Why does my 2 way binding not work - wpf

Question

I'm trying to bind 2 ways
In my ViewModel, I have
private Temporary _selectedCompany;
public Temporary SelectedCompany
{
get
{
return this._selectedCompany;
}
set
{
if (this._selectedCompany == value || value == null)
return;
this._selectedCompany = value;
this.SelectedCompany.CompanyName = "TestName";
base.OnPropertyChanged("SelectedCompany");
}
}
Temporary is actually a class similar to a class you'd do for a CompanyAddress (name, country, phone etc) and was created by EntityFramework.
In the corresponding View, the XAML is
<local:CompanyDetail CompanyName="{Binding SelectedCompany.CompanyName}"/>
In code behind of the custom control
// Dependency Property
public static readonly DependencyProperty CompanyNameProperty =
DependencyProperty.Register("CompanyName", typeof(string),
typeof(CompanyDetail), null);
// .NET Property wrapper
public string CompanyName
{
get { return (string)GetValue(CompanyNameProperty); }
set { SetValue(CompanyNameProperty, value); }
}
There is nothing in the ViewModel. There is the following XAML
<TextBox Text="{Binding CompanyName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, RelativeSource={RelativeSource AncestorType=UserControl, AncestorLevel=1, Mode=FindAncestor}}" />
So, when the controls are loaded and shown on screen, I see the value "TestName" in the TextBox but, if I change the value by typing and then click an OK button I can see the value has not been updated.
I'm assuming it has nothing to do with INotifyPropertyChanged because I think it's all a reference type any way?
What am I doing wrong?

Answer 1

The CompanyName binding must be made two-way, either by setting it explicitly
<local:CompanyDetail
CompanyName="{Binding SelectedCompany.CompanyName, Mode=TwoWay}"/>
or by declaring the dependency property to bind two-way by default
public static readonly DependencyProperty CompanyNameProperty =
DependencyProperty.Register(
"CompanyName", typeof(string), typeof(CompanyDetail),
new FrameworkPropertyMetadata(
null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));