How to: Implement Validation Logic on Custom Objects


This example shows how to implement validation logic on a custom object and then bind to it.


You can provide validation logic on the business layer if your source object implements IDataErrorInfo, as in the following example:

Public Class Person
       Implements IDataErrorInfo

       Private _age As Integer
       Public Property Age() As Integer
               Return _age
           End Get
           Set(ByVal value As Integer)
               _age = value
           End Set
       End Property

       Public ReadOnly Property [Error]() As String Implements IDataErrorInfo.Error
               Return Nothing
           End Get
       End Property

       Default Public ReadOnly Property Item(ByVal columnName As String) As String Implements IDataErrorInfo.Item
               Dim result As String = Nothing

               If columnName = "Age" Then
                   If Me._age < 0 OrElse Me._age > 150 Then
                       result = "Age must not be less than 0 or greater than 150."
                   End If
               End If
               Return result
           End Get
       End Property
   End Class

In the following example, the text property of the text box binds to the Age property of the Person object, which has been made available for binding through a resource declaration that is given the x:Key data. The DataErrorValidationRule checks for the validation errors raised by the IDataErrorInfo implementation.

<TextBox Style="{StaticResource textBoxInError}">
        <!--By setting ValidatesOnExceptions to True, it checks for exceptions
        that are thrown during the update of the source property.
        An alternative syntax is to add <ExceptionValidationRule/> within
        the <Binding.ValidationRules> section.-->
        <Binding Path="Age" Source="{StaticResource data}"
                <!--DataErrorValidationRule checks for validation 
                    errors raised by the IDataErrorInfo object.-->
                <!--Alternatively, you can set ValidationOnDataErrors="True" on the Binding.-->

Alternatively, instead of using the DataErrorValidationRule, you can set the ValidatesOnDataErrors property to true.