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DataBinding in .NET - Another cool trick |   | You are probaly familiar with .NET's databinding when you are using a DataTable or similar System.Data object. Anyway, let's say that you aren't. Can you still take advantange of .NET's databinding. Well, until today I didn't think there was much you could do. I did know that you could bind to a Strongly typed collection, but this seemed just like the same face of binding to a datatable, after all, isn't it a collection?
Earlier today I was playing with an application and I noticed that when you call the Add method of the Listbox or any other Complexly bound object, the Items property takes type 'Object'. Well in VB6 I seem to remember this being of type string and I was wondering what would be the benefit of adding type object instead when you'd just have to downcast it anyway. I figured it out and the reason is that you don't have to downcast it.
Take the following Object definition:
Imports System
Public Class Customer
Private _firstName As String
Private _lastName As String
Public Property FirstName() As String
Get
Return _firstName
End Get
Set(ByVal Value As String)
_firstName = Value
End Set
End Property
Public Property LastName() As String
Get
Return _lastName
End Get
Set(ByVal Value As String)
_lastName = Value
End Set
End Property
Public ReadOnly Property FullName() As String
Get
Return Me.LastName & ", " & Me.FirstName
End Get
End Property
Public Sub New(ByVal first As String, ByVal last As String)
_firstName = first
_lastName = last
End Sub
End Class |
This is a pretty simple example and the main reason I used it is that I want to bind to FullName as the DisplayMember. I didn't think this was possible but it is. All you need to do is specify the field of the object as the DisplayMember, and then add the object to the Items collection. NOTE that you don't need to add the collection as the datasource, you add the items just like you normally would, you just add the whole object as opposed to simply the text of it. By specifying the DisplayMember, what you want to display will show in the control but you can do a lot more with it now. This gives you access to the whole object and you don't have to parse pieces to the text to get a piece of it (provided you built the object that way) but all in all, this gives you a lot of power.
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim bill As New Customer("Bill", "Ryan")
Dim brian As New Customer("Brian", "Davis")
Dim les As New Customer("Lester", "Smith")
lb.ValueMember = "FullName"
lb.DisplayMember = "FullName"
lb.Items.Add(bill)
lb.Items.Add(brian)
lb.Items.Add(les)
End Sub
Private Sub lb_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lb.SelectedIndexChanged
MessageBox.Show(CType(lb.SelectedItem, Customer).FirstName)
End Sub |
This is a simplistic example, but as you can see, you can have it both ways now. Your control will show only the text you want displayed, but you have full access to all of the properties of the object because, well, that's what been added to the listbox. Once again, there was a method to the apparent madness.
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