Why does data binding break in OneWay mode?

Question

Here's a little XAML fragment. You will see

<StackPanel>
     <TextBox x:Name="txtValue">250</TextBox>
     <Slider x:Name="slide" 
             Value="{Binding ElementName=txtValue, Path=Text, 
                             Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" 
            Minimum="0" Maximum="500"></Slider>
</StackPanel>

1. when you change the textbox value, the slider updates
2. If you change the slider value explicitly, the previous behavior breaks a.k.a. stops working.

If I remove the Mode=OneWay set directive, (defaults to two-way) everything works perfectly.

Why is this happening?

Answer 1

Use mode=TwoWay and set the UpdateSourceTrigger=Explicit.

Answer 2

Your data binding is not broken but deactivated (http://en.wikipedia.org/wiki/Euphemism):

System.Windows.Data Warning: 75 : BindingExpression (hash=52697953): Deactivate
System.Windows.Data Warning: 99 : BindingExpression (hash=52697953): Replace item at level 0 with {NullDataItem}
System.Windows.Data Warning: 59 : BindingExpression (hash=52697953): Detach

Setting the trace level to high will produce this message in the VS output window in case you move the slider:

<Slider xmlns:trace="clr-namespace:System.Diagnostics;assembly=WindowsBase"
        Value="{Binding trace:PresentationTraceSources.TraceLevel=High,
            ElementName=txtValue, Path=Text, Mode=OneWay,
            UpdateSourceTrigger=PropertyChanged}"
        Minimum="0" Maximum="500"></Slider>