Question

I tried following code but it gave null reference error for second line. I deleted second line but now it gave same error for third line. Any idea?

Binding binding = BindingOperations.GetBinding(txt_density, TextBox.TextProperty);
binding.ValidationRules.Clear();
binding.ValidationRules.Add(new MainWindow.Float_Positive_ValidationRule());

Answer 1

You haven’t included the relevant XAML so it is hard to say definitively what the issue is.

If you set a break point on the line binding.ValidationRules.Clear(); you will most likely find your binding is null, probably because there has been no binding set on the Text property.

If there is a valid binding set on the property then the ValidationRules collection should be initialised but empty (i.e. not null), so this shouldn’t be your problem, which leaves the binding itself as the problem.

If you use this simple example you will see what I mean, just remove the Text binding from the XAML to cause the error.

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBox x:Name="txt_density" Text="{Binding SomeText}"  />
    </Grid>
</Window>


namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            SomeText = "blah";
            this.DataContext = this;
            this.Loaded += new RoutedEventHandler(MainWindow_Loaded);
        }

        void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            Binding binding = BindingOperations.GetBinding(txt_density, TextBox.TextProperty);
            binding.ValidationRules.Clear();
            //binding.ValidationRules.Add(new MainWindow.Float_Positive_ValidationRule());
        }

        public string SomeText { get; set; }

    }
}