Question

I’m trying to validate the text in a textbox when a key is pressed. Here’s the shortest code sample that shows what I’m trying to do:

<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 FontSize="15" HorizontalAlignment="Left" Name="txtEmail" VerticalAlignment="Top" Width="135"
                 Text="{Binding ValidationRules.EmailAddress, ValidatesOnExceptions=True, UpdateSourceTrigger=PropertyChanged}"/>
    </Grid>
</Window>

“ValidationRules” class:

 class ValidationRules
    {
        string email = "";
        public string EmailAddress
        {
            get
            {
                return email;
            }

            set
            {
                Console.WriteLine("Setting!");
                //Only check if there is any text for now...
                if (String.IsNullOrWhiteSpace(value))
                {
                    throw new Exception();
                }

                email = value;
            }
        }

    }

When I start typing in the textbox, I don’t get “Setting” as console output, even though I’m using UpdateSourceTrigger=PropertyChanged. I’ve done my research but all the examples I could find are long winding and confusing. I would also appreciate it if you could point out any other mistakes I have in validation, but try to explain in simple terms if possible because I’m new to WPF.

Answer 1

It must be an issue on where you are setting your DataContext.

This example seems to work fine:

Code:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        ValidationRules = new ValidationRules();
    }

    private ValidationRules _validation;
    public ValidationRules ValidationRules
    {
        get { return _validation; }
        set { _validation = value; NotifyPropertyChanged("ValidationRules"); }
    }


    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(string property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }
 }

public class ValidationRules : INotifyPropertyChanged
{
    string email = "";
    public string EmailAddress
    {
        get
        {
            return email;
        }

        set
        {
            Console.WriteLine("Setting!");
            //Only check if there is any text for now...
            if (String.IsNullOrWhiteSpace(value))
            {
                throw new Exception();
            }
            email = value;
            NotifyPropertyChanged("EmailAddress");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(string property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }

}

Xaml

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Class="WpfApplication1.MainWindow"
        Title="MainWindow" Height="125.078" Width="236.441" x:Name="UI" >
    <Grid DataContext="{Binding ElementName=UI}">
        <TextBox FontSize="15" HorizontalAlignment="Left" Name="txtEmail" VerticalAlignment="Top" Width="135"
                 Text="{Binding ValidationRules.EmailAddress, ValidatesOnExceptions=True, UpdateSourceTrigger=PropertyChanged}"/>
    </Grid>
</Window>