I took [Praetorian's answer][answer] and made an extension class that inherits `TextBox` so you don't have to muddle up your view's code behind with this behavior.

**C-Sharp**:



    public class TextBoxUpdate : TextBox
    {
        public TextBoxUpdate()
        {
            TextChanged += OnTextBoxTextChanged;
        }
        private void OnTextBoxTextChanged(object sender, TextChangedEventArgs e)
        {
            TextBox senderText = (TextBox)sender;
            BindingExpression bindingExp = senderText.GetBindingExpression(TextBox.TextProperty);
            bindingExp.UpdateSource();
        }
    }

**VisualBasic**:



    Public Class TextBoxUpdate : Inherits TextBox

        Private Sub OnTextBoxTextChanged(sender As Object, e As TextChangedEventArgs) Handles Me.TextChanged
            Dim senderText As TextBox = DirectCast(sender, TextBox)
            Dim bindingExp As BindingExpression = senderText.GetBindingExpression(TextBox.TextProperty)
            bindingExp.UpdateSource()
        End Sub

    End Class

Then call like this in **XAML**:



    <local:TextBoxUpdate Text="{Binding PersonName, Mode=TwoWay}"/>


[answer]: https://stackoverflow.com/a/4833196/1366033

Is there a way to get a TextBox in Windows Phone 7 to update the Binding as the user types each letter rather than after losing focus? 

Like the following WPF TextBox would do:



    <TextBox Text="{Binding Path=TextProperty, UpdateSourceTrigger=PropertyChanged}"/>