CODEWITHSUNDEEP
c#.netwpf
Tameen Malik
Tameen Malik

Reputation: 1268

object reference is null

There is one MainWindow and one user-control in my WPF application. I want to call function of my MainWindow in user-control, without creating new instance of MainWindow. For this i made main-window parent of user-control. I wrote this code below for calling function of Parent.

Child User-Control

public partial class AppLogo : UserControl
    {
    public MainWindow myparent { get; set; }
       private void activate_Click_1(object sender, RoutedEventArgs e)
        {
           myparent.function();

         }
          . . .
     }

Parent Window:

      public MainWindow()
          {
            InitializeComponent();
             AppLogo childWindow = new AppLogo(); 
       . . .

Questions:

  1. Is It Possible to create Window a parent of user-control?
  2. If answer of above question is Yes then why it is generating error that Object Reference is Null.
  3. If answer is No it is not possible then how can i achieve this goal. As it is necessary to create user-control in my application as it is requirement.

Upvotes: 0

Views: 434

Answers (5)

kmatyaszek
kmatyaszek

Reputation: 19296

If you want to have reference in UserControl to MainWindow use following code:

MainWindow mw = Application.Current.MainWindow as MainWindow;

http://msdn.microsoft.com/en-us/library/system.windows.application.mainwindow.aspx

private void activate_Click_1(object sender, RoutedEventArgs e)
{
    MainWindow mw = Application.Current.MainWindow as MainWindow;
    if(mw != null)
    {
        mw.function();
    }
}

Second solution:

In your code you should set myparent property in MainWindow constructor:

public MainWindow()
{
    InitializeComponent();
    AppLogo childWindow = new AppLogo(); 
    childWindow.myparent = this;
    ...
}

In activate_Click_1 event handler the good habit is check if myparent is not null:

private void activate_Click_1(object sender, RoutedEventArgs e)
{
    if(myparent != null)
        myparent.function();
    else
        ...
}

Upvotes: 1

AgentFire
AgentFire

Reputation: 9790

  1. If you locate your UserControl directly inside your Window, then its Parent property will reference your window.

  2. The exception is called when you try to access the field which contains null value. It contains null because noone placed there anything else. You might want to set it:

    AppLogo childWindow = new AppLogo(); 
childWindow.myparent = <something>;

  3. You just need to recursively search your UserControl's parents until you get an instance of a Window, which will be your goal.

public static Window GetWindow(FrameworkElement element)
{
    return (element.Parent as Window) ?? GetWindow(element.Parent);
}

Upvotes: 0

musical_coder
musical_coder

Reputation: 3896

You need to pass a reference to the instance of MainWindow as an argument to AppLogo's constructor, then set it to AppLogo's variable for MainWindow.

public AppLogo(MainWindow mainWindow)
{
    this.myparent = mainWindow;
}

Upvotes: 0

Gary C.
Gary C.

Reputation: 116

You may introduce a child parent dependecy as suggested, however since you didn't instantiate MainWindow you should expect a null reference exception when calling myparent.function();

First, you need to instantiate MainWindow, and then set the child parent relationship by calling AppLogo .set_myparent, only then your call won't fail.

Upvotes: 0

Trevor Pilley
Trevor Pilley

Reputation: 16423

I'm assuming that the null reference is for the myparent property on AppLogo?

After this line AppLogo childWindow = new AppLogo(); add one saying childWindow.myparent = this;

Upvotes: 1

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