According to the .NET Framework documentation, to bind a Windows Forms
ListControl (e.g. a ListBox or ComboBox) as a lookup (that is, the control's display value maps to a different underlying value), you need to set three properties:
DisplayMember. What it neither tells nor shows you, as far as I have been able to determine, is that the order in which you set these properties may make a huge difference in the performance and/or behavior of your code. In this sample, I will attempt to clarify this issue and show what I believe to be the proper way to create lookup controls in code.
In Windows Forms, we consider subclasses of the abstract class
ListControl to be "complex-bound" controls. In more everyday terms, that means they are useful as lookups, i.e. if their data source is an ADO.NET
DataTable, they can display values from one column (perhaps a person's name) and use values from another column (perhaps a person's unique identifier) as the corresponding value of the control. The two complex-bound controls you've most likely encountered are the ComboBox and the Listbox.
To complex-bind one of these controls, you need to set the
DataSource (where values originate), the
DisplayMember (the name of the column of data that supplies the visible list items), and the
ValueMember (the name of the column of data that supplies the possible control values). You might think that the order in which you set these properties never matters, and samples from Microsoft and others would tend to back up such a belief. However, there are things going on underneath the covers that can cause trouble for your code. In particular, when the
DataSource is changed, or when
ValueMember is changed after
DataSource has been set, the binding infrastructure forces the control to rebind. Logically, this makes sense. But it poses a problem if you handle certain control events, in particular
SelectedValueChanged. In the worst case, the following simple sequence will raise
SelectedIndexChanged three times in a row:
listbox.DataSource = dataTable
listbox.ValueMember = "id"
listbox.DisplayMember = "name"
Obviously if you're doing anything non-trivial in your handlers, this could turn into a serious performance problem.
And it's even worse than that, because until
DisplayMember is set, the
SelectedValue of the control will not be the type you're probably expecting, but rather it will be a
DataRowView. At that point, you have two choices if you're handling the aforementioned Selected-Changed events: Special-case for when
SelectedValue returns a
DataRowView, or handle and eat the exception when you try to do something incorrect with the
DataRowView. I have actually seen instances of each of these in professional code. You don't want to have to do this, so in the following sections, I will show you how to sidestep this issue.
Why You Might Not Have This Problem
If you're using the Windows Forms designer to set up all of your lookup wiring, you don't have this problem. There are at least two reasons for this, as far as I can tell. First, in all of my testing, the designer always hooks up events -after- setting properties on form controls, so at the point the troublesome properties are set, the events aren't even there yet to be called. Second, the designer adds controls to the form at the end of
InitializeComponents, and no events can be fired before that point.
Unfortunately, you frequently don't have the luxury of either of these workarounds. You may have to bind the ListControl long after all controls have been added to the form, and a workaround which temporarily removes the event handler would be a pain. There is probably a workaround by using Suspend and Resume on the binding context, but that would be too complicated a fix for what should really be a simple procedure.
The solution is simple and can be summed up in three words: SET DATASOURCE LAST. Contrary to what you might expect, setting
ValueMember does not trigger any internal validation on the part of the control that might fail if there is not yet a
DataSource. So you're safe in doing this. But the vast majority of samples (including those from Microsoft) set
DataSource first and thus are prone to the difficulties I have mentioned.
Using the code
The included project contains code in C# and in VB.NET that demonstrates two different flavors of the problem, as well as two different solutions. The demo form contains four listboxes with four buttons that bind and unbind the listboxes by setting the lookup properties. If you're looking at the demo form, I will go over the listboxes from top to bottom by name.
- "HORRIBLE". This version sets the properties in the following order:
DisplayMember. This gives you three
SelectedIndexChanged events with
SelectedValue equal to a
DataRowView, and an
Int32 respectively. Not fun to robustly handle.
- "MERELY BAD". This version sets the properties in the following order:
ValueMember. I called this one "merely bad" because it only fires two
SelectedIndexChanged events instead of three, but it's arguably not much better than "horrible" because in each of the events,
SelectedValue for the listbox is still a
DataRowView. You only get two events because it's apparently ok to set
ValueMember without a rebind if
DisplayMember is already set.
- "GOOD". This is the recommended order of
DataSource. Here you only get one
SelectedIndexChanged event, and it contains the type you really want, in this case an
- "GOOD HELPER FUNCTION". To enforce the good behavior in your code, you might want to write a helper function that takes a ListControl and values to plug into
DisplayMember in the recommended order. This solution demonstrates such a function, found in LookupUtility.cs or LookupUtility.vb.
Here is the source code for the helper function in C#:
public static void SetLookupBinding( ListControl toBind,
object dataSource, string displayMember,
string valueMember )
toBind.DisplayMember = displayMember;
toBind.ValueMember = valueMember;
toBind.DataSource = dataSource;
And Visual Basic.NET:
Public Shared Sub SetLookupBinding(ByVal toBind As ListControl,
ByVal dataSource As Object, ByVal displayMember As String,
ByVal valueMember As String)
toBind.DisplayMember = displayMember
toBind.ValueMember = valueMember
toBind.DataSource = dataSource
- 09/25/2004 Initial Release.