Databinding a nullable value to a combobox lookup – Silverlight – The Missing Notes

(This is a series of unspoken Silverlight best practices, designed to avoid specific pain points when developing Silverlight applications. Refer to this URL http://www.redmountainsw.com/wordpress/?p=1270#list for a complete listing).

Scenario

You have a field CountryID which could be set to null. If you set the ComboBox.ItemsSource=Countries then the user could not select a null value.
Another scenario is you want to present a filter of a list of countries. However, one of the items have be to “(Show all)”.

Solution

Use a NullableItemsSource (code below). The usage is like this:

this.LookupCountries = new Helpers.NullableItemsSource(
  from c in this._context.Countries orderby c.CountryName select c, 
  new Country { CountryID=0, CountryName = "(None)" });

namespace ProjectNavigation.Helpers
{
    using System.Collections;

    public class NullableItemsSource : IEnumerable
    {
        #region Constructor
        public NullableItemsSource(IEnumerable ItemsSource, object NullItem)
        {
            m_NullDisplayListItem = NullItem;
            this.m_ItemsSource = ItemsSource;
        }

        public object First
        {
            get
            {
                IEnumerator it = GetEnumerator();
                it.MoveNext();
                return it.Current;            
            }
        }

        #endregion

        #region Private members
        private IEnumerable m_ItemsSource;
        private object m_NullDisplayListItem;
        #endregion

        public IEnumerable ItemsSource
        {
            get { return m_ItemsSource; }
            set { m_ItemsSource = value; }
        }

        public IEnumerator GetEnumerator()
        {
            yield return m_NullDisplayListItem;
            foreach (var item in m_ItemsSource)
            {
                yield return item;
            }
        }
    }
}