Public Class spTextBox
    Implements IDataErrorInfo

    Private validationErrors As New Dictionary(Of String, String)

    Protected Sub AddError(ByVal columnName As String, ByVal msg As String)
        If Not validationErrors.ContainsKey(columnName) Then
            validationErrors.Add(columnName, msg)
        End If
    End Sub

    Protected Sub RemoveError(ByVal columnName As String)
        If validationErrors.ContainsKey(columnName) Then
            validationErrors.Remove(columnName)
        End If
    End Sub

    Public Overridable ReadOnly Property HasErrors() As Boolean
        Get
            Return (validationErrors.Count > 0)
        End Get
    End Property

    Public ReadOnly Property [Error]() As String _
        Implements System.ComponentModel.IDataErrorInfo.Error
        Get
            If validationErrors.Count > 0 Then
                Return String.Format("{0} data is invalid.", TypeName(Me))
            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 validationErrors.ContainsKey(columnName) Then
                Return validationErrors(columnName).ToString
            Else
                Return Nothing
            End If
        End Get
    End Property

    Private Sub OnTextChanging(ByVal value As String)
        Me.CheckText(txt.Text)
    End Sub

    Private Sub CheckText(ByVal value As String)
        value = txt.Text
        If value = "" Then
            Me.AddError("Text", "La casella di testo non può essere vuota")
        Else
            Me.RemoveError("Text")
        End If
    End Sub

    Public Property Text As String
        Get
            Return txt.Text
        End Get
        Set(ByVal value As String)
            txt.Text = value
        End Set
    End Property

    Public Property TextAlignment As System.Windows.TextAlignment
        Get
            Return txt.TextAlignment
        End Get
        Set(ByVal value As System.Windows.TextAlignment)
            txt.TextAlignment = TextAlignment
        End Set
    End Property

    Private Sub txt_GotFocus(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles txt.GotFocus

        txt.SelectAll()

    End Sub

End Class
 

  <TextBox Name="txt" BorderBrush="Black" HorizontalContentAlignment="Left" VerticalContentAlignment="Center"
              Text="{Binding Path=Text, ValidatesOnDataErrors=True}">
        <TextBox.Style>
            <Style>
                <Style.Triggers>
                    <Trigger Property="Validation.HasError" Value="true">
                        <Setter Property="Control.ToolTip"
                        Value="{Binding RelativeSource={x:Static RelativeSource.Self},
                        Path=(Validation.Errors)[0].ErrorContent}"/>
                    </Trigger>
                    <Trigger Property="Control.IsFocused" Value="True">
                        <Setter Property="Control.Background" Value="Yellow"/>
                        <Setter Property="Control.FontWeight" Value="Bold"/>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </TextBox.Style>
    </TextBox> 

    <TextBox Name="txt" BorderBrush="Black" HorizontalContentAlignment="Left" VerticalContentAlignment="Center">
        <TextBox.Text>
            <Binding Path="Text" ElementName="txt" ValidatesOnDataErrors="True" ValidatesOnExceptions="True">
                <Binding.ValidationRules>
                    <local:TextBoxValidationRule ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
        <TextBox.Style>
            <Style>
                <Style.Triggers>
                    <Trigger Property="Control.IsFocused" Value="True">
                        <Setter Property="Control.Background" Value="Yellow"/>
                        <Setter Property="Control.FontWeight" Value="Bold"/>
                    </Trigger>
                    <Trigger Property="Validation.HasError" Value="true">
                        <Setter Property="Control.Background" Value="Red" />
                    </Trigger>
                </Style.Triggers>
            </Style>
        </TextBox.Style>
    </TextBox>
 

Public Class TextBoxValidationRule
    Inherits ValidationRule

    Public Overrides Function Validate(ByVal value As Object, ByVal cultureInfo As System.Globalization.CultureInfo) As System.Windows.Controls.ValidationResult

        Dim testo As String = value.ToString

        If value = String.Empty Then
            Return New ValidationResult(False, "Stringa vuota!")
        End If

        Return New ValidationResult(True, DBNull.Value)

    End Function
End Class