Why can't I select a null value in a ComboBox?

Question

In WPF, it seems to be impossible to select (with the mouse) a \"null\" value from a ComboBox. Edit To clarify, this is .NET 3.5 SP1.

Here\'s some c

Answer 1

I know this answer isn't what you asked for (an explanation of why it doesn't work with the mouse), but I think the premise is flawed:

From my perspective as a programmer and user (not .NET), selecting a null value is a bad thing. "null" is supposed to be the absence of a value, not something you select.

If you need the ability explicitly not to select something, I would suggest either the work-around you mentioned ("-", "n.a." or "none" as a value), or better

• wrap the combobox with a checkbox that can be unchecked to disable the combobox. This strikes me as the cleanest design both from a user's perspective and programmatically.

Answer 2

I had the same kind of problem we did some work around like adding a value property to the collection item like this :

 public class Bar

   {
      public string Name { get; set; }
      public Bar Value
      {
         get { return String.IsNullOrEmpty(Name) ?  null :  this; } // you can define here your criteria for being null
      }
   }

Then while adding items instead of null I use the same object :

  comboBox1.ItemsSource=  new ObservableCollection<Bar> 
        {
            new Bar(),
            new Bar { Name = "Hello" }, 
            new Bar { Name = "World" } 
        };

And instead of selecteditem I bind it to selectedvalue :

<ComboBox Height="23" Margin="25,40,133,0" DisplayMemberPath="Name"
              SelectedValuePath="Value" 
              SelectedValue="{Binding Bar}"
              Name="comboBox1" VerticalAlignment="Top" />

I know It is not a complete solution, just one workaround I use

Answer 3

I got a new solution for this question. "USING Mahapps"

  xmlns:controls="http://metro.mahapps.com/winfx/xaml/controls"


  <ComboBox x:Name="bars"  **controls:TextBoxHelper.ClearTextButton="True"**
              DisplayMemberPath="Name" 
              Height="21" 
              SelectedItem="{Binding Bar}"/>

You can use the close button to clear the content.

Thanks.

Answer 4

Just a guess, but I think it sounds reasonable.

Assume combobox is using "ListCollectionView" (lcv as its instance) as its item collection, which it should be. If you are a programmer, what you gonna do?

I will respons to both Keyboard and Mouse.

Once I get Keyboard input, I use

lcv.MoveCurrentToNext();

or

lcv.MoveCurrentToPrevious();

So, sure keyboard works well.

Then I am working on respons Mouse inputs. And it comes the problem.

1. I want to listen 'MouseClick' event of my item. But probably, my Item doesn't generated, it is just a placeholder. So when user click on this placeholder, I get nothing.

2. If I get the event successfully, what's next. I will invoke

   lcv.MoveCurrentTo(selectedItem);

the "selectedItem" which would be null is not an acceptable parameter here I think.

Anyway, it's just guessing. I don't have time to debug into it though I am able to. I have a bunch of defects to fix. Good Luck. :)