How can I create a binding in code-behind that doesn't specify a Path?

Question

I was answering another question about creating a binding in code-behind, and my original attempt at answering it was to post binding code that didn't specify a Path. This binding compiles fine, however the value is never updated. If I change the binding to use a Path, it works fine.

Why is this? And what is the correct way to create a binding in code-behind that doesn't have a Path? For example, how would I recreate Value="{Binding }" in code-behind?

Non-Working code:

Binding b = new Binding(); b.Source = SomeInt; b.Mode = BindingMode.OneWay; MyProgressBar.SetBinding(ProgressBar.ValueProperty, b); SomeInt = 50;

Working code:

Binding b = new Binding(); b.Source = this; b.Path = new PropertyPath("SomeInt"); MyProgressBar.SetBinding(ProgressBar.ValueProperty, b); SomeInt = 50;

brunnerh · Accepted Answer · 2013-11-06 02:42:07Z

3

The binding engine subscribes to INPC and DP-changes on the Source object (and non-leaves on the Path), and checks whether the Path property/properties was/were changed. If there is no Path there are no notifications. A rather unfortunate drawback.

(I might be oversimplifying the system a bit but the essence is that there are no updates to source-changes, they are not and cannot be monitored)

{Binding} is equivalent to new Binding() (no additional properties), this binding may update as there are events for DataContext changes.

edited Nov 6, 2013 at 2:42

answered Dec 1, 2011 at 15:36

brunnerh

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9 Comments

Joe White Over a year ago

That's probably true too: INPC says which property changed. There's no mechanism to say "the whole object changed".

Rachel Over a year ago

@JoeWhite However I can set my binding to nothing ({Binding }), and the PropertyChange notification works just fine. Isn't there a way to do that in code-behind?

brunnerh Over a year ago

@Rachel: That is because there are notifications for the DataContext changes, that is an exception to the rule though.

Rachel Over a year ago

@H.B. Could you suggest the code-behind syntax which would be the equivalent to {Binding }?

brunnerh Over a year ago

@Rachel: That's just new Binding(), no source no path, nothing else.

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Joe White · Accepted Answer · 2011-12-01 15:37:58Z

Binding.Source is typed as System.Object. So when you do:

b.Source = SomeInt;

you're assigning a value type (System.Int32) into something of type System.Object, which means you'll get a boxed copy of the original value.

The boxed copy lives on the heap and has no relation to the original variable. When you modify the original variable, nothing happens to the boxed copy.

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