Question

I’m not quite sure how to deal with this problem. I’m using a bunch of comboboxes with dropdown lists of values we allow the user to set a property too. (i.e. Currencies = “USD, CAD, EUR”).

Every now and then, when we load data, we’ll find the currency is something not in our list, like “AUD”. In this case, we still want the combobox to display the loaded value, and the current selected Currency should remain “AUD” unless the user chooses to change it, in which case their only options will still be “USD, CAD, EUR”.

My problem is that as soon as the control becomes visible, the ComboBox is calling the setter on my SelectedCurrency property and setting it to null, presumably because the current value “AUD” isn’t in it’s list. How can I disable this behaviour without making it possible for the user to type whatever they want into the Currency field?

Answer 1

This seems to be a reasonably common problem. Imagine you have a lookup list in the database, maybe a list of employees. The employee table has a ‘works here’ flag. Another table references the employee lookup list. When a person leaves the company, you want your views to show the name of the old employee, but not allow the old employee to be assigned in future.

Here’s my solution to the similar currency problem:

Xaml

<Page.DataContext>
    <Samples:ComboBoxWithObsoleteItemsViewModel/>
</Page.DataContext>
<Grid>
    <ComboBox Height="23" ItemsSource="{Binding Items}" 
              SelectedItem="{Binding SelectedItem}"/>
</Grid>

C#

// ViewModelBase and Set() are from MVVM Light Toolkit
public class ComboBoxWithObsoleteItemsViewModel : ViewModelBase
{
    private readonly string _originalCurrency;
    private ObservableCollection<string> _items;
    private readonly bool _removeOriginalWhenNotSelected;
    private string _selectedItem;

    public ComboBoxWithObsoleteItemsViewModel()
    {
        // This value might be passed in to the VM as a parameter
        // or obtained from a data service
        _originalCurrency = "AUD";

        // This list is hard-coded or obtained from your data service
        var collection = new ObservableCollection<string> {"USD", "CAD", "EUR"};

        // If the value to display isn't in the list, then add it
        if (!collection.Contains(_originalCurrency))
        {
            // Record the fact that we may need to remove this
            // value from the list later.
            _removeOriginalWhenNotSelected = true;
            collection.Add(_originalCurrency);
        }

        Items = collection;

        SelectedItem = _originalCurrency;
    }

    public string SelectedItem
    {
        get { return _selectedItem; }
        set
        {
            // Remove the original value from the list if necessary
            if(_removeOriginalWhenNotSelected && value != _originalCurrency)
            {
                Items.Remove(_originalCurrency);
            }

            Set(()=>SelectedItem, ref _selectedItem, value);
        }
    }

    public ObservableCollection<string> Items
    {
        get { return _items; }
        private set { Set(()=>Items, ref _items, value); }
    }
}