MVVM: list in viewmodel consists of entities but I don't think it's right to place logic in entity poco classes - entity-framework

Question

I have a viewmodel and it exists out of entity framework classes (just plain classes). I Read on stackoverflow that it's okey to put an inotifypropertychanged in the model. So I did that in order not to have a lot of extra code.
So, now I have to perform an action when a property changes in one of my list items... the thing is, I can't react on this, in my viewmodel...
What do I do?

Answer 1

I have come across this issue once I started making larger projects with the MVVM pattern.
In the end I just shifted my INotifyPropertyChanged event calls to the ViewModel properties. I also decorated these properties with validation notification as that is where most of the validation that needs a user to react to will be held, any common elements can also be pulled out to a base ViewModel where relevant.
Another issue that you might come across as time goes by is where to put some of your processing logic. I used to have mine at the Model level, but then some of them required a bit closer interaction with the UI (validation reasons mainly), but if I put it in the ViewModel then I would have to have references to my repositories etc. In the end my ViewModels have access to another layer (a Service Layer) where this takes place. (A prime example for the use of this would be if you have a column in a database which is unique that you need to check when a user has created/modified data belonging to this column.)
Anyway, that is just a bit of extra information that I had to figure out shortly after the issue your currently having!

Answer 2

i fought with the best stategy for this for a while. I found that there are a number of different ways to do this.
First thing to note, if you add the property validation/notification directly to your model classes then the notifictaion and validation will happen when the class is created and for each object that gets created in observable collections, which in turn can cause performance issues or other challenges. So for that reason I moved my validation and notification into a "helper" partial class for the model.
So the model gets created through the Entity framework, lets say for an object called Job. I create public partial class called job as well. Here is an example (in vb, I can convert to C# if you need)
Partial Public Class job
Inherits ValidationBase
#Region "CONSTRUCTORS"
Public Sub New()
''default values
Me.FTC_Type = 4
Me.dtCreated = Now
Me.dtUpdated = Now
HasChanges = False
End Sub
Public Sub New(bValidate As Boolean)
PropertyValitaion(bValidate)
''default values
Me.FTC_Type = 4
Me.dtCreated = Now
Me.dtUpdated = Now
HasChanges = False
End Sub
Public ReadOnly Property DisplayPath
Get
Return "W" + idJob.ToString + ": " + chrTitle + " - " + client.chrCompany
End Get
End Property
Public ReadOnly Property SearchPath
Get
Return "W" + idJob.ToString + " " + chrTitle + " " + client.chrCompany + " " + chrContact
End Get
End Property
#End Region
#Region "VALIDATION FUNCTIONS"
Public Overrides Function Validate(validationContext As ComponentModel.DataAnnotations.ValidationContext) As IEnumerable(Of ComponentModel.DataAnnotations.ValidationResult)
Return MyBase.Validate(validationContext)
PropertyValitaion(True)
End Function
Public Sub PropertyValitaion(bAllProperties As Boolean, Optional sProperty As String = "")
'initialize validation helper
If bAllProperties OrElse sProperty = "chrTitle" Then
If String.IsNullOrEmpty(chrTitle) Then
AddError("chrTitle", "You must enter a Job Title")
Else
RemoveError("chrTitle")
End If
End If
End Sub
#End Region
End Class
So you can see that I can create other readonly properties (Like display title etc) and create my own custom validation that gets called when I want. THis validation uses the IDataErrorInfo for getting tied back into the UI. I have two constructors, one the calls the property validation and one that does not. That way I can control when it happens.
I created a ValidationBase class that my Objects partial class inhertis from. THis saves me from having to manually implement IdataErrorInfo and INotifyPropertyCHnaged in every partial class for every object.
Here is my validation base:
Imports System.ComponentModel
Imports System.Collections.Concurrent
Imports System.ComponentModel.DataAnnotations
Imports System.ComponentModel.DataAnnotations.Schema
Public Class ValidationBase
Implements IValidatableObject, IDataErrorInfo, INotifyPropertyChanged
#Region "DECLARATIONS"
Protected _propertyErrors As New Dictionary(Of String, String)
Protected _validationResults As New List(Of ValidationResult)
Public ReadOnly Property HasErrors() As Boolean
Get
Return (_propertyErrors.Count + _validationResults.Count) > 0
End Get
End Property
#End Region
#Region "IValidatableObject IMPLEMENTATION"
Public Overridable Function Validate(validationContext As ValidationContext) As IEnumerable(Of ValidationResult) Implements IValidatableObject.Validate
Return Nothing
End Function
#End Region
#Region "iDataError OBJECTS"
'Returns an error message
'In this case it is a general message, which is
'returned if the list contains elements of errors
Public ReadOnly Property [Error] As String Implements System.ComponentModel.IDataErrorInfo.Error
Get
If _propertyErrors.Count > 0 Then
Return "Object data is invalid"
Else
Return Nothing
End If
End Get
End Property
Default Public ReadOnly Property Item(ByVal columnName As String) As String Implements System.ComponentModel.IDataErrorInfo.Item
Get
If _propertyErrors.ContainsKey(columnName) Then
Return _propertyErrors(columnName).ToString
Else
Return Nothing
End If
End Get
End Property
#End Region
#Region "IDataError FUNCTIONS"
'Adds an error to the collection, if not already present
'with the same key
Protected Sub AddError(ByVal columnName As String, ByVal msg As String)
If Not _propertyErrors.ContainsKey(columnName) Then
_propertyErrors.Add(columnName, msg)
OnPropertyChanged(columnName)
End If
End Sub
'Removes an error from the collection, if present
Protected Sub RemoveError(ByVal columnName As String)
If _propertyErrors.ContainsKey(columnName) Then
_propertyErrors.Remove(columnName)
OnPropertyChanged(columnName)
End If
End Sub
Public Sub ClearErrors()
_propertyErrors.Clear()
End Sub
#End Region
#Region "INotifyPropertyChanged IMPLEMENTATION"
Public Event PropertyChanged(sender As Object, e As PropertyChangedEventArgs) Implements INotifyPropertyChanged.PropertyChanged
Public Overridable Sub OnPropertyChanged(ByVal propertyName As String)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
End Sub
#End Region
End Class
SO i can't say this is the totally right way because there does not seem to be a clear best pratcie defined out there. But this works for me and I hope it can help you.