Property in a Windows Forms form class only accessable after Load event

Question

I'm instantiating and calling Form B from Form A. FormB has some custom properties, to allow me to pass it things like sqlAdaptors and dataset instances.

When i instantiate and show Form B from Form A as a dialog form with a Using statement, it all works fine, but i find the properties i pass are not available in Form B until after the form_load event has fired.

I was under the impression the properties when passed to a instantiated class should be available from a constructor, but this is not the case. If it try to access the properties before the form load event i get a null reference exception.

Is this correct behavior ?

Answer 1

Move all the Form B internal variable initialization to its constructor

Here is how your formA will look like. It has 2 buttons: One simply initializes an instance of formb and call the public property. The other button displays formB. Form_load is only called when you show the form to the user via Show() or ShowDialog() calls.

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

namespace formload
{
    public partial class FormA : Form
    {
        public FormA()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            FormB frm = new FormB();
            MessageBox.Show(frm.MyProperty);
            frm = null;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            FormB frm = new FormB();
            frm.ShowDialog();
            MessageBox.Show(frm.MyProperty);
            frm = null;
        }
    }
}

Here is how formb would look:

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

namespace formload
{
    public partial class FormB : Form
    {
        public FormB()
        {
            InitializeComponent();
            myPropString = "Default set via constructor";
        }

        private void FormB_Load(object sender, EventArgs e)
        {
            myPropString = "Set from form load";
        }

        private string myPropString;

        public string MyProperty
        {
            get { return myPropString; }
            set { myPropString = value; }
        }

    }
}

Answer 2

If you're doing

FormB frm = new FormB();      ' Constructor runs now
frm.MyProperty = "whatever";  ' Now you set the property
frm.ShowDialog()

Obviously, you're running the constructor before you set the property! You can't access the property until after you set it.