WPF ComboboxItem binding

Question

I want to learn WPF programming. I want to know binding the gender of staff to the ComboBox:

class Staff {
    public int Gender {get; set;}
}

class ViewModel {
    private Staff _staff
    public Staff Staff {
        get {return _staff;}
        set {
            _staff = value;
            RaisePropertyChangedEvent("Staff");
        }
    }
}

<ComboBox SelectedItem="{Binding Staff.Gender, Converter={StaticResource GenderConverter}}">
    <ComboBoxItem IsSelected="True" Content="{StaticResource strGenderMale}"/>
    <ComboBoxItem Content="{StaticResource strGenderFemale}"/>
    <ComboBoxItem  Content="{StaticResource strGenderOther}"/>
</ComboBox>

GenderConverter is my customized Converter to convert int <-> string (0: male, 1: female, 2: other)

public class IntegerToGenderConverter : IValueConverter {
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
        if (value is int) {
            if ((int)value == MyConstants.GENDER_MALE) { //GENDER_MALE is 1
                return MyConstants.GENDER_MALE_STR; //GENDER_MALE_STR is the string: "Male"
            }
            if ((int)value == MyConstants.GENDER_FEMALE) {
                return MyConstants.GENDER_FEMALE_STR;
            }
        }
        return MyConstants.GENDER_OTHER_STR;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
        switch (value.ToString()) {
            case MyConstants.GENDER_MALE_STR:
                return MyConstants.GENDER_MALE;
            case MyConstants.GENDER_FEMALE_STR:
                return MyConstants.GENDER_FEMALE;
            default:
                return MyConstants.GENDER_OTHER;
        }
    }
}

When i run the application, the staff is not null (everything else (such as name, date of birth...) is bound well except Gender :(

Edit:

When I created a List Genders and bound it to the ComboBox's property ItemsSource instead of using tags, it worked well. Why???

Answer 1

A ComboBox uses ComboBoxItem as item type when you explicitly add them like

<ComboBox>
    <ComboBoxItem Content="..."/>
    ...
</ComboBox>

This means that the SelectedItem property returns a ComboBoxItem, which is then passed to your converter. However, your converter does not expect values of type ComboBoxItem.

When you add integers - as in your bound Genders list - the item type is int, which you successfully handled in your converter. In this case the ComboBox class creates and uses ComboBoxItems only internally.

This is a common behaviour in all WPF controls that are derived from ItemSource. They use the item type that is provided by the user.

---

That said, you would usually use an enumeration type for your Gender property:

public enum Gender
{
    Male, Female, Other
}

public class Staff
{
    public Gender Gender { get; set; }
}

Then you would add Gender values to your ComboBox and bind the SelectedItem without a converter:

<ComboBox SelectedItem="{Binding Staff.Gender}">
    <local:Gender>Male</local:Gender>
    <local:Gender>Female</local:Gender>
    <local:Gender>Other</local:Gender>
</ComboBox>

where local is a XAML namespace declaration of the C# namespace that contains the Gender type.