Question

I have a View that is linked to my ViewModel using a DataTemplate, like this

<DataTemplate DataType="{x:Type ViewModels:ViewModel}">
    <Views:View />
</DataTemplate>

The ViewModel holds a property ProcessOption that is of type MyEnum?, where MyEnum is a custom enumeration that has let’s say 3 values: Single, Multiple and All. I am trying to bind a combobox to this property, so the approach I am following is:

ViewModel has a property of List<string> that is

public List<string> Options
    {
        get 
        {
            return Enum.GetNames(typeof(MyEnum)).ToList();
        }
    }

to which I bind the ItemsSource property of the Combobox. Then, in addition to the ProcessOption property, the ViewModel also has an OptionName property (of string), which is intended to hold the selected option’s name. The ViewModel implements INotifyPropertyChanged and both properties raise the PropertyChanged event in their setters. The binding I am using then is:

<ComboBox ItemsSource="{Binding Options}"
    SelectedItem="{Binding OptionName}"
    SelectedValue="{Binding ProcessOption}"/>

This works fine up to this point. Initially the combobox is empty and both properties are null, and when the user selects an option this is propagated to the ViewModel as it should.

The problem appears when I load the data from a Database, and I want to load the controls with initial values. In this case, in the ViewModel’s constructor I have this:

this.ProcessOption = objectFromDB.ProcessOption // this is the value restored from DB,        let's say it is MyEnum.Multiple
this.OptionName = Options.First(x => x.Equals(Enum.GetName(typeof(MyEnum), objectFromDB.ProcessOption)));

The problem is, although the above sets the two properties to their correct values, they are later set to null from the Combobox binding, so the initial values are not kept. I have also tried to do something like if (value == null) { return; } in the properties’ setters, in which case they have the correct values after the View loads, however the Combobox still does not display the correct option, it is empty.

I should also note that I’ve also tried setting IsSynchronisedWithCurrentItem and it doesn’t make any difference, apart from the fact that the first element is displayed instead of the combobox being empty.

Can anyone help with this binding? Any help will be very much appreciated, this is driving me crazy!

Answer 1

<ComboBox ItemsSource="{Binding Options}"
    SelectedItem="{Binding OptionName}"
    SelectedValue="{Binding ProcessOption}"/>

Your binding doesn’t look like it should work at all — you don’t have TwoWay binding set up, and I think SelectedItem and SelectedValue is an either/or proposition.

I suggest that you get rid of OptionName and just bind SelectedItem to ProcessOption (TwoWay) with an IValueConverter that will convert to/from string.