OneWayToSource Binding seems broken in .NET 4.0

OneWayToSource Binding seems broken in .NET 4.0

I have this simple piece of Xaml

<StackPanel>
    <TextBox Text="{Binding TextProperty, Mode=OneWayToSource}"/>
    <Button/>
</StackPanel>

And my code behind looks like this

public MainWindow()
{
    InitializeComponent();
    this.DataContext = this;
}
private string m_textProperty;
public string TextProperty
{
    get
    {
        return "Should not be used in OneWayToSource Binding";
    }
    set
    {
        m_textProperty = value;
    }
}

In .NET 3.5 this works as you might except. Put some text in the TextBox, press Tab to make it lose Focus, and the TextProperty updates with whatever text that was entered in the TextBox

In .NET 4.0, if I type some text in the TextBox and then press Tab to make it lose Focus, the TextBox reverts to the value for TextProperty (meaning "Should not be used in OneWayToSource Binding"). Is this re-reading intended for a OneWayToSource Binding in .NET 4.0? I just want the TextBox to push its value into the TextProperty and not the other way around.

Update
Adding a Bounty to this question as this has become a mayor inconvenience in my project and I would like to know the reason that this has changed. It seems that get is called after the Binding has updated the source. Is this the desired behavior for a OneWayToSource Binding in .NET 4.0?

If Yes

What was the problem with the way it worked in 3.5?
In what scenarios is this new behavior better?

Or is this in fact a bug that we can hope to get fixed in a future release?

c# wpf xaml binding

Edit Report

Is this the desired behavior for a OneWayToSource Binding in .NET 4.0?

Yes. This is done for developer's ability to change provided value without clumsy converters.

What was the problem with the way it worked in 3.5?

No problem. The way it worked in 3.5 didn't allow to correct provided values.

In what scenarios is this new behavior better?

When you need to correct provided values. If you don't need it, then you should just write correct property's getter and setter.

public string TextProperty
{
    get;
    set;
}

However, as I can see, changing UpdateSourceTrigger to "PropertyChanged" preserves values from being reread (so you could leave old property declaration):

<StackPanel>
    <TextBox Text="{Binding TextProperty, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"/>
    <Button/>
</StackPanel>

    private string m_textProperty;
    public string TextProperty
    {
        get
        {
            return "Should not be used in OneWayToSource Binding";
        }
        set
        {
            m_textProperty = value;
        }
    }

answered 2011-02-07T08:29:38.257

OneWayToSource Binding seems broken in .NET 4.0

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