IDataErrorInfo Interface

Imports System.ComponentModel

Public Class Product
    Implements IDataErrorInfo

    Private idValue As Integer
    Public Property Id As Integer
        Get
            Return idValue
        End Get
        Set(ByVal value As Integer)
            If IsIdValid(value) AndAlso
                idValue <> value Then idValue = value
        End Set
    End Property

    Private nameValue As String
    Public Property Name As String
        Get
            Return nameValue
        End Get
        Set(ByVal value As String)
            If IsNameValid(value) AndAlso
                nameValue <> value Then nameValue = value
        End Set
    End Property

    ' Validates the Id property, updating the errors collection as needed.
    Public Function IsIdValid(ByVal value As Integer) As Boolean

        Dim isValid = True

        If value < 5 Then
            AddError("Id", ID_ERROR, False)
            isValid = False
        Else
            RemoveError("Id", ID_ERROR)
        End If

        If value < 10 Then
            AddError("Id", ID_WARNING, True)
        Else
            RemoveError("Id", ID_WARNING)
        End If

        Return isValid

    End Function

    ' Validates the Name property, updating the errors collection as needed.
    Public Function IsNameValid(ByVal value As String) As Boolean

        Dim isValid = True

        If value.Contains(" ") Then
            AddError("Name", NAME_ERROR, False)
            isValid = False
        Else
            RemoveError("Name", NAME_ERROR)
        End If

        If (value.Length > 5) Then
            AddError("Name", NAME_WARNING, True)
        Else
            RemoveError("Name", NAME_WARNING)
        End If

        Return isValid

    End Function

    Private errors As New Dictionary(Of String, List(Of String))
    Private ReadOnly ID_ERROR = "Value cannot be less than 5."
    Private ReadOnly ID_WARNING = "Value should not be less than 10."
    Private ReadOnly NAME_ERROR = "Value must not contain any spaces."
    Private ReadOnly NAME_WARNING = "Value should be 5 characters or less."

    ' Adds the specified error to the errors collection if it is not already 
    ' present, inserting it in the first position if isWarning is false. 
    Public Sub AddError(ByVal propertyName As String, ByVal [error] As String,
                        ByVal isWarning As Boolean)

        If Not errors.ContainsKey(propertyName) Then _
            errors(propertyName) = New List(Of String)()

        If Not errors(propertyName).Contains([error]) Then
            If isWarning Then
                errors(propertyName).Add([error])
            Else
                errors(propertyName).Insert(0, [error])
            End If
        End If

    End Sub

    ' Removes the specified error from the errors collection if it is present. 
    Public Sub RemoveError(ByVal propertyName As String, ByVal [error] As String)

        If errors.ContainsKey(propertyName) AndAlso
            errors(propertyName).Contains([error]) Then

            errors(propertyName).Remove([error])
            If errors(propertyName).Count = 0 Then errors.Remove(propertyName)

        End If

    End Sub

    Public ReadOnly Property [Error] As String Implements IDataErrorInfo.Error
        Get
            Throw New NotImplementedException()
        End Get
    End Property

    ReadOnly Property Item(ByVal propertyName As String) As String _
        Implements IDataErrorInfo.Item
        Get
            Return If(Not errors.ContainsKey(propertyName), Nothing,
                String.Join(Environment.NewLine, errors(propertyName)))
        End Get
    End Property

End Class

using System;
using System.ComponentModel;
using System.Collections.Generic;

namespace IDataErrorInfoExample
{

    public class Product : IDataErrorInfo
    {
        private int idValue;

        public int Id
        {
            get { return idValue; }
            set { if (IsIdValid(value) && idValue != value) idValue = value; }
        }

        private string nameValue;
        public string Name
        {
            get { return nameValue; }
            set { if (IsNameValid(value) && nameValue != value) nameValue = value; }
        }

        // Validates the Id property, updating the errors collection as needed.
        public bool IsIdValid(int value)
        {
            bool isValid = true;

            if (value < 5)
            {
                AddError("Id", ID_ERROR, false);
                isValid = false;
            }
            else RemoveError("Id", ID_ERROR);

            if (value < 10) AddError("Id", ID_WARNING, true);
            else RemoveError("Id", ID_WARNING);

            return isValid;
        }

        // Validates the Name property, updating the errors collection as needed.
        public bool IsNameValid(string value)
        {
            bool isValid = true;

            if (value.Contains(" "))
            {
                AddError("Name", NAME_ERROR, false);
                isValid = false;
            }
            else RemoveError("Name", NAME_ERROR);

            if (value.Length > 5) AddError("Name", NAME_WARNING, true);
            else RemoveError("Name", NAME_WARNING);

            return isValid;
        }

        private Dictionary<String, List<String>> errors =
            new Dictionary<string, List<string>>();
        private const string ID_ERROR = "Value cannot be less than 5.";
        private const string ID_WARNING = "Value should not be less than 10.";
        private const string NAME_ERROR = "Value must not contain any spaces.";
        private const string NAME_WARNING = "Value should be 5 characters or less.";

        // Adds the specified error to the errors collection if it is not already 
        // present, inserting it in the first position if isWarning is false. 
        public void AddError(string propertyName, string error, bool isWarning)
        {
            if (!errors.ContainsKey(propertyName))
                errors[propertyName] = new List<string>();

            if (!errors[propertyName].Contains(error))
            {
                if (isWarning) errors[propertyName].Add(error);
                else errors[propertyName].Insert(0, error);
            }
        }

        // Removes the specified error from the errors collection if it is present. 
        public void RemoveError(string propertyName, string error)
        {
            if (errors.ContainsKey(propertyName) &&
                errors[propertyName].Contains(error))
            {
                errors[propertyName].Remove(error);
                if (errors[propertyName].Count == 0) errors.Remove(propertyName);
            }
        }

        #region IDataErrorInfo Members

        public string Error
        {
            get { throw new NotImplementedException(); }
        }

        public string this[string propertyName]
        {
            get
            {
                return (!errors.ContainsKey(propertyName) ? null :
                    String.Join(Environment.NewLine, errors[propertyName]));
            }
        }

        #endregion
    }
}

<UserControl x:Class="IDataErrorInfoExample.MainPage"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="https://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="https://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:sdk="https://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

  <Grid x:Name="LayoutRoot" Background="White" Margin="10">

    <Grid.RowDefinitions>
      <RowDefinition Height="Auto"/>
      <RowDefinition Height="Auto"/>
      <RowDefinition Height="Auto"/>
      <RowDefinition Height="Auto"/>
      <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>

    <Grid.ColumnDefinitions>
      <ColumnDefinition Width="Auto"/>
      <ColumnDefinition Width="200"/>
      <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinitions>

    <TextBlock Grid.Column="1" Margin="3" Text="Product" FontWeight="Bold"/>

    <sdk:Label Grid.Row="1" Target="{Binding ElementName=IdTextBox}" 
      Margin="3" HorizontalAlignment="Right"/>
    <TextBox x:Name="IdTextBox" KeyDown="TextBox_KeyDown"
      Grid.Column="1" Grid.Row="1" Margin="3" MaxLength="5" 
      Text="{Binding Id, Mode=TwoWay, ValidatesOnExceptions=True,
      ValidatesOnDataErrors=True, NotifyOnValidationError=True}"/>
    <sdk:DescriptionViewer Grid.Column="2" Grid.Row="1" 
      Description="Id must be greater than 4 and should be greater than 9."/>

    <sdk:Label Grid.Row="2" Target="{Binding ElementName=NameTextBox}" 
      Margin="3" HorizontalAlignment="Right"/>
    <TextBox x:Name="NameTextBox" KeyDown="TextBox_KeyDown"  
      Grid.Column="1" Grid.Row="2" Margin="3" MaxLength="10" 
      Text="{Binding Name, Mode=TwoWay, ValidatesOnExceptions=True,
      ValidatesOnDataErrors=True, NotifyOnValidationError=True}"/>
    <sdk:DescriptionViewer Grid.Column="2" Grid.Row="2" Description=
      "Name must not contain spaces and should be 5 characters or less."/>

    <sdk:ValidationSummary Grid.Row="3" Grid.ColumnSpan="2" Margin="3"/>

  </Grid>
</UserControl>

Partial Public Class MainPage
    Inherits UserControl

    Public Sub New()
        InitializeComponent()
        LayoutRoot.DataContext = New Product() With {.Id = 10, .Name = "food"}
    End Sub

    ' Commits text box values when the user presses ENTER. This makes it 
    ' easier to experiment with different values in the text boxes.
    Private Sub TextBox_KeyDown(ByVal sender As System.Object,
                                ByVal e As System.Windows.Input.KeyEventArgs)
        If e.Key = System.Windows.Input.Key.Enter Then CType(sender, TextBox) _
            .GetBindingExpression(TextBox.TextProperty).UpdateSource()
    End Sub

End Class

using System.Windows.Controls;

namespace IDataErrorInfoExample
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
            LayoutRoot.DataContext = new Product() { Id = 10, Name = "food" };
        }

        // Commits text box values when the user presses ENTER. This makes it 
        // easier to experiment with different values in the text boxes.
        private void TextBox_KeyDown(object sender,
            System.Windows.Input.KeyEventArgs e)
        {
            if (e.Key == System.Windows.Input.Key.Enter) (sender as TextBox)
                .GetBindingExpression(TextBox.TextProperty).UpdateSource();
        }
    }
}