Can my binding source tell me if a change has occurred?

Question

I have a BindingSource that I'm using in winforms data binding and I'd like to have some sort of prompt for when the user attempts to close the form after they've made changes to the data. A sort of "Are you sure you want to exit without saving changes?"

I'm aware that I can do this via the BindingSource's CurrencyManager.ItemChanged event by just flipping a "has changed" boolean.

However, I want a more robust functionality. I'd like to know when the current data is different from the original data. The event just tells me if somethings changed. A user could still change a property, hit undo, and I would still think that there is a change in the data to save.

I want to mimic this similar functionality of notepad

• open notepad
• type something
• delete everything (essentially undoing what you did)
• close notepad, notepad closes, no prompt to save changes because it knows the end state == the initial state

If this is not possible, then should I go with the ItemChanged event handler as outlined above or is there a better way?

For the record, I'm looking for something along the lines of

bool HasChanged()
{
    return this.currentState != this.initialState;
}

not this

bool HasChanged()
{
    // this._hasChanged is set to true via event handlers
    return this._hasChanged;
}

I'd just rather not have to manage the current state and the initial state myself, I'm looking for a way to grab that info from the BindingSource If I can get this functionality from the BindingSource its way more ideal since I will be able to use the functionality on many different data sources, regardless of type, etc.

Ascanius

+1 I find this question very interesting – Cask

If you're talking about the DataTable/DataSet as a binding source each DataRow can have multiple versions. Use HasVersion to check. You can access the values in the various versions by using myDataRow["FieldName", DataRowVersion.Original] or myDataRow.Field<string>("FieldName", DataRowVersion.Current], etc. – Domineer

Answer 1

You'll have to implement the INotifyPropertyChanged interface from within your object classes, then catch whenever a change occurs through proper event handlers for your type class within your DataSource BindingSource property.

The one object offering what you require is the DataSet, containing both the Original and Current (changed) state of an persistent entity. Then, when one cancels, all you need to call is the Rollback() method. When one accepts the changes, then a call to the AcceptChanges() method will do.

Besides the DataSet, perhaps considering an ORM like NHibernate will do the job for you, plus allowing you to use custom defined objects, instead of a DataSet. Keeping the ISession API alive while in your form will allow the ISession to keep track of your changes whatever it may be to whatever object it is, as long as it is know by NHibernate.

Another solution implementing the INotifyPropertyChanged interface, is at the property setter, you could stock the Original value within a private field or for every property of an object. You could simple have an abstract class with the HasChanges property return whether each property is as its Original state, then return true or false accordingly.

I have a question regarding our interesting initial discussion. I just want to make sure of one thing. Let's call it language barrier if we like. But publishing the PropertyChanged event through the INotifyPropertyChanged interface will also somehow "rollback" an object to its original state. The only detail you had to take care is that if the user says he doesn't want to keep the changes, then reload this CurrentItem from the underlying database via the BackgroundWorker class and its done! No lagging from your GUI, your user has canceled the changes, and you resetted the object to its default/original state!

Well, I guess here's enough details to make yourself an idea, plus all of the other good answers provided by the others. I am confident you will find your way to accomplish what you want.

Best of success! =)

Maximo

In what way is this different than flipping a bit with CurrencyManager.ItemChanged? It doesnt compare the origional state with the current state – Ascanius

Your object only should know whether it has changed or not. Implementing it directly into your object, you won't bother playing around with the CurrencyManager. Anyway, I don't get really what Brett means by "Instead of flipping a bit", though I get the idea of comparing the snapshot against the initial state. In fact, the real question is, do you want to be able to compare if whether there was a change compared to your initial state, or do you want to publish there was a change, so you may validate whether the user wants to save changes or not while closing the form? – Maximo

Brett means that instead of doing bool hasChanged() { return this._hasChanged; }, I would do bool hasChanged() { return currentValues != initialValues; } – Ascanius

Thanks for enlightening my lantern. =) This might be a better approach for your situation, I can't say, only you know. Despite, I use the approach suggested regularly and works fine. Doing Brett's way means you have to have two sets of your objects in memory. But that is fine, really! =) – Maximo

@Will, I've read elsewhere that the BindingSource already does this, which is why I'm hesitant to go in and implement it myself. I'd like to know where / how it keeps the current and initial states, so I can compare those, rather than keeping track of it myself – Ascanius

@Allen: I guess you'll have to clone the CurrentItem to be edited into another DataSource, then compare the changes against your primary object from the initial DataSource. Otherwise, I think CurrencyManager.CurrentItemChanged can do it, even though this is something you would like to avoid. I'm sorry, I can't help more than that even if I would have liked to. I don't get the point of really comparing between an initial and current object state. If a property publishes that it has changed, than its older value is already overwritten. For the behaviour of FormClosing, PropertyChanged is fine. – Maximo

@Allen: I hope this won't get too hard to maintain for the next newbie programmer that will come in and work with it. You must know what you're doing, for sure! =) – Maximo

@will, the point of all this is explained in the question, I think we're running into a language barrier. What if they change the value and then undo their change. At that point, you will think that they have changed something when they really haven't. Unfortunately, its looking like it will be impossible to accomplish this without alot of type specific code, which I was trying to avoid and wont be interested in implementing. – Ascanius

@Allen: You may be right at some point about the language barrier. =) See my EDIT, please. – Maximo

Answer 2

Will is right, you should implement INotifyPropertyChanged, ideally in conjunction with IDataInfoError to get visisble information for your users.

For your Objects to get a state and a notification on Editing, try using the IEditableObject interface.

All three interfaces are used by default from WinForms and help make the programmers life easier.

Absentminded

I may be missing something, but like I said, in what way is this different than the event I outlined in my question. Its STILL just telling me when a user had made a change, its not comparing the initial state to current state. – Ascanius

@Allen: It just depends on your implementation if it holds an initial state internally. The reference implementation for IEditableObject shows a clean way to make your objects editable and undoable. – Absentminded

Answer 3

Instead of flipping a bit, you could check the state against a snapshot of your initial state.

Mc

yes yes yes, this is what i want to do, but how do i do it? lol I dont want to do it manually, surely the currencymanager or the bindingsource has this built in. That or maybe it keeps track of the initial and current states – Ascanius

Answer 4

When you open your detail, you could make a clone of the entity that you are going to modify.

Then, when the user attempts to close the form, you could compare the clone (the entity in its original state) with the modified (or not) entity. If the clone and the entity are not equals, you could prompt the user.

Cask

I was under the opinion that the CurrencyManager or the BindingSource did this already, internally. – Ascanius

Aha, yeah..., it has all the sense. I´ll wait for an answer too your question, too. – Cask

Answer 5

You could roll your own binding source and implement it to do what you want that way you do not need INotifyChange handling on every form - you just let the BindingSource give you the changed element - this works when the BindingSource is updated - bindable control .UpdateSourceTrigger is set to UpdateOnPropertyChanged. is instant(well almost).

Here is something to get you started - I found it on the net years ago I do not remember the originator of the code , I have modified it slightly for my purpose.

Imports System.ComponentModel.Design
Imports System.Windows.Forms
Imports System.ComponentModel

Public Class BindingSourceExIsDirty
    Inherits System.Windows.Forms.BindingSource
    Implements INotifyPropertyChanged

    #Region "DECLARATIONS AND PROPERTIES"

    Private _displayMember As String
    Private _dataTable As DataTable
    Private _dataSet As DataSet
    Private _parentBindingSource As BindingSource
    Private _form As System.Windows.Forms.Form
    Private _usercontrol As System.Windows.Forms.Control

    Private _isCurrentDirtyFlag As Boolean = False

    Public Property IsCurrentDirty() As Boolean
        Get
            Return _isCurrentDirtyFlag
        End Get
        Set(ByVal value As Boolean)
            If _isCurrentDirtyFlag <> value Then
                _isCurrentDirtyFlag = value
                Me.OnPropertyChanged(value.ToString())
                If value = True Then 'call the event when flag is set
                    OnCurrentIsDirty(New EventArgs)

                End If
            End If
        End Set
    End Property

    Private _objectSource As String

    Public Property ObjectSource() As String
        Get
            Return _objectSource
        End Get
        Set(ByVal value As String)
            _objectSource = value
            Me.OnPropertyChanged(value)
        End Set
    End Property   

'    Private _autoSaveFlag As Boolean
'
'    Public Property AutoSave() As Boolean
'        Get
'            Return _autoSaveFlag
'        End Get
'        Set(ByVal value As Boolean)
'           _autoSaveFlag = value
'           Me.OnPropertyChanged(value.ToString())
'        End Set
'    End Property  

    #End Region

    #Region "EVENTS"

    'Current Is Dirty Event
    Public Event CurrentIsDirty As CurrentIsDirtyEventHandler

    ' Delegate declaration.
    Public Delegate Sub CurrentIsDirtyEventHandler(ByVal sender As Object, ByVal e As EventArgs)

    Protected Overridable Sub OnCurrentIsDirty(ByVal e As EventArgs)
        RaiseEvent CurrentIsDirty(Me, e)
    End Sub

     'PropertyChanged Event 
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged

    Protected Overridable Sub OnPropertyChanged(ByVal info As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(info))
    End Sub 

    #End Region

    #Region "METHODS"

    Private Sub _BindingComplete(ByVal sender As System.Object, ByVal e As System.Windows.Forms.BindingCompleteEventArgs) Handles Me.BindingComplete
        If e.BindingCompleteContext = BindingCompleteContext.DataSourceUpdate Then
            If e.BindingCompleteState = BindingCompleteState.Success And Not e.Binding.Control.BindingContext.IsReadOnly Then

                'Make sure the data source value is refreshed (fixes problem mousing off control)
                e.Binding.ReadValue()
                'if not focused then not a user edit.
                If Not e.Binding.Control.Focused Then Exit Sub

                'check for the lookup type of combobox that changes position instead of value
                If TryCast(e.Binding.Control, ComboBox) IsNot Nothing Then
                    'if the combo box has the same data member table as the binding source, ignore it
                    If CType(e.Binding.Control, ComboBox).DataSource IsNot Nothing Then
                        If TryCast(CType(e.Binding.Control, ComboBox).DataSource, BindingSource) IsNot Nothing Then
                            If CType(CType(e.Binding.Control, ComboBox).DataSource, BindingSource).DataMember = (Me.DataMember) Then
                                Exit Sub
                            End If

                        End If

                    End If
                End If
                IsCurrentDirty = True 'set the dirty flag because data was changed
            End If
        End If
    End Sub

    Private Sub _DataSourceChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Me.DataSourceChanged
        _parentBindingSource = Nothing
        If Me.DataSource Is Nothing Then
            _dataSet = Nothing
        Else
            'get a reference to the dataset
            Dim bsTest As BindingSource = Me
            Dim dsType As Type = bsTest.DataSource.GetType
            'try to cast the data source as a binding source
            Do While Not TryCast(bsTest.DataSource, BindingSource) Is Nothing
                'set the parent binding source reference
                If _parentBindingSource Is Nothing Then _parentBindingSource = bsTest
                'if cast was successful, walk up the chain until dataset is reached
                bsTest = CType(bsTest.DataSource, BindingSource)
            Loop
            'since it is no longer a binding source, it must be a dataset or something else
            If TryCast(bsTest.DataSource, DataSet) Is Nothing Then
                'Cast as dataset did not work

                If dsType.IsClass = False Then
                    Throw New ApplicationException("Invalid Binding Source ")
                Else
                    _dataSet = Nothing

                End If
            Else

                _dataSet = CType(bsTest.DataSource, DataSet)
            End If


            'is there a data member - find the datatable
            If Me.DataMember <> "" Then
                _DataMemberChanged(sender, e)
            End If 'CType(value.GetService(GetType(IDesignerHost)), IDesignerHost)
            If _form Is Nothing Then GetFormInstance()
            If _usercontrol Is Nothing Then GetUserControlInstance()
        End If
    End Sub

    Private Sub _DataMemberChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Me.DataMemberChanged
        If Me.DataMember = "" Or _dataSet Is Nothing Then
            _dataTable = Nothing
        Else
            'check to see if the Data Member is the name of a table in the dataset
            If _dataSet.Tables(Me.DataMember) Is Nothing Then
                'it must be a relationship instead of a table
                Dim rel As System.Data.DataRelation = _dataSet.Relations(Me.DataMember)
                If Not rel Is Nothing Then
                    _dataTable = rel.ChildTable
                Else
                    Throw New ApplicationException("Invalid Data Member")
                End If
            Else
                _dataTable = _dataSet.Tables(Me.DataMember)
            End If
        End If
    End Sub

    Public Overrides Property Site() As System.ComponentModel.ISite
        Get
            Return MyBase.Site
        End Get
        Set(ByVal value As System.ComponentModel.ISite)
            'runs at design time to initiate ContainerControl
            MyBase.Site = value
            If value Is Nothing Then Return
            ' Requests an IDesignerHost service using Component.Site.GetService()
            Dim service As IDesignerHost = CType(value.GetService(GetType(IDesignerHost)), IDesignerHost)
            If service Is Nothing Then Return
            If Not TryCast(service.RootComponent, Form) Is Nothing Then
                _form = CType(service.RootComponent, Form)
            ElseIf Not TryCast(service.RootComponent, UserControl) Is Nothing Then
                _usercontrol = CType(service.RootComponent, UserControl)
            End If

        End Set
    End Property

    Public Function GetFormInstance() As System.Windows.Forms.Form
        If _form Is Nothing And Me.CurrencyManager.Bindings.Count > 0 Then
            _form = Me.CurrencyManager.Bindings(0).Control.FindForm()

        End If
        Return _form
    End Function

    ''' <summary>
    ''' Returns the First Instance of the specified User Control
    ''' </summary>
    ''' <returns>System.Windows.Forms.Control</returns>
    Public Function GetUserControlInstance() As System.Windows.Forms.Control
        If _usercontrol Is Nothing And Me.CurrencyManager.Bindings.Count > 0 Then
            Dim _uControls() As System.Windows.Forms.Control
            _uControls = Me.CurrencyManager.Bindings(0).Control.FindForm.Controls.Find(Me.Site.Name.ToString(), True)
            _usercontrol = _uControls(0)

        End If
        Return _usercontrol
    End Function

    '============================================================================

    'Private Sub _PositionChanged(ByVal sender As Object, ByVal e As EventArgs) Handles Me.PositionChanged

    '    If IsCurrentDirty Then
    '        If AutoSave Then  ' IsAutoSavingEvent
    '            Try
    '                'cast table as ITableUpdate to get the Update method
    '                '  CType(_dataTable, ITableUpdate).Update()
    '            Catch ex As Exception
    '               ' - needs to raise an event 
    '            End Try
    '        Else
    '            Me.CancelEdit()
    '            _dataTable.RejectChanges()
    '        End If
    '        IsCurrentDirty = False
    '    End If
    'End Sub

    #End Region

End Class

Pointillism

Answer 6

Yes, but there is some work involved. I know, it's a late answer, but I asked myself the same question resecently and came up with the following soltion that I wrapped up into the class UpdateManager. I only accounted for binding to a single object so far.

This works with plain POCO objects. Implementing INotifyPropertyChanged is not required; however, it works only if the changes are made through the UI. Changes made through code in the business object are not detected. But this enough in most cases to detect whether the object is dirty or in a saved state.

public class UpdateManager
{
    public event EventHandler DirtyChanged;

    private readonly BindingSource _bindingSource;

    // Stores original and current values of all bindings.
    private readonly Dictionary<string, (object original, object current)> _values =
        new Dictionary<string, (object original, object current)>();

    public UpdateManager(BindingSource bindingSource)
    {
        _bindingSource = bindingSource;
        bindingSource.CurrencyManager.Bindings.CollectionChanged += Bindings_CollectionChanged;
        bindingSource.BindingComplete += BindingSource_BindingComplete;
    }

    private bool _dirty;
    public bool Dirty
    {
        get {
            return _dirty;
        }
        set {
            if (value != _dirty) {
                _dirty = value;
                DirtyChanged?.Invoke(this, EventArgs.Empty);
            }
        }
    }

    private void Bindings_CollectionChanged(object sender, CollectionChangeEventArgs e)
    {
        // Initialize the values information for the binding.
        if (e.Element is Binding binding && GetCurrentValue(binding, out object value)) {
            _values[binding.BindingMemberInfo.BindingField] = (value, value);
        }
    }

    private void BindingSource_BindingComplete(object sender, BindingCompleteEventArgs e)
    {
        if (e.BindingCompleteContext == BindingCompleteContext.DataSourceUpdate &&
            e.BindingCompleteState == BindingCompleteState.Success) {

            UpdateDirty(e.Binding);
        }
    }

    private void UpdateDirty(Binding binding)
    {
        if (GetCurrentValue(binding, out object currentValue)) {
            string propertyName = binding.BindingMemberInfo.BindingField;
            var valueInfo = _values[propertyName];
            _values[propertyName] = (valueInfo.original, currentValue);
            if (Object.Equals(valueInfo.original, currentValue)) {
                Dirty = _values.Any(kvp => !Object.Equals(kvp.Value.original, kvp.Value.current));
            } else {
                Dirty = true;
            }
        }
    }

    private bool GetCurrentValue(Binding binding, out object value)
    {
        object model = binding.BindingManagerBase?.Current;
        if (model != null) {
            // Get current value in business object (model) with Reflection.
            Type modelType = model.GetType();
            string propertyName = binding.BindingMemberInfo.BindingField;
            PropertyInfo modelProp = modelType.GetProperty(propertyName);
            value = modelProp.GetValue(model);
            return true;
        }
        value = null;
        return false;
    }
}

In the form I used it like this:

private UpdateManager _updateManager;
private Person _person = new Person();

public frmBindingNotification()
{
    InitializeComponent();
    _updateManager = new UpdateManager(personBindingSource);
    _updateManager.DirtyChanged += UpdateManager_DirtyChanged;
    personBindingSource.DataSource = _person; // Assign the current business object.
}

private void UpdateManager_DirtyChanged(object sender, EventArgs e)
{
    Console.WriteLine(_updateManager.Dirty ? "Dirty" : "Saved"); // Testing only.
}

Whenever the Dirty status changes, this prints either "Dirty" or "Saved" in the Output window.

Penley