MVVM SelectionChanged Combobox doesn't call 'Set' - mvvm

Question

my problem is that I want to call the 'SelectionChanged' Event in my ViewModel.
I have a ComboBox (here called ListPicker, it's a phone application):
<tool:ListPicker Name="txt_LZZ"
ItemsSource="{Binding ZZR}" SelectedItem="{Binding MySelectedItem}" />
My Property in the ViewModel looks like this:
private List<string> _zzr;
public List<string> ZZR
{
get
{
_zzr = new List<string>();
_zzr.Add("Jahr");
_zzr.Add("Monat");
_zzr.Add("Woche");
_zzr.Add("Tag");
return _zzr;
}
set
{
_zzr = value;
RaisePropertyChanged(() => ZZR);
}
}
private string _mySelectedItem;
public string MySelectedItem
{
get
{
return _mySelectedItem;
}
set
{
if (value == _mySelectedItem)
return;
_mySelectedItem = value;
RaisePropertyChanged(() => MySelectedItem);
GetValues();
}
}
The program only calls the get method once, while _mySelectedItem has the value 'null'. What I want is that when I change the SelectedItem in my Combobox (ListPicker), the ViewModel has to call the method GetValues, which is in my setter for MySelectedItem. Problem: -> ViewModel doesn't call setter. Why?

Answer 1

Try setting the mode of the binding to TwoWay:
<tool:ListPicker Name="txt_LZZ"
ItemsSource="{Binding ZZR}" SelectedItem="{Binding MySelectedItem, Mode=TwoWay}" />
I also wouldn't instantiate your value collection in the getter, but instead make the setter private, and assign a value to the property in your view model (e.g. the constructor or when the view model is activated).