XML Feeds
Lets say I have a class teacher which inherits from a class person (hard to believe I know ;-)). And we want to bind a list of Iteachers (the interface) to a datagridview because that is the easiest way of getting things working for the datagridview. This will give a surprising (perhaps not) result.
First lets look at the Person class and IPerson interface.
- Public Interface IPerson
- Property Name() As String
- Property FirstName() As String
- End Interface
- Public Class Person
- Implements IPerson
- Private _name As String
- Private _firstname As String
- Public Sub New()
- End Sub
- Public Sub New(ByVal Name As String, ByVal Firstname As String)
- _name = Name
- _firstname = Firstname
- End Sub
- Public Property Name() As String Implements IPerson.Name
- Get
- Return _name
- End Get
- Set(ByVal value As String)
- _name = value
- End Set
- End Property
- Public Property FirstName() As String Implements IPerson.FirstName
- Get
- Return _firstname
- End Get
- Set(ByVal value As String)
- _firstname = value
- End Set
- End Property
- End Class
And then the Teacher class and ITeacher interface which inherits from IPerson.
- Public Interface ITeacher
- Inherits IPerson
- Property ClassRoom() As String
- End Interface
- Public Class Teacher
- Inherits Person
- Private _classroom As String
- Public Sub New()
- End Sub
- Public Sub New(ByVal name As String, ByVal FirstName As String, ByVal ClassRoom As String)
- MyBase.New(Name, FirstName)
- _classroom = ClassRoom
- End Sub
- Public Property ClassRoom() As String
- Get
- Return _classroom
- End Get
- Set(ByVal value As String)
- _classroom = value
- End Set
- End Property
- End Class
And then we bind it to the datagridview via a bindingsource. Like so
- Public Class Form1
- Dim _bs As BindingSource
- Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
- Dim _l As New List(Of ITeacher)
- _l.Add(New Teacher("t1", "f1", "A"))
- _l.Add(New Teacher("t2", "f2", "B"))
- _l.Add(New Teacher("t3", "f3", "C"))
- _l.Add(New Teacher("t4", "f4", "D"))
- _l.Add(New Teacher("t5", "f5", "E"))
- _l.Add(New Teacher("t6", "f6", "F"))
- _l.Add(New Teacher("t7", "f7", "G"))
- _bs = New BindingSource
- _bs.DataSource = _l
- Me.DataGridView1.DataSource = _bs.DataSource
- End Sub
- End Class
If we run this we will get this.
So where did the person data go?
And know watch even more closely and find the difference.
- Public Class Form1
- Dim _bs As BindingSource
- Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
- Dim _l As New List(Of Teacher)
- _l.Add(New Teacher("t1", "f1", "A"))
- _l.Add(New Teacher("t2", "f2", "B"))
- _l.Add(New Teacher("t3", "f3", "C"))
- _l.Add(New Teacher("t4", "f4", "D"))
- _l.Add(New Teacher("t5", "f5", "E"))
- _l.Add(New Teacher("t6", "f6", "F"))
- _l.Add(New Teacher("t7", "f7", "G"))
- _bs = New BindingSource
- _bs.DataSource = _l
- Me.DataGridView1.DataSource = _bs.DataSource
- End Sub
- End Class
And see the result.
Yes the fields are there. Why does this happen? Well reflection and a bug in the interface implementation. Reflection should pick up the properties from IPerson in ITeacher like it does for the class but it doesn’t so your ……. The only solution that works is either bind to teacher or (like in my case) make a private class that accepts an ITeacher and has all the properties needed).
and why in my case? Because I only pass interface between my different layers, never implmentations.
Something like this.
- Public Class LocalTeacher
- Implements ITeacher
- Private _teacher As ITeacher
- Public Sub New(ByVal Teacher As ITeacher)
- _teacher = Teacher
- End Sub
- Public Property Name() As String Implements IPerson.Name
- Get
- Return _teacher.Name
- End Get
- Set(ByVal value As String)
- _teacher.Name = value
- End Set
- End Property
- Public Property FirstName() As String Implements IPerson.FirstName
- Get
- Return _teacher.FirstName
- End Get
- Set(ByVal value As String)
- _teacher.FirstName = value
- End Set
- End Property
- Public Property ClassRoom() As String Implements ITeacher.ClassRoom
- Get
- Return _teacher.ClassRoom
- End Get
- Set(ByVal value As String)
- _teacher.ClassRoom = value
- End Set
- End Property
- Public ReadOnly Property Teacher() As ITeacher
- Get
- Return _teacher
- End Get
- End Property
- End Class
And then this for the form.
- Public Class Form1
- Dim _bs As BindingSource
- Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
- ‘normally I would get this from the DAL
- Dim _l As New List(Of ITeacher)
- _l.Add(New Teacher("t1", "f1", "A"))
- _l.Add(New Teacher("t2", "f2", "B"))
- _l.Add(New Teacher("t3", "f3", "C"))
- _l.Add(New Teacher("t4", "f4", "D"))
- _l.Add(New Teacher("t5", "f5", "E"))
- _l.Add(New Teacher("t6", "f6", "F"))
- _l.Add(New Teacher("t7", "f7", "G"))
- ‘Then I have to do this to it
- Dim _l2 As New List(Of LocalTeacher)
- For Each T As ITeacher In _l
- _l2.Add(New LocalTeacher(T))
- Next
- _bs = New BindingSource
- _bs.DataSource = _l2
- Me.DataGridView1.DataSource = _bs.DataSource
- End Sub
- End Class
and then of course it’s a mess to save it again ;-). Call me nuts 
and tell me I should just pass the objects. But trust me I won’t. I’ll keep the interfaces and live with this kind of thing.
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