Advanced combobox binding from code

Suppose I have a business object '`obj1`' that has property '`P`'.

Let's also assume that I have a list of business objects: '`List<BussObj> list`' and each BussObj object contains 2 properties: '`A`' and '`B`'.

This list is bound to combobox: `combobox.ItemsSource = list;`.

I would like to specify binding (in C# code) that would bind combobox.SelectedItem.B to my obj.P. How to do it?

I tried something like that but it does not work:

Binding bind= new Binding("B");
bind.Source = obj.P;
comboSubject.SetBinding(ComboBox.SelectedItemProperty, bind);

Thank you in advance for any help