WPF: ComboBox with reset item

Question

I want to make a ComboBox in WPF that has one null item on the top, when this gets selected, the SelectedItem should be set to null (reset to default state). I\

Answer 1

While I agree there are plenty solutions to WPF ComboBox's null item issue, Andrei Zubov's reference to Null Object Pattern inspired me to try a less overkilling alternative, which consists on wrapping every source item allow with a null value (also wrapped) before injecting the whole wrapped collection into ComboBox.ItemsSource property. Selected item will be available into SelectedWrappedItem property.

So, first you define your generic Wrapper...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ComboBoxWrapperSample
{

    /// 

    /// Wrapper that adds supports to null values upon ComboBox.ItemsSource
    /// 
    /// Source combobox items collection datatype
    public class ComboBoxNullableItemWrapper
    {
        string _nullValueText;

        private T _value;

        public T Value
        {
            get { return _value; }
            set { _value = value; }
        }

        /// 

        /// 
        /// 
        /// Source object
        /// Text to be presented whenever Value argument object is NULL
        public ComboBoxNullableItemWrapper(T Value, string NullValueText = "(none)")
        {
            this._value = Value;
            this._nullValueText = NullValueText;
        }

        /// 

        /// Text that will be shown on combobox items
        /// 
        /// 
        public override string ToString()
        {
            string result;
            if (this._value == null)
                result = _nullValueText;
            else
                result = _value.ToString();
            return result;
        }

    }
}

Define your item model...

using System.ComponentModel;

namespace ComboBoxWrapperSample
{
    public class Person : INotifyPropertyChanged
    {
        // Declare the event
        public event PropertyChangedEventHandler PropertyChanged;

        public Person()
        {
        }

        // Name property
        private string _name;

        public string Name
        {
            get { return _name; }
            set
            {
                _name = value;
                OnPropertyChanged("Name");
            }
        }

        // Age property
        private int _age;

        public int Age
        {
            get { return _age; }
            set
            {
                _age = value;
                OnPropertyChanged("Age");
            }
        }

        protected void OnPropertyChanged(string name)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(name));
            }
        }

        // Don't forget this override, since it's what defines ao each combo item is shown
        public override string ToString()
        {
            return string.Format("{0} (age {1})", Name, Age);
        }
    }
}

Define your ViewModel...

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;

namespace ComboBoxWrapperSample
{
    public partial class SampleViewModel : INotifyPropertyChanged
    {

        // SelectedWrappedItem- This property stores selected wrapped item
        public ComboBoxNullableItemWrapper _SelectedWrappedItem { get; set; }

        public ComboBoxNullableItemWrapper SelectedWrappedItem
        {
            get { return _SelectedWrappedItem; }
            set
            {
                _SelectedWrappedItem = value;
                OnPropertyChanged("SelectedWrappedItem");
            }
        }

        // ListOfPersons - Collection to be injected into ComboBox.ItemsSource property
        public ObservableCollection> ListOfPersons { get; set; }

        public SampleViewModel()
        {

            // Setup a regular items collection
            var person1 = new Person() { Name = "Foo", Age = 31 };
            var person2 = new Person() { Name = "Bar", Age = 42 };

            List RegularList = new List();
            RegularList.Add(person1);
            RegularList.Add(person2);

            // Convert regular collection into a wrapped collection
            ListOfPersons = new ObservableCollection>();
            ListOfPersons.Add(new ComboBoxNullableItemWrapper(null));
            RegularList.ForEach(x => ListOfPersons.Add(new ComboBoxNullableItemWrapper(x)));

            // Set UserSelectedItem so it targes null item
            this.SelectedWrappedItem = ListOfPersons.Single(x => x.Value ==null);

        }

        // INotifyPropertyChanged related stuff
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string name)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(name));
            }
        }
    }
}

And, finnaly your View (ok, it's a Window)

    
        Favorite teacher
        
        
        
            Selected wrapped value:
            
        
    

Reaching this point, did I mention that you could retrieve unwrapped selected item thru SelectedWrappedItem.Value property ?

Here you can get a working sample

Hope it helps someone else