UWP Binding to a property - xaml

Question

I am making a UWP and cannot correctly grasp DataBinding and INotifyPropertyChanged
I am trying to bind some TextBox in a ContentDialog to properties in my code-behind cs file.
Here's my view model:
class UserViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged = delegate { };
public string _fname { get; set; }
public string _lname { get; set; }
public string Fname
{
get { return _fname; }
set
{
_fname = value;
this.OnPropertyChanged();
}
}
public string Lname
{
get { return _lname; }
set
{
_lname = value;
this.OnPropertyChanged();
}
}
public void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Code behind:
public sealed partial class MainPage : Page
{
UserViewModel User { get; set; }
public MainPage()
{
this.InitializeComponent();
User = new UserViewModel();
}
....
....
private void SomeButton_Click(object sender, TappedRoutedEventArgs e)
{
//GetUserDetails is a static method that returns UserViewModel
User = UserStore.GetUserDetails();
//show the content dialog
ContentDialogResult result = await UpdateUserDialog.ShowAsync();
}
}
Here's the XAML for the ContentDialog:
<ContentDialog Name="UpdateUserDialog">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*"></ColumnDefinition>
<ColumnDefinition Width="1*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<TextBox Grid.Row="0"
Grid.Column="0"
Grid.ColumnSpan="2"
Name="tbFirstNameUpdate"
Text="{x:Bind Path=User.Fname, Mode=OneWay}"
Style="{StaticResource SignUpTextBox}"/>
<TextBox Grid.Row="1"
Grid.Column="0"
Grid.ColumnSpan="2"
Name="tbLastNameUpdate"
Text="{x:Bind Path=User.Lname, Mode=OneWay}"
Style="{StaticResource SignUpTextBox}"/>
</ContentDialog>
NOTE: Binding works well when I initialize the view model in the MainPage constructor itself like this:
User = new UserViewModel { Fname = "name", Lname = "name" };

Answer 1

You don't fire a PropertyChanged event when you replace the value of the User property with a new view model instance.
You may however simply replace
User = UserStore.GetUserDetails();
by
var user = UserStore.GetUserDetails();
User.Fname = user.Fname;
User.Lname = user.Lname;
and hence update the existing instance of your view model.

Answer 2

You should set the DataContext property to the view model instance:
public MainPage()
{
this.InitializeComponent();
User = new UserViewModel();
DataContext = User;
}
See: https://learn.microsoft.com/en-us/windows/uwp/data-binding/data-binding-in-depth