I created a new project, very simple, to test only the Microsoft WPF DataGrid behavior. Nothing else involved, I only use the standard DataGrid:
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<DataGrid ItemsSource="{Binding Employees, Mode=TwoWay}"
x:Name="tlv"
AutoGenerateColumns="False"
SelectionMode="Extended"
CanUserAddRows="true"
SelectionUnit="CellOrRowHeader">
<DataGrid.Columns>
<DataGridTextColumn Header="First Name" Binding="{Binding FirstName, Mode=TwoWay}"/>
<DataGridTextColumn Header="Last Name" Binding="{Binding LastName, Mode=TwoWay}"/>
<DataGridTextColumn Header="Salary" Binding="{Binding Salary, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=LostFocus}"/>
</DataGrid.Columns>
</DataGrid>
</Window>
Code behind:
Imports System.Collections.ObjectModel
Imports DataGridTest.Data
Class MainWindow
Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
Me.DataContext = Me
Employees = New ObservableCollection(Of Employee)(EmployeeRepository.GetFlatListData())
BindableSelectedItems = New ObservableCollection(Of Object)
End Sub
Private _employees As ObservableCollection(Of Employee)
Public Property Employees() As ObservableCollection(Of Employee)
Get
Return _employees
End Get
Set(ByVal value As ObservableCollection(Of Employee))
_employees = value
End Set
End Property
Private _bindableSelectedItems As ObservableCollection(Of Object)
Public Property BindableSelectedItems() As ObservableCollection(Of Object)
Get
Return _bindableSelectedItems
End Get
Set(value As ObservableCollection(Of Object))
'Set the new value of BindableSelectedItems
_bindableSelectedItems = value
End Set
End Property
Private _selectedEmployeeForSelectedItemsSimulation As Employee
Public Property SelectedEmployeeForSelectedItemsSimulation() As Employee
Get
Return _selectedEmployeeForSelectedItemsSimulation
End Get
Set(value As Employee)
'Set the new value of SelectedEmployeeForSelectedItemsSimulation
_selectedEmployeeForSelectedItemsSimulation = value
If _selectedEmployeeForSelectedItemsSimulation IsNot Nothing AndAlso BindableSelectedItems IsNot Nothing Then
BindableSelectedItems.Clear()
BindableSelectedItems.Add(value)
End If
End Set
End Property
End Class
The Employee class, which implements IDataErrorInfo:
Imports Microsoft.Practices.Prism.ViewModel
Imports System.Collections.ObjectModel
Imports System.ComponentModel
Namespace Data
Public Class Employee
Inherits NotificationObject
Implements IDataErrorInfo
Public Sub New()
End Sub
Public Sub New(ByVal fName As String, ByVal lName As String, ByVal salary As Double)
FirstName = fName
LastName = lName
Me.Salary = salary
End Sub
Private _firstName As String
Public Property FirstName() As String
Get
Return _firstName
End Get
Set(value As String)
'Set the new value of FirstName
_firstName = value
'Warn any Observers that the FirstName have changed.
RaisePropertyChanged(Function() Me.FirstName)
End Set
End Property
Private _lastName As String
Public Property LastName() As String
Get
Return _lastName
End Get
Set(value As String)
'Set the new value of LastName
_lastName = value
'Warn any Observers that the LastName have changed.
RaisePropertyChanged(Function() Me.LastName)
End Set
End Property
Private _isSelected As Boolean
Public Property IsSelected() As Boolean
Get
Return _isSelected
End Get
Set(value As Boolean)
'Set the new value of IsSelected
_isSelected = value
'Warn any Observers that the IsSelected have changed.
RaisePropertyChanged(Function() Me.IsSelected)
End Set
End Property
Private _salary As Double
Public Property Salary() As Double
Get
Return _salary
End Get
Set(value As Double)
'Set the new value of Salary
_salary = value
'Warn any Observers that the Salary have changed.
RaisePropertyChanged(Function() Me.Salary)
End Set
End Property
Public ReadOnly Property [Error] As String Implements IDataErrorInfo.Error
Get
Return String.Empty
End Get
End Property
Default Public ReadOnly Property Item(columnName As String) As String Implements IDataErrorInfo.Item
Get
If Me.Salary <= 0 Then
Return "The salary must be positive."
End If
Return String.Empty
End Get
End Property
End Class
End Namespace
So far, so good. Then, here is what happens when I try to update the salary value after having added a new row:
The value resets to 0!
- Why is it doing this? Something wrong with the way I am using DataGrids?
- Is there a workaround?
[EDIT] ... and WORKAROUND
I finally found what looks like a workaround, even if I still don't know the reason of what I now consider as a bug in the Microsoft DataGrid.
If a binding mode is specified in the datagrid column with a Validation, the bug occurs; if instead I do not specify the binding mode, everything is fine.
<DataGridTextColumn Header="Salary" Binding="{Binding Salary, ValidatesOnDataErrors=True, UpdateSourceTrigger=LostFocus}"/>
What I do not quite understand though, is that as far as I know the default binding mode is TwoWay... Well, at least I solved my problem.
I opened a bug report on Microsoft Connect, here
