Reset combobox selected item on set using MVVM

Question

I am using a ComboBox in my WPF application and following MVVM. There is a list of strings which I want to show in my ComboBox.

XAML:

<ComboBox ItemsSource="{Binding ItemsCollection}" SelectedItem="{Binding SelectedItem}" />

View Model:

public Collection<string> ItemsCollection; // Suppose this has 10 values.
private string _selectedItem;
public string SelectedItem
{
    get { return _selectedItem; }
    set
    {
        _selectedItem = value;
        Trigger Notify of property changed.
    }
}

Now this code is working absolutely fine. I am able to select from view and I can get changes in ViewModel and if I change SelectedItem from my ViewModel I can see it in my view.

Now here is what I am trying to achieve. When I change selected item from my view I need to put a check that value is good/bad (or anything) set selected item else do not set it. So my view model changes like to this.

public string SelectedItem
{
    get { return _selectedItem; }
    set
    {
        if (SomeCondition(value))
            _selectedItem = value;           // Update selected item.
        else
            _selectedItem = _selectedItem;   // Do not update selected item.
        Trigger Notify of property changed.
    }
}

Now when I execute this code and SomeCondition(value) returns false, SelectedItem returns old string value, but in my view selected item in ComboBox is the the value which I selected. So lets assume I have collection of 10 strings showing in my ComboBox. All values are good except second and fourth element (SomeCondition returns false for 2nd and 4th value). What I want that if I select 2nd or 4th element selectedItem do not change. But my code is not doing this properly. If I select 2nd element then view still displays 2nd element as selected. I know there is something wrong in my code. But what is it?

Answer 1

I know this is a bit late but as of WPF 4.5 you can use the Delay command like so:

    <ComboBox ItemsSource="{Binding ItemsCollection}" SelectedItem="{Binding SelectedItem, Mode=TwoWay, Delay=1, UpdateSourceTrigger=PropertyChanged}" />

This saved me after hours of looking up stuff the other day. For other methods which may or may not work you can read this post and its comments.