Form returning null after coming back to the initial user control

Question

I am new to Windows forms and having an issue handling all the user controls. I have 3 User Controls, and when I click a accept button it takes me to the second screen (which is user control 2) but then when I click cancel on the second screen it brings me back to the first screen (I load the first user control again) the issue now is that when I click again "Accept" the welcome user control returns null and errors.

private void Viewer_Load(object sender, EventArgs e) { formPanel.Controls.Clear(); formPanel.Controls.Add(wel); }

    private void SwapControls(object sender, EventArgs e)
    {
        if (formPanel.Controls.Contains(wel))
        {
            formPanel.Controls.Remove(wel);
            formPanel.Controls.Add(p);
        }
        else if (formPanel.Controls.Contains(pin) && IsAuthenticated)
        {
            formPanel.Controls.Remove(p);
            formPanel.Controls.Add(m);
        }
        else if(formPanel.Controls.Contains(pin) && !Global.IsAuthenticated)
        {
            formPanel.Controls.Remove(p);
            formPanel.Controls.Add(wel);
        }

So the first time around it loads up the welcome user control, then there I click "Accept" and it clears the User control and loads up the second one "Enter Pin Control", from there when I click "Cancel" I remove that User Control and Load up again Welcome. BUT now, when I click Accept, I get a null in this line in the welcome user control

 this.AddControl(this, new EventArgs());

Again, I am new to windows forms and I am learning, any inputs would be much appreciated.

Mark Hall · Accepted Answer · 2012-04-01 05:11:54Z

3

Since you are reusing your UserControls don't remove the handlers when you remove them from the Form, just make sure you remove them when you are finished using your UserControls.

Try something like this.

private void SwapControls(object sender, EventArgs e) 
{ 
    if (formPanel.Controls.Contains(wel)) 
    { 
        formPanel.Controls.Remove(wel); 
        formPanel.Controls.Add(pin); 
    } 
    else if (formPanel.Controls.Contains(pin) && Global.Instance.IsAuthenticated) 
    { 
        formPanel.Controls.Remove(pin); 
        formPanel.Controls.Add(mmenu); 
    } 
    else 
    { 
        formPanel.Controls.Remove(pin); 
        formPanel.Controls.Add(wel); 
    } 
}

/// <summary>
/// Clean up any resources being used.
/// </summary>
/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
protected override void Dispose(bool disposing)
{
    wel.AddControl -= new EventHandler(SwapControls);
    pin.AddControl -= new EventHandler(SwapControls);
    pin.ReturnWelcome -= new EventHandler(SwapControls); 

    if (disposing && (components != null))
    {
        components.Dispose();
    }
    base.Dispose(disposing);
}

edited Apr 1, 2012 at 5:11

answered Apr 1, 2012 at 4:51

Mark Hall

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4 Comments

user710502 Over a year ago

Thank you that worked, the only thing i dont understand is when i need to remove the handlers... can I just leave them there?.

Mark Hall Over a year ago

If your UserControls are always instantiated I would just remove them in the Dispose Method of your Form

user710502 Over a year ago

I am sorry, how would i remove a user control? in the dispose section?

Mark Hall Over a year ago

See this MSDN article. I was talking about the Handlers not the Controls.

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