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