How to stop a custom control bound to a business object from updating unnecessarily?

I have a Windows Forms application with a custom control for entering dates. This is inherits from Textbox and implements a Value property for binding to the underlying business object.

Everything works well except that when the control validates it updates the bound property, even when it hasn't changed. This is a problem because I'm using Entity Framework and the change to the entity's property causes the corresponding field in the database to be updated every time a user opens and close开发者_开发百科s the form hosting this control.

Here is the code:

Public Class TextBoxDate
Inherits TextBox

Public ValueChanged As EventHandler

Private _dateValue As Nullable(Of Date) = Nothing
<Bindable(True)> _
<Category("Appearance")> _
Public Property Value() As Nullable(Of Date)
    ' Bind to the Value property instead of the Text property, as the latter will not allow
    ' the user to delete the contents of the textbox. The Value property provides support for nulls.
    Get
        Value = _dateValue
    End Get
    Set(ByVal value As Nullable(Of Date))
        _dateValue = value
        ' Update the text in the textbox
        If value.HasValue Then
            Text = CDate(value).ToShortDateString
        Else
            Text = vbNullString
        End If
        OnValueChanged()
    End Set
End Property

Private Sub OnValueChanged()
    If (ValueChanged IsNot Nothing) Then
        ValueChanged(Me, New EventArgs())
    End If

End Sub

Private Sub TextBoxDate_Enter(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Enter

    _userEntered = True

End Sub

Private Sub TextBoxDate_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Leave

    _userEntered = False

End Sub

Protected Overrides Sub OnValidating(ByVal e As System.ComponentModel.CancelEventArgs)

    If _userEntered Then
        If Me.TextLength = 0 Then
            ' Null value will be saved to the database via the bound Value property
            If Value IsNot Nothing Then
                Value = Nothing
            End If
        Else
            Dim dateValue As Date
            If Date.TryParseExact(Text, "d/M/yyyy", System.Globalization.DateTimeFormatInfo.InvariantInfo, Globalization.DateTimeStyles.None, dateValue) Then
                If Value Is Nothing Or (dateValue <> Value) Then
                    Value = CDate(Text)
                End If
            Else
                e.Cancel = True
            End If
        End If
    End If

    MyBase.OnValidating(e)

End Sub

End Class

This is driving me mad. Any help would be much appreciated.

Scott

When you define the binding of the textbox you can specify the DataSourceUpdateMode of the Binding object, for example

Binding custNameBinding = new Binding("Text", ds, "customers.custName");
custNameBinding.DataSourceUpdateMode = DataSourceUpdateMode.OnPropertyChanged;
custNameTextBox.DataBindings.Add(custNameBinding);

The MSDN documentation says "Data source is updated whenever the value of the control property changes."