Give value to TextBlock.Text of ViewModel in Window1 and use that value of TextBlock.Text to Window2

Question

I am running into a peculiar problem with MVVM and WPF application. I will explain it briefly to make it as clear as possible.

First of all I have binded the text of a TextBlock property in the XAML file

XAML

<TextBlock
    x:Name="ProjectCredentials"
    Text="{Binding Path=HeaderName}"/>

View Model - created in MainWindow

namespace Test
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

    public class MainWindowViewModel : INotifyPropertyChanged
    {
        private string _tabitemHeader;
        public string HeaderName
        {
            get
            {   
                return _tabitemHeader;
            }
            set
            {
                _tabitemHeader = value;
                OnPropertyChanged("HeaderName");
            }
        }
    }
    // Continue in next code block
}

Below is the LoginScreen Window or Window 1 from where I want to give value to my TextBlock.Text property.

namespace Test
{
    public partial class LoginScreen : Window
    {
        public LoginScreen()
        {
            InitializeComponent();
        }

        private void LoginButton_Click(object sender, RoutedEventArgs e)
        {
            MainWindow win2 = new MainWindow();

            win2.DataContext = new MainWindow.MainWindowViewModel();

            MainWindow.MainWindowViewModel object_change = win2.DataContext as MainWindow.MainWindowViewModel;

            object_change.HeaderName = "Project Name: " + SQLSeverName.Text + " - " + SQLDatabasesComboBox.SelectedItem;
            Debug.WriteLine(object_change.HeaderName);
            //Successfully returns the string
        }
    }
}

Now that I have given the value "Project Name: ..." I should call this string in MainWindow inside the method GetHeaderDetails()

namespace Test
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

    public class MainWindowViewModel : INotifyPropertyChanged
    {
        private string _tabitemHeader;
        public string HeaderName
        {
            get
            {   
                return _tabitemHeader;
            }
            set
            {
                _tabitemHeader = value;
                OnPropertyChanged("HeaderName");
            }
        }
    }
    public List<string> GetHeaderDetails()
    {
        string projectdetails_option1 = ?.HeaderName; //how can  I call the HeaderName of MainWindow DataContext which is already filled from LoginScreen window?

        Debug.Assert(projectdetails_option1 != null, "Project name not found!"); //this returns null

        string connectiondetails = projectdetails_option1.Split(": ")[1];

        string servernamedetails = connectiondetails.Split(" - ")[0].Trim();
        string projectnamedetails = connectiondetails.Split(" - ")[1].Trim();

        List<string> connectiondetailslist = new List<string>();
        connectiondetailslist.Add(servernamedetails);
        connectiondetailslist.Add(projectnamedetails);

        return connectiondetailslist;
    }

    public List<string> ConnectionDetailsList { get => GetHeaderDetails(); set => ConnectionDetailsList = value; }
}

The problem here is how can I call the HeaderName of MainWindow DataContext which is already filled from the LoginScreen window?

I know that if I open a new instance of the MainWindow DataContext in the GetHeaderDetails() method I will get a NullReferenceException because the HeaderName will be lost and a new DataContext will be introduced. So I want to find a way to bypass this.

Please note that I am aware of the NullReferenceException . Although, I am looking for answers that may help me to understand how to locate the problem.

Regards.

Answer 1

I have figured it out, finally. To retrieve the value of the TextBlock.Text value in MainWindow, the only thing that I had to do is:

Instead of this:

string projectdetails_option1 = ?.HeaderName;

I tried this:

string projectdetails_option1 = ProjectCredentials.Text; //The TextBlock that got its value from LoginScreen.

And successfully I retrieved the value of the ProjectCredentials textblock, without opening a new instance of the DataContext.