Prevent SelectedValue from being NULL with RadComboBox

[Brian Sayatovic]

I have a combobox, shown below.  The MyItems collection has a series of states with verbose display names like "State of Alaska", "State of Alabama", etc.  I want the selected value -- bound to MyModel.ItemId -- to NEVER be null.

<telerik:RadComboBox x:Name="MyComboBox"
        IsReadOnly="True"
        IsEditable="True"
        CanAutocompleteSelectItems="False"
        IsFilteringEnabled="True"
        TextSearchMode="Contains"
        StaysOpenOnEdit="True"
        OpenDropDownOnFocus="True"
        ItemsSource="{Binding MyItems}"
        DisplayMemberPath="DisplayName"
        SelectedValuePath="Id"
        SelectedValue="{Binding MyModel.ItemId, Mode=TwoWay, NotifyOnValidationError=True, ValidatesOnDataErrors=true}"
        />

When the view is loaded, the Id starts out as not-null (defaulted by server long before), e.g. "Territory of Puerto Rico".  When I focus the box, the dropdown opens with "Territory of Puerto Rico" selected.  I can start backspacing just fine.  But when I backspace all the way to blank, it's updating my SelectedValue with NULL.

How can I make the RadComboBox to require one of the items to be selected and NOT update the binding with NULL?

[Yana]

Hi Brian,

You can implement validation using IDataErrorInfo interface, the approach is demonstrated in the following example:
http://demos.telerik.com/silverlight/#ComboBox/Validation

All the best,
Yana
the Telerik team

[Brian Sayatovic]

Unfortunately, that example simply highlights it in red using the standard validation techniques.  I don't want to simply tell the user when they've selected nothing/blank/null... I want to PREVENT IT from being nothing/blank/null.  Binding a null back to my model is problematic for me.  I'm investigating what it would take to make my model more forgiving, but so far that's a ton of work.

I was hoping the RadComboBox itself had a feature or trick to retain the previous value if someone tried to change it to nothing.  As it is, it seems the binding is updated as I type and it lands on other selected items, or null when I clear out the search text completely.

[Yana]

Hello Brian,

RadComboBox doesn't provide such a feature. The only way is to check the value in the ViewModel and set a default value in case it's null.

We're sorry for the inconvenience.

Kind regards,
Yana
the Telerik team

[Kunal]

How can I do that in ViewModel can you describe that.

 

 

[Martin Ivanov]

I believe that Yana had something like this in mind:

private string id;
public string Id
{
	get { return id; }
	set
	{
		string newId = value;
		if (string.IsNullOrEmpty(newId))
		{
			newId = "myDefaultValueHere";
		}
		id = newId;
		OnPropertyChanged(nameof(Id));
	}
}