Does setting a control property remove any binding?

Question

I've read that setting a control property to a specific value will replace any binding on that property, and I could swear I've run into that being the case.  But, recently, I was quite surprised to find that a case like this appears to work just fine - it sets the value and keeps the binding intact:

This is in SL 4 - In a view, I've got something like this: (actually in a UserControl inside a view) (DataContext is my VM)

                <CheckBox Name="checkBox1"
                          Content="Check Me"
                          IsChecked="{Binding MyBool, Mode=TwoWay}" />

In the codebehind for the UserControl, we have something like this:

checkBox1.IsChecked = true;

I would have expected that this would erase the binding, but it seems to carry on just fine.

Are there some cases where setting a property directly in code will undo a binding?

Maybe I'm just having flashbacks of fighting with ComboBox's SelectedValue issues...

Thanks,

Dave

Answer 1

What you have read is wrong. Setting value directly coresponds to some user interaction, more or less. Has no influence on binding.

Regards, Greg

Answer 2

Hi,

That doesn't clear any binding. Because we use clearbing class for that.

Answer 3

So just to make sure...this quip should go on the stack of MSDN Silverlight stuff that isn't quite right, or is there some difference I'm missing?

 Bindings are treated as a local value for purposes of dependency property value precedence, which means that if you set another local value, you will eliminate the binding.

Pasted from <http://msdn.microsoft.com/en-us/library/cc221408(v=VS.95).aspx>

 

 

Thanks,

 

Dave

Answer 4

Maybe I'm missing something, but I think the key is that you have a two-way binding.  Being able to set a value from the target control is really the whole purpose of a two-way binding.

If you change it to a one-way binding and then set the value, I think you'll see the behavior that you expected.  MSDN says "eliminate", but honestly I usually think of it as "breaking the binding", because when this happens it's usually not what I intended.

Answer 5

Maybe now I understand your point. For example you can set in XAML Text property to some expression involving Binding. When you now set in your code Text to some local string - what happens? I bet your binding set in XAML is eliminated.

            var be1 = textBox1.GetBindingExpression(TextBox.TextProperty);
            textBox1.Text = "LoLO";
            var be2 = textBox1.GetBindingExpression(TextBox.TextProperty);
           // here be2 is null

Answer 6

I would have expected that this would erase the binding, but it seems to carry on just fine.

Hi Dave,

of course it erases the binding:

            var ce1 = checkBox1.GetBindingExpression(CheckBox.IsCheckedProperty);
            // ce1 is NOT null
            checkBox1.IsChecked = false;
            var ce2 = checkBox1.GetBindingExpression(CheckBox.IsCheckedProperty);
            // ce2 is null
            // will binding still work?
            IsChecked = true;
            // of course not

My previous reply was wrong, sorry

Answer 7

Thanks Greg,

I think the whole issue was that I was expecting the XAML bindings to work just the same as adding coded bindings.  Think I've got it all straightened out. 

 

-Dave