class.cs:
using System;
using System.Collections.Generic;
using System.Text;

namespace FormAccessTest
{
    class external_class
    {
        Form1 frm_UI;
        public external_class()
        {
            frm_UI = new Form1();
        }

        public void chng_lbl(string desired_label)
        {
                frm_UI.accessor_method(desired_label);
        }
    }
}

mehdi62b · December 17th, 2005, 04:50 PM

pass the correct argument to the constructor

Code:

namespace FormAccessTest
{
    class external_class
    {
        Form1 frm_UI;
        public external_class(Form1 form1)
        {
            frm_UI = form1;
        }

        public void chng_lbl(string desired_label)
        {
                frm_UI.accessor_method(desired_label);
        }
    }
}

Code:

        private void btn_change_Click(object sender, EventArgs e)
        {
            external_class obj = new external_class(this);
            obj.chng_lbl("Change via External class");
        }

_____________
Mehdi.
software student.

planoie · December 18th, 2005, 03:15 PM

Mehdi's suggestion is only good for when you initially create the form.

You can leave the form's controls private but provide access to them thru standard property accessors. Chances are that you only need a few properties of a few controls so there's not reason to publicize the whole control. Instead just make the properties you need public. This also reduces later problems if you need to change the control types.

-Peter

Philip Taylor · July 31st, 2007, 01:40 AM

Does anyone know how to do this but access the contents of text field:
ex
frm_UI = new Form1();

frm_UI.textBox1.text;

doesn't appear to be immediately possible

David_0223 · July 31st, 2007, 08:14 AM

Phillip,

The earlier messages in this thread explain this. To elaborate, textBox1 is a private member of Form1 so cannot be accessed from an instance of Form1 (frm_UI is an instance).

To make the information available from the instance set up a 'get' accessor ...
public string textBox1Text {
  get {
    return(this.textBox1.text);
  }
}

What you don't know can hurt you!

nyasha_c · August 1st, 2007, 04:06 AM

Exactly what i was about to say

www.xhydra.com

Philip Taylor · August 1st, 2007, 06:18 AM

Thanks ever so much. guess I wasn't reading through properly. Sorry.