Access to the elements of parent form from child in MDI

Question

The problem is that i need to access different forms elements from other forms, or for example access MdiContainer form menu from some child windows and do some operations with it. How to correctly implement such feature? I'm using Windows Forms now.

Some sample code below to demonstrate how I tried to do it.

Form1 (is an Mdi container)

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

    private void menu2ToolStripMenuItem_Click(object sender, EventArgs e)
    {
        Form2 chWin = new Form2(this);
        chWin.Show();
    }

    public void disableMenu()
    {
        menuStrip1.Enabled = false;
    }
}

Form2 (is a child of Form1)

public partial class Form2 : Form
{
    private Form1 parent;

    public Form2(Form1 parent)
    {
        InitializeComponent();
        MdiParent = parent;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        parent.disableMenu();
    }
}

And the exception: Object reference not set to an instance of an object

Tried to google on it, but actually nothing helpful for my occasion.

Thanks in advance!

Mikael Koskinen Mikael Koskinen · Accepted Answer · 2010-10-02T19:34:06

You're trying to access a member which hasn't been initialized. In the Form2 constructor you're just setting the MdiParent-property, but you have forgotten this line:

this.parent = parent;

Also, you don't have to pass the Form1 into the Form2's constructor. You can avoid this by setting the MdiParent before showing the Form2:

private void menu2ToolStripMenuItem_Click(object sender, EventArgs e)
{
    Form2 chWin = new Form2(this);
    chWin.MdiParent = this;
    chWin.Show();
}

Then in Form2 you can call the method of Form1 by casting the MdiParent to Form1:

private void button1_Click(object sender, EventArgs e)
{
    ((Form1)MdiParent).disableMenu();
}

Though instead of doing this you probably should first create an interface:

public interface IMenuHandler
{
    void DisableMenu();
}

Then make the Form1 to implement this interface and pass it to the Form2 in its constructor. The whole implementation of Form2 would then look like this:

public partial class Form2 : Form
{
    private IMenuHandler menuHandler;

    public Form2(IMenuHandler handler)
    {
        InitializeComponent();
        this.menuHandler = handler;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        menuHandler.DisableMenu();
    }
}

This way your Form2 doesn't directly depend on the Form1 and it's easier for you to change either of the forms.

Access to the elements of parent form from child in MDI

2 Answers