WPF Dependency Property - set from XAML - wpf

Question

I am trying to achieve the following in a WPF personal finance app:
In various places I want to display a user control giving details of a asset holding (usually a share, bond etc), the target asset may be changed dynamically by the user in which case the control must be refreshed. Each Asset has a unique identifier, AssetId.
I am using MVVM and I've developed a single window with a View Model that takes AssetID as a parameter (property) and retrieves the relevant details for binding to the View. This work fine. What I'd like to do is make a generic user control with the same functionality so I can basically drop that 'window' inside other windows.
So I pretty much copy-pasted the XAML from that form into a User Control, where I'm struggling is passing in the AssetId from the parent window to the child control.
Google tells me I need a dependency property and here's where I am
Public Class HoldingView
Private _AssetId As Integer
Public AssetIdProperty As DependencyProperty = DependencyProperty.Register("AssetId",
GetType(Integer),
GetType(HoldingView),
New FrameworkPropertyMetadata(New PropertyChangedCallback(AddressOf AssetIDChanged)))
Public Property AssetId As Integer
Get
Return GetValue(AssetIdProperty)
End Get
Set(value As Integer)
SetValue(AssetIdProperty, value)
End Set
End Property
Private Sub AssetIDChanged(d As DependencyObject, e As DependencyPropertyChangedEventArgs)
Dim NewAssetId As Integer
NewAssetId = e.NewValue
Me.DataContext.AssetId = NewAssetId
End Sub
Public Sub New()
' This call is required by the designer.
InitializeComponent()
Me.DataContext = New HoldingViewmodel
End Sub
End Class
Called like this:
<Grid>
<local:HoldingView AssetId="{Binding AssetId}"/>
</Grid>
The code compiles and runs but when I try and load the window that has the user control, the app crashes with this message:
A 'Binding' cannot be set on the 'AssetId' property of type 'HoldingView'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.
Which is not that helpful. From my Googling, you can also get this message if the syntax of the DP registration is not spot on, but it looks Ok to my inexperienced eye...
Anybody else had this?

Answer 1

Public AssetIdProperty As DependencyProperty
should be
Public Shared ReadOnly AssetIdProperty As DependencyProperty
Please take a look at Custom Dependency Properties.
Also remove
Me.DataContext = New HoldingViewmodel
because that will effectively break any DataContext-based Bindings like
AssetId="{Binding AssetId}"
where the source property is supposed to be owned by the object in the inherited DataContext, which usually is an object in the application's view model.
Controls should never have their own, "private" view model, but instead handle property changes in code behind. In case of UserControls, there could simply be UI elements in their XAML that would be bound to the UserConrol's own properties.
Hence
Me.DataContext.AssetId = NewAssetId
in the PropertyChangedCallback is pointless and should be removed, as well as
Private _AssetId As Integer
To summarize, it should look like this:
Public Class HoldingView
Public Shared ReadOnly AssetIdProperty As DependencyProperty =
DependencyProperty.Register(
"AssetId",
GetType(Integer),
GetType(HoldingView),
New FrameworkPropertyMetadata(
New PropertyChangedCallback(AddressOf AssetIdPropertyChanged)))
Public Property AssetId As Integer
Get
Return GetValue(AssetIdProperty)
End Get
Set(value As Integer)
SetValue(AssetIdProperty, value)
End Set
End Property
Private Shared Sub AssetIdPropertyChanged(
d As DependencyObject, e As DependencyPropertyChangedEventArgs)
CType(d, HoldingView).AssetIdChanged(e.NewValue)
End Sub
Private Sub AssetIdChanged(id As Integer)
...
End Sub
Public Sub New()
InitializeComponent()
End Sub
End Class