WPF I cannot update a combobox value in code behind

Question

I would want to update the following combobox selected item:

<ComboBox ItemsSource="{Binding Path=DictUsers, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="Key" SelectedValuePath="Value" SelectionChanged="ComboUser_SelectionChanged" SelectedValue="{Binding Edit.ProtLevel, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" />

The ItemsSource, DictUsers, comes from a dictionary:

DictUsers = new Dictionary<string, int>();
DictUsers.Add(string.Empty, 0); 
foreach (cUser user in App.Users)
    DictUsers.Add(user.Name, user.ShowLevel);

I cannot access directly to the ComboBox as it's inside a datatemplate, so I tought up update the binded value (Edit.ProtLevel):

// Test
Edit.ProtLevel = 5;

But the combobox doesn't update itself, which means that the selected items doesn't change. What's wrong?

Answer 1

For your binding to work correctly, the bound property should be a DependencyProperty or be inside of a class that implements INotifyPropertyChanged, something like this should work:

public class Edit : INotifyPropertyChanged {
    private int _protLevel;

    public int ProtLevel {
        get => _protLevel;
        set {
            if (_protLevel != (_protLevel = value)) {
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(ProtLevel)));
            }
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
}

You can read the binding documentation here.

And a simple INotifyPropertyChanged implementation here.