Data Validation with MVVM-Light WPF and Linq to Entity Framework - wpf

Question

I think I have read every article google returns when I search wpf mvvm-light data validation and I dont know which way to go. I am aware of josh smith, Karl Shifflett's, and MVVM LIGHT's own demo techniques for data validation. What I see is that most validation requires me to fully "re-abstract" my model in my view model. Meaning that I have to create a property in my viewmodel for each property of my model that I want to validate (and in some cases convert all these into string values for binding/validation). This seems like a lot or redundancy when all I want to do is mark most fields as required.
I am using LINQ to entity framework(with self tracking) for my model classes which come from a SQL server DB. As a result I would prefer to keep my business data validation/rules within my viewmodels. I write a simple service interface to get the data from the model and pass it to my viewmodel.
Most of the examples I can find are from as far back as 2008 (ie josh smith). Are these techniques still valid or are there more up to date best practices for mvvm data validation with .NET 4.5 etc.
So I am asking:
1) What methods do you suggest I use
2) What methods work best in a LINQ to EF with MVVM-Light Environment.
3) EDIT: I want to provide feedback to user as they enter data, not just when they submit form
thanks

Answer 1

The way I do this (not necessarily correct) is to do my validation in the ViewModel (where the CRUD operations typically take place), then if there are validation errors, abort saving/adding any data and use Messenger.Default.Send to send a custom message type to my view. Then I alert the user via a DialogBox or otherwise.
I've experimented with Binding ValidationRules in the past but found by far the most reliable and consistent method to be simple if statements.

Answer 2

I eventually ended up using the following. I changed my model to use LINQ to self tracking entities (see this article for info about STE http://msdn.microsoft.com/en-us/library/vstudio/ff407090%28v=vs.100%29.aspx).
LINQ to STE creates an OnPropertyChanged event that implements the iNotifyPropertyChanged interface.
I just created a public partial class for the matching model object (linq entity generated code) I wanted and added an event handler for the OnPropertyChanged event. I then used the IDataErrorInfo interface to validate and throw errors as I needed. This allows me to validate the fields as they change which gets reflected to the user. This also allows you to perform more advanced validation logic that may need to requery the database (i.e. to look for if a username is already used etc.) or throw a dialog box
Also, having the data validation in the model allows me to still have validation if i perform direct "batch" operations that bypass the UI.
I then used an HasErrors and HasChanges property and used them to create a Boolean value that gets attached to the relay commands, disabling the crud command buttons if errors are present.
I will post some simple code to outline what I just described, comment if you want more detail.
Here is the Entity Framework extension of the model class:
Imports System.ComponentModel
Partial Public Class client
Implements IDataErrorInfo
#Region "Properties / Declarations"
'Collection / error description
Private m_validationErrors As New Dictionary(Of String, String)
Private _HasChanges As Boolean = False
''Marks object as dirty, requires saving
Public Property HasChanges() As Boolean
Get
Return _HasChanges
End Get
Set(value As Boolean)
If Not Equals(_HasChanges, value) Then
_HasChanges = value
OnPropertyChanged("HasChanges")
End If
End Set
End Property
'Extends the class with a property that determines
'if the instance has validation errors
Public ReadOnly Property HasErrors() As Boolean
Get
Return m_validationErrors.Count > 0
End Get
End Property
#End Region
#Region "Base Error 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 m_validationErrors.Count > 0 Then
Return "Client 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 m_validationErrors.ContainsKey(columnName) Then
Return m_validationErrors(columnName).ToString
Else
Return Nothing
End If
End Get
End Property
#End Region
#Region "Base Error Methods"
'Adds an error to the collection, if not already present
'with the same key
Private Sub AddError(ByVal columnName As String, ByVal msg As String)
If Not m_validationErrors.ContainsKey(columnName) Then
m_validationErrors.Add(columnName, msg)
End If
End Sub
'Removes an error from the collection, if present
Private Sub RemoveError(ByVal columnName As String)
If m_validationErrors.ContainsKey(columnName) Then
m_validationErrors.Remove(columnName)
End If
End Sub
#End Region
Public Sub New()
Me.HasChanges = False
End Sub
#Region "Data Validation Methods"
''handles event and calls function that does the actual validation so that it can be called explicitly for batch processes
Private Sub ValidateProperty(ByVal sender As Object, ByVal e As PropertyChangedEventArgs) Handles Me.PropertyChanged
If e.PropertyName = "HasChanges" Then
Exit Sub
End If
IsPropertyValid(e.PropertyName)
HasChanges = True
End Sub
Public Function IsPropertyValid(sProperty As String) As Boolean
Select Case sProperty
''add validation by column name here
Case "chrLast"
If Me.chrLast.Length < 4 Then
Me.AddError("chrLast", "The last name is too short")
Return True
Else
Me.RemoveError("chrLast")
Return False
End If
Case Else
Return False
End Select
End Function
#End Region
End Class
then in the view model I included the following code to bind thecommand and evaluate whether or not it can be executed.
Public ReadOnly Property SaveCommand() As RelayCommand
Get
If _SaveCommand Is Nothing Then
_SaveCommand = New RelayCommand(AddressOf SaveExecute, AddressOf CanSaveExecute)
End If
Return _SaveCommand
End Get
End Property
Private Function CanSaveExecute() As Boolean
Try
If Selection.HasErrors = False And Selection.HasChanges = True Then
Return True
Else
Return False
End If
Catch ex As Exception
Return False
End Try
End Function
Private Sub SaveExecute()
''this is my LINQ to Self Tracking Entities DataContext
FTC_Context.SaveChanges()
End Sub
the following is how I bound my button (has custom styling in WPF)
<Button Content="" Height="40" Style="{DynamicResource ButtonAdd}" Command="{Binding SaveCommand}" Width="40" Cursor="Hand" ToolTip="Save Changes" Margin="0,0,10,10"/>
so, when there are no validation errors and the current client record "isDirty" the save button automatically becomes enabled, and disabled if any of those two conditions fail. This way I now have a simple way of validating any type of column/data I want for the entity, and I can provide user feedback as they enter data in the form, and only enable CRUD command buttons once all my "conditions" have been met.
This was quite a battle to figure out.