Como posso ativar um método na mainwindow de uma outra janela em C# WPF?

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Hello, I'm new to the language, I would like to know if anyone has a solution or example to help me.

I would like that when I click on a button in a secondary window, a new tab is created by executing a method in the main window MainWindow. I'm trying to use the code below but it throws an exception saying that the MainWindow returned null.

Message:

System.NullReferenceException: 'Referência de objeto não definida para uma instância de um objeto.' (... as Projeto.MainWindow) retornou null.

Thank you for your help

What I have tried:

Secondary window button

private void Selecionar_Click(object sender, RoutedEventArgs e)
        {
            (Application.Current.MainWindow as MainWindow).CrianovaAba();
        }

Method in MainWindow

public void CrianovaAba()
        {
            UserControl Nuser = new UserControl();

            ClosableTab theTabItem = new ClosableTab();
            theTabItem.Title = UserControl._Novomaterial;
            theTabItem.Content = Nuser;
            TabControlMaterial.Items.Add(theTabItem);
            theTabItem.Focus();
        }

Posted 5-Sep-22 4:34am

Rodrigo_F_P

Updated 5-Sep-22 5:47am

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Comments

Chris Copeland 6-Sep-22 5:05am

As Griff said, use the debugger by putting a break-point on the line where you call CrianovaAba(); and see what the value of Application.Current.MainWindow is. The as cast will return null if the value of that property is not a MainWindow instance.

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OriginalGriff · Answer 1 · 2022-09-05T05:47:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!