WPF/C# - Content Binding Was Working, Now Isn't

Question

EDIT: THIS CODE IS AN UPDATED VERSION USING THE RECOMMENDATIONS BELOW BUT STILL RESULTING IN THE SAME PROBLEM. LINK TO SAMPLE IMAGE OF PROBLEM, [EXAMPLE IMAGE].

This code is meant to update the "AddUserNewUsername" label on a tabitem for creating a new user account for my little program. It use to update the label once the user left the "First Name" and "Last Name" fields. However, after some testing, it looks like the "AddUserNewUsername" string IS being populated, but not updating from blank on the GUI.

I'm really confused. I'm new to binding but I don't really understand how this broke. The only thing I changed was the variable from "NewUsername" to "AddUserNewUsername" and updated everything along with it. Below is a snippet of the relevant code.

MainWindow XAML:

    <Label Name="add_user_uname" Grid.Column="5" Grid.ColumnSpan="6" Grid.Row="7" 
           Content="{Binding Path=AddUserNewUsername, Mode=OneWay}" 
           Style="{DynamicResource add_user_label_uname}"/>

    <TextBox Name="add_user_finame" Grid.Column="5" Grid.ColumnSpan="6" Grid.Row="2" 
             Text="{Binding Path=AddUserFirstName, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}" 
             Style="{DynamicResource add_user_text_box_std}"/>

    <TextBox Name="add_user_lname" Grid.Column="5" Grid.ColumnSpan="6" Grid.Row="3" 
             Text="{Binding Path=AddUserLastName, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}" 
             Style="{DynamicResource add_user_text_box_std}"/>

MainWindow Code Behind:

    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = id;
    }

    public Instance_Data id = new Instance_Data();

User_Management Class (Just to show where the message box comes from.):

public static void AttemptUserCreation(MainWindow parent, string finame, string lname, string email, string pword, string verify_pword, string uname, string current_user)
    {
        MessageBox.Show(parent.id.AddUserNewUsername);
        return;

Instance_Data Class:

public class Instance_Data : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    private string _addUserFirstName;
    private string _addUserLastName;

    public string AddUserFirstName
    {
        get { return _addUserFirstName; }
        set
        {
            _addUserFirstName = value;
            OnPropertyChanged("AddUserFirstName");
            OnPropertyChanged("AddUserNewUsername");
        }

    }
    public string AddUserLastName
    {
        get { return _addUserLastName; }
        set
        {
            _addUserLastName = value;
            OnPropertyChanged("AddUserLastName");
            OnPropertyChanged("AddUserNewUsername");
        }
    }
    public string AddUserNewUsername
    {
        get
        {
            if (!String.IsNullOrEmpty(AddUserFirstName))
            {
                return AddUserFirstName[0] + AddUserLastName;
            }
            else
            {
                return AddUserFirstName;
            }
        }
    }

add_user_label_uname Style as requested:

    <Style x:Key="add_user_label_uname" TargetType="{x:Type Label}">
        <Setter Property="HorizontalAlignment" Value="Left"/>
        <Setter Property="FontFamily" Value="Tahoma"/>
        <Setter Property="FontSize" Value="10"/>
        <Setter Property="Foreground" Value="#B45A00"/>
    </Style>

I spent the end of my work day yesterday trying to figure this out and cannot seem to find my hiccup. Hopefully I provided enough data but should you need more just ask! Thank you!

Answer 1

After much probing, we discovered that a seemingly innocuous line in your C# code was clearing the Binding on your Label:

parent.add_user_uname.Content = "";

When you set a dependency property using its set accessor, you set a local value. That value will clear any OneWay bindings that had been applied to the same property.

As a rule, if you are going to be binding against some sort of view model or data model, you should stick to manipulating the model rather than the view that binds to it. Following that approach would have saved you hours of frustrated debugging. On the upside, you won't make that mistake again :).

Dependency properties have many subtleties. Not only will setting a local value fully replace the previous value (including a one-way, source-to-target binding), but it will override values being supplied by setters and triggers too. These subtleties invariably cause some confusion for new WPF developers. If you want to succeed at WPF development, you should dive into the detailed MSDN documentation. I also recommend Adam Nathan's excellent book WPF 4.5 Unleashed. Chapter 3, the most relevant to this subject, used to be available for free online, and it still might be.

As @Iqon points out, you also had a potential NullReferenceException in your AddUserNewUsername accessor

Answer 2

The only possible way of breaking I see is in the getter of AddUserNewUsername. If AddUserFirstName is null, you will get a NullReferenceException, because you only check if the string is empty.

Change the getter to:

public string AddUserNewUsername
{
    get
    {
        if (!String.IsNullOrEmpty(AddUserFirstName))
        {
            return AddUserFirstName[0] + AddUserLastName;
        }
        else
        {
            return AddUserFirstName;
        }
    }
}