ComboBox with SelectedValuePath

11-28-2008 12:09 PM |

Unless I missed a property somewhere, a SelectedValue property is needed for combo boxes and list boxes.  I extended a combo box to do this.

 If you have business objects that store data in a database via databinding, you've probably already found it a pain not to be able to associate a value with an item in a dropdown when selected.  Currently you have to do it manually.  I was adding bussiness objects to the list and having to determine the value based on the selection.  The DisplayMemberPath and SelectedItem works great but with the SelectedValuePath its complete.  Check out the code:

 http://www.engineserver.com/silverlightcombobox/

Re: ComboBox with SelectedValuePath

01-15-2009 9:26 PM |

That makes sense.  Thanks for the improvement.

Re: ComboBox with SelectedValuePath

01-16-2009 5:59 AM |

Hmm, can't get it to work.

I get an error on Items on the Linq query.

And could you please update your code example with the improvement that Andrej posted?

Thanks,
Håkan

Re: ComboBox with SelectedValuePath

01-16-2009 6:08 AM |

Håkan,

have you included the "using System.Linq" directive in your derived control?

Andrej

Re: ComboBox with SelectedValuePath

01-16-2009 6:16 AM |

Any chance you could share the actual exception message with us?
Thanks,

Re: ComboBox with SelectedValuePath

01-16-2009 7:40 AM |

Aaah, it was the using System.Linq; that was missing, thanks!

- Håkan

Re: ComboBox with SelectedValuePath

02-21-2009 11:09 AM |

Hello,

I can't get it working!

I have in my xaml:

...

 <StackPanel x:Name="spClient" DataContext="{Binding}" Orientation="Vertical">

     <TextBox Width="150" Text="{Binding nom, Mode=TwoWay}" />

     <custom:ComboBoxClassic x:Name="cbLangue" DisplayMemberPath="langue" 
      SelectedValuePath="langueId" ItemsSource="{Binding}"
      SelectedValue="{Binding langueId, Mode=TwoWay}" />
...

The ItemsSource of Combo is IEnumerable<Langue>:

    public class Langue
    {
        public int langueId { get; set; }
        public String langue { get; set; }
    } 

 The DataContext of StackPanel is an object of type Client:

    public class Client : INotifyPropertyChanged
    {
        public int clientId { get; set; }
        public String nom { get; set; }
        public int langueId { get; set; }

        public event PropertyChangedEventHandler PropertyChanged;
    }

If I edit the textbox then the client object have it's "nom" property changed automatically.

If I select a value in the comboBox client.langueId is not changed...

What am I doing wrong?

Thanks.

Re: ComboBox with SelectedValuePath

02-23-2009 2:52 PM |

Hi again,

It works now: I was populating the DataContext of the comboBox instead of its ItemsSource in the code behind.

But I made a little change also to be able to set the SelectedValue BEFORE the comboBox is filled of items. The DropDownList in ASP.NET has this behaviour too and I think it's important because of the asynchronous environment of SilverLight. I also filled the SelectedValuePropertyChanged to have the good value on first binding.

using System;
using System.Linq;
using System.Windows;
using System.Windows.Controls;


/// <summary>
/// Extended ComboBox that provide SelectedValueProperty and SelectedValue for asp:DropDownList-like capabilities.
/// Original version by ZeekJones: http://www.engineserver.com/silverlightcombobox/
/// Improved by andrejt: http://silverlight.net/forums/p/53720/139118.aspx#139118 : DependencyProperty.
/// Improved by cyse: http://silverlight.net/forums/p/53720/139118.aspx#178204: SelectedValue cache.
/// </summary>
public class ComboBoxClassic : ComboBox
{
	private Object _selectedValue;
	public static readonly DependencyProperty SelectedValuePathProperty = DependencyProperty.Register("SelectedValuePath", typeof(String), typeof(ComboBoxClassic), new PropertyMetadata(new PropertyChangedCallback(SelectedValuePathPropertyChanged)));
	public static readonly DependencyProperty SelectedValueProperty = DependencyProperty.Register("SelectedValue", typeof(Object), typeof(ComboBoxClassic), new PropertyMetadata(new PropertyChangedCallback(SelectedValuePropertyChanged)));

	public ComboBoxClassic()
		: base()
	{
		base.SelectionChanged += new SelectionChangedEventHandler(ComboBoxClassic_SelectionChanged);
	}

	private void ComboBoxClassic_SelectionChanged(Object sender, SelectionChangedEventArgs e)
	{
		if (SelectedItem != null && !string.IsNullOrEmpty(SelectedValuePath))
		{
			SetValue(ComboBoxClassic.SelectedValueProperty, SelectedItem.GetType().GetProperty(SelectedValuePath).GetValue(SelectedItem, null));
		}
	}

	protected override void OnItemsChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
	{
		base.OnItemsChanged(e);
		if (_selectedValue != null)
		{
			SelectedValue = _selectedValue;
		}
	}


	private static void SelectedValuePathPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
	{
		ComboBoxClassic cb = d as ComboBoxClassic;
		cb.SelectedValuePath = e.NewValue.ToString();
		cb.SelectedValue = cb.SelectedValue;
	}

	private static void SelectedValuePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
	{
		ComboBoxClassic cb = d as ComboBoxClassic;
		cb.SelectedValue = e.NewValue;
	}

	/// <summary>
	/// The name of the property in the ItemsSource objects that "SelectedValue" will return.
	/// </summary>
	public string SelectedValuePath
	{
		get
		{ 
			return (string)GetValue(ComboBoxClassic.SelectedValuePathProperty);
		}
		set 
		{
			SetValue(ComboBoxClassic.SelectedValuePathProperty, value); 
		}
	}

	/// <summary>
	/// The value of the property specified by SelectedValuePath in the selected item.
	/// </summary>
	public object SelectedValue
	{
		get
		{
			return GetValue(SelectedValueProperty);
		}
		set
		{
			_selectedValue = value;

			var q = (from item in Items
					 where item.GetType().GetProperty(SelectedValuePath).GetValue(item, null).Equals(value)
					 select item).FirstOrDefault();
			if ( q!= null)
			{
				SelectedItem = q;
			}
		}
	}

}

Re: Re: ComboBox with SelectedValuePath

03-27-2009 12:51 PM |

I get the following error when attempting to bind to the itemsource : 

Items collection must be empty before using ItemsSource.

So I cleared the items before binding to itemsource and got this error "Object reference not set to an instance of an object." on second line below (from your code) 

 So I commented those three lines and it is working..

Just to clarify, if I leave those three lines in, I get the error: Items collection must be empty before using ItemsSource.

And if I clear the items, then I get an Object reference not set to an instance of an object stemming from those three lines.

Re: ComboBox with SelectedValuePath

08-20-2009 4:55 PM |

Thanks, creating a custom ComboBox worked great!