Star date: 2009.300
WPF data binding has built in support for IDataErrorInfo, so it is easy to display a validation error when a property has invalid data. However, sometimes we need to manually find all the validation errors on an object. A perfect example of this is when trying to save data. Often you will want to verify that there are no validation errors before allowing the save operation to proceed. Using the MVVM pattern it would be ideal to determine whether there are any errors purely within your model view. The default behavior of IDataErrorInfo does not give a collection of all current errors. Instead, it will just tell you if a given property has an error (ie: using IDataErrorInfo.Item), so all we have to do to find all the errors is enumerate through each property on the class and call the Item property with that property name as the argument. The GetValidationErrors function below does exactly that:
Option Strict On
Imports System.ComponentModel
Public Class ValidationHelper
''' <summary>
''' Checks for errors in <c>validatable</c> and returns all the errors found.
''' </summary>
Public Shared Function GetValidationErrors(
ByVal validatable As IDataErrorInfo) As IEnumerable(Of DataError)
Dim errors As New List(Of DataError)
' Iterate through every property in the class
For Each prop In validatable.GetType.GetProperties
' If the property has an error, then add it to the list
Dim errorMessage = validatable(prop.Name)
If errorMessage IsNot Nothing Then
errors.Add(New DataError(prop.Name, errorMessage))
End If
Next
Return errors
End Function
''' <summary>
''' Represents a single error. It's a propertyName, errorMessage pair
''' </summary>
Public Class DataError
Private _propertyName As String
Private _errorMessage As String
Public Sub New(ByVal propertyName As String, ByVal errorMessage As String)
_propertyName = propertyName
_errorMessage = errorMessage
End Sub
''' <summary>
''' The name of the property that has the error.
''' </summary>
Public ReadOnly Property PropertyName() As String
Get
Return _propertyName
End Get
End Property
''' <summary>
''' A description of the property's error.
''' </summary>
Public ReadOnly Property ErrorMessage() As String
Get
Return _errorMessage
End Get
End Property
End Class
End Class