title: MVVM Validation in UWP
body: <p>In the last week I've been trying to apply MVVM Pattern to Universal Windows Plataform, in  the most elegant possible way, that means apply SOLID principles and some popular design patterns.</p>
  
  <p>I've been trying to reproduce this exercise from this link:
  <a href="http://www.sullinger.us/blog/2014/7/4/custom-object-validation-in-winrt" rel="nofollow noreferrer">http://www.sullinger.us/blog/2014/7/4/custom-object-validation-in-winrt</a></p>
  
  <p>Also the link is for Windows 8 apps it also applies to Windows 10 apps according to MSDN answer at this forum: <a href="https://social.msdn.microsoft.com/Forums/windowsapps/en-US/05690519-1937-4e3b-aa12-c6ca89e57266/uwp-what-is-the-recommended-approach-for-data-validation-in-uwp-windows-10?forum=wpdevelop" rel="nofollow noreferrer">https://social.msdn.microsoft.com/Forums/windowsapps/en-US/05690519-1937-4e3b-aa12-c6ca89e57266/uwp-what-is-the-recommended-approach-for-data-validation-in-uwp-windows-10?forum=wpdevelop</a></p>
  
  <p>Let me show you my classes, this is my view my final view:</p>
  
  <pre><code>&lt;Page
  x:Class="ValidationTestUWP.MainPage"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:local="using:ValidationTestUWP"
  xmlns:conv="using:ValidationTestUWP.Converters"
  xmlns:viewmodels="using:ValidationTestUWP.ViewModel"
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  mc:Ignorable="d"&gt;
  
  &lt;Page.DataContext&gt;
      &lt;viewmodels:AccountCreationViewModel/&gt;
  &lt;/Page.DataContext&gt;
  
  &lt;Page.Resources&gt;
      &lt;conv:ValidationMessageConverter x:Key="ValidationMessageConverter"/&gt;
  &lt;/Page.Resources&gt;
  
      &lt;StackPanel Grid.Row="1"
              VerticalAlignment="Center"
              HorizontalAlignment="Center"&gt;
  
      &lt;!-- E-Mail address input --&gt;
      &lt;TextBlock Text="Email"
                 Style="{StaticResource TitleTextBlockStyle}" /&gt;
      &lt;TextBox x:Name="EmailTextBox"
               Margin="0 5 0 0"
               MinWidth="200"
               Text="{Binding Path=AppUser.Email, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/&gt;
  
      &lt;!--We now have one more thing to do. We need to update our XAML. 
          The Error TextBlocks will now bind to the ValidationMessages property within the model,
          using an index matching the property they are bound to.--&gt;
      &lt;TextBlock x:Name="EmailValidationErrorTextBlock"
                 Text="{Binding AppUser.ValidationMessages[Email], Converter={StaticResource ValidationMessageConverter}}"
                 Foreground="Red" /&gt;
  
          &lt;!-- Password input --&gt;
      &lt;TextBlock Text="Password"
                 Margin="0 30 0 0"
                 Style="{StaticResource TitleTextBlockStyle}"/&gt;
      &lt;TextBox x:Name="PasswordTextBox"
               Margin="0 5 0 0"
               MinWidth="{Binding ElementName=EmailTextBox, Path=MinWidth}"
               Text="{Binding Path=AppUser.ValidationMessages[Password], Converter={StaticResource ValidationMessageConverter}}"/&gt;
  
      &lt;TextBlock x:Name="PasswordValidationToShortErrorTextBlock"
                 Text="{Binding PasswordToShortError}"
                 Foreground="Red" /&gt;
      &lt;TextBlock x:Name="PasswordValidationToLongErrorTextBlock"
                 Text="{Binding PasswordToLongError}"
                 Foreground="Red" /&gt;
  
      &lt;!-- Login command button --&gt;
      &lt;Button Content="Create Account"
              Margin="0,10, 0, 0"
              Command="{Binding CreateAccount}"/&gt;
  &lt;/StackPanel&gt;
  &lt;/Page&gt;
  </code></pre>
  
  <p>My model end up looking like this: (also I'm adding explanation of this classes within the comments in the classes.)</p>
  
  <pre><code>public class User : ValidatableBase
  {
      private string email = string.Empty;
  
      public string Email
      {
          get { return email; }
          set
          {
              email = value;
              OnPropertyChanged("Email");
          }
      }
  
      private string password = string.Empty;
  
      public string Password
      {
          get { return password; }
          set
          {
              password = value;
              OnPropertyChanged("Password");
          }
      }
  
      /*Now that we are inheriting from our base class, we need to implement the required Validate() method.
       * In order to keep with the Single-Responsibility-Principle, we will invoke other methods from within
       * the Validate() method. 
       * Since we have to validate multiple properties, we should have each property validation be contained 
       * within it's own method. This makes it easier to test.*/
      public override void Validate()
      {
          ValidatePassword("Password");
          //base.OnPropertyChanged("Password");
          ValidateEmail("Email");
          //base.OnPropertyChanged("Email");
  
          // Passing in an empty string will cause the ValidatableBase indexer to be hit.
          // This will let the UI refresh it's error bindings.
          base.OnPropertyChanged(string.Empty);
      }
  
      /*Here we just invoke a ValidatePassword and ValidateEmail method. 
       * When we are done, we notify any observers that the entire object has changed by not specifying a property name
       * in the call to OnPropertyChanged. 
       * This lets the observers (in this case, the View) know its bindings need to be refreshed.*/
      private IValidationMessage ValidateEmail(string property)
      {
          const string emailAddressEmptyError = "Email address can not be blank.";
          if (string.IsNullOrEmpty(this.Email))
          {
              var msg = new ValidationErrorMessage(emailAddressEmptyError);
              return msg;
          }
  
          return null;
      }
  
      private IValidationMessage ValidatePassword(string property)
      {
          const string passwordToShortError = "Password must a minimum of 8 characters in length.";
          const string passwordToLongError = "Password must not exceed 16 characters in length.";
          if (this.Password.Length &lt; 8)
          {
              var msg = new ValidationErrorMessage(passwordToShortError);
              return msg;
          }
          if (this.Password.Length &gt; 16)
          {
              var msg = new ValidationErrorMessage(passwordToLongError);
              return msg;
          }
  
          return null;
      }
  </code></pre>
  
  <p>This is my ViewModel:</p>
  
  <pre><code>/*View Model 
   * 
   * Next, we need to revise our View Model.
   * We will delete all of the error properties within it, along with the INotifyPropertyChanged implementation.
   * We will only need the AppUser property and the ICommand implementation.*/
  public class AccountCreationViewModel
  {
      public AccountCreationViewModel()
      {
          this.AppUser = new User();
          CreateAccount = new MyCommand(CreateUserAccount);
      }
  
      private User appUser;
  
      public event EventHandler CanExecuteChanged = delegate { };
      public MyCommand CreateAccount { get; set; }
  
      public User AppUser
      {
          get { return appUser; }
          set
          {
              appUser = value;
          }
      }
  
      private void CreateUserAccount()
      {
          AppUser.Validate();
  
          if (AppUser.HasValidationMessageType&lt;ValidationErrorMessage&gt;())
          {
              return;
          }
          // Create the user
          // ......
      }
  
      /*Now, when you run the app and enter an invalid Email or Password,
       * the UI will automatically inform you of the validation errors when you press the Create Account button. 
       * If you ever need to add more Email validation (such as the proper email format) 
       * or more Password validation (such as not allowing specific characters) you can do so without needing
       * to modify your View Model or your View.
       * 
       * If you need to add a whole new property to the Model, with validation, you can. You don't need to modify
       * your View Model, you only need to add a TextBlock to the View to display the validation.*/
  }
  </code></pre>
  
  <p>Also I've applied a RelayCommand Pattern:</p>
  
  <pre><code>public class MyCommand : ICommand
  {
      Action _TargetExecuteMethod;
      Func&lt;bool&gt; _TargetCanExecuteMethod;
  
      public MyCommand(Action executeMethod)
      {
          _TargetExecuteMethod = executeMethod;
      }
  
      public MyCommand(Action executeMethod, Func&lt;bool&gt; canExecuteMethod)
      {
          _TargetExecuteMethod = executeMethod;
          _TargetCanExecuteMethod = canExecuteMethod;
      }
  
      public void RaiseCanExecuteChanged()
      {
          CanExecuteChanged?.Invoke(this, EventArgs.Empty);
      }
  
      /*Beware - should use weak references if command instance lifetime 
      is longer than lifetime of UI objects that get hooked up to command*/
      // Prism commands solve this in their implementation public event 
      public event EventHandler CanExecuteChanged = delegate { };
  
      public bool CanExecute(object parameter)
      {
          if (_TargetCanExecuteMethod != null)
              return _TargetCanExecuteMethod();
  
          if (_TargetExecuteMethod != null)
              return true;
  
          return false;
      }
  
      public void Execute(object parameter)
      {
          /*This sintax use the null*/
          _TargetExecuteMethod?.Invoke();
      }
  }
  </code></pre>
  
  <p>This is where the fun begins, I'll introduce the ValidatableBase created at the blog I showed you earlier:</p>
  
  <pre><code>public abstract class ValidatableBase : IValidatable, INotifyPropertyChanged
  {
      /*Our initial class contains the Dictionary that will hold our validation messages. 
       * Next, we implement the read-only property required by our interface.*/
      private Dictionary&lt;string, List&lt;IValidationMessage&gt;&gt; _validationMessages = 
          new Dictionary&lt;string, List&lt;IValidationMessage&gt;&gt;();
  
      /*The call to OnPropertyChanged will let the UI know that this collection has changed. 
       * This in most cases won't be used since the collection is read-only, but since it is going in to a base class,
       * we want to provide support for that.*/
      public Dictionary&lt;string, List&lt;IValidationMessage&gt;&gt; ValidationMessages
      {
          get { return _validationMessages; }
          set
          {
              _validationMessages = value;
              OnPropertyChanged("ValidationMessages");
          }
      }
  
      public event PropertyChangedEventHandler PropertyChanged = delegate { };
  
      /*our base class implements the INotifyPropertyChanged method, 
       * so we will remove it from our model and put the implementation in to our base class.*/
      public void OnPropertyChanged(string propertyName)
      {
          PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
      }
  
      /*In this method, we check if the collection contains a Key matching the property supplied.
       * If it does, then we check it's values to see if any of them match the Type specified in &lt; T&gt;.
       * This lets you do something like
       * 
       * HasValidationMessageType&lt; ValidationErrorMessage&gt;("Email");
       * 
       * to check if the model has a validation error on the email property.*/
      public bool HasValidationMessageType&lt;T&gt;(string property = "")
      {
          if (string.IsNullOrEmpty(property))
          {
              bool result = _validationMessages.Values.Any(collection =&gt;
                  collection.Any(msg =&gt; msg is T));
              return result;
          }
  
          return _validationMessages.ContainsKey(property);
      }
  
      /*In this method we create a new collection if the key doesn't exist yet,
       * we then double check to ensure this validation message does not already exist in the collection.
       * If not, we add it.*/
      public void AddValidationMessage(IValidationMessage message, string property = "")
      {
          if (string.IsNullOrEmpty(property))
          {
              return;
          }
  
          // If the key does not exist, then we create one.
          if (!_validationMessages.ContainsKey(property))
          {
              _validationMessages[property] = new List&lt;IValidationMessage&gt;();
          }
  
          if (_validationMessages[property].Any(msg =&gt; msg.Message.Equals(message.Message) || msg == message))
          {
              return;
          }
  
          _validationMessages[property].Add(message);
      }
  
      /*Here we just check if there is any message for the supplied Key and remove it.
       * At the moment, this does not do any Type checking to see if there is more 
       * than one Type of object (Warning and Error) in the collection with the same message.
       * The method just removes the first thing it finds and calls it good.*/
      public void RemoveValidationMessage(string message, string property = "")
      {
          if (string.IsNullOrEmpty(property))
          {
              return;
          }
  
          if (!_validationMessages.ContainsKey(property))
          {
              return;
          }
  
          if (_validationMessages[property].Any(msg =&gt; msg.Message.Equals(message)))
          {
              // Remove the error from the key's collection.
              _validationMessages[property].Remove(
                  _validationMessages[property].FirstOrDefault(msg =&gt; msg.Message.Equals(message)));
          }
      }
  
      /*We just check if a key exists that matches the property name and then clear out its messages contents
       * and remove the key from the Dictionary.*/
      public void RemoveValidationMessages(string property = "")
      {
          if (string.IsNullOrEmpty(property))
          {
              return;
          }
  
          if (!_validationMessages.ContainsKey(property))
          {
              return;
          }
  
          _validationMessages[property].Clear();
          _validationMessages.Remove(property);
      }
  
      /*Finally, we finish implementing the interface by building the ValidateProperty method.
       * In this method, we just invoke the delegate we are provided, and accept a IValidationMessage object in return.
       * If the return value is not null, then we add it to the ValidationMessages collection.
       * If it is null, then we can assume that the validation passed and there are no issues.
       * Since that is the case, we remove it from the validation collection.*/
      public IValidationMessage ValidateProperty(Func&lt;string, IValidationMessage&gt; validationDelegate,
          string failureMessage, string propertyName = "")
      {
          IValidationMessage result = validationDelegate(failureMessage);
          if (result != null)
          {
              this.AddValidationMessage(result, propertyName);
          }
          else
          {
              this.RemoveValidationMessage(failureMessage, propertyName);
          }
  
          return result;
      }
  
      /*We have satisfied the requirements of the IValidatable interface, but there is one more method
       * we need to add to the base class. This will let us group all of our property validations in to a single call.
       * 
       * We mark it as abstract, since the base class has nothing to validate, 
       * and we want to force any object that inherits from the base class to implement the method. 
       * If you don't want to do this, you can opt out of in your code. Not everyone needs to have this feature,
       * thus the reason why it was left out of the interface.*/
      public abstract void Validate();
  }
  </code></pre>
  
  <p>Lastly this are my interfaces:</p>
  
  <pre><code>//The first thing I did was created an interface that all models needing validation would be required to implement. 
  public interface IValidatable
  {
      /*This is a read-only property, that will contain all of our validation messages. 
       * The property has a Key typed to a string, which will be the Models property name. 
       * The value is a collection of IValidationMessage objects (We will discuss what the IValidationMessage is later).
       * The idea being that for each property in the model, we can store more than 1 error.*/
      Dictionary&lt;string, List&lt;IValidationMessage&gt;&gt; ValidationMessages { get; }
  
      /*This method is used to add a validation message to the ValidationMessages collection.
       * The property will be assigned as the Key, with the message being added as the value.*/
      void AddValidationMessage(IValidationMessage message, string property = "");
  
      /*Just like we can add a validation message, we will provide ourselves with the ability to remove it.*/
      void RemoveValidationMessage(string message, string property = "");
  
      /*We can use this method to completely clear out all validation messages in one shot for a single property.*/
      void RemoveValidationMessages(string property = "");
  
      /*This method will return true if the object has validation messages matching &lt;T&gt; and false if it does not.*/
      bool HasValidationMessageType&lt;T&gt;(string property = "");
  
      /*This method can be called to actually perform validation on a property within the object and 
       * build the collection of errors. The arguments require a method delegate that returns an IValidationMessage object.
       * This is how the validation becomes reusable. Each individual object can pass in a method delegate that performs
       * the actual validation. The IValidatable implementation will take the results and determine if it must go in to
       * the ValidationMessages collection or not.*/
      IValidationMessage ValidateProperty(Func&lt;string, IValidationMessage&gt; validationDelegate, 
          string failureMessage,
          string propertyName = "");
  }
  
  /*The idea with this, is that we can create objects that implement this interface,
   * but containing different types of messages. For instance, in this post, we will create a ValidationErrorMessage
   * and a ValidationWarningMessage. You could go on and create any kind of messaging you want and use it
   * for binding to the View.*/
  public interface IValidationMessage
  {
      string Message { get; }
  }
  </code></pre>
  
  <p>This is my Converter:</p>
  
  <pre><code>/*The idea with this, is that we can create objects that implement this interface,
   * but containing different types of messages. For instance, in this post, we will create a ValidationErrorMessage
   * and a ValidationWarningMessage. You could go on and create any kind of messaging you want and use it
   * for binding to the View.*/
  public interface IValidationMessage
  {
      string Message { get; }
  }
  </code></pre>
  
  <p>and finally my ValidationErrorMessages:</p>
  
  <pre><code> /*Before we end the post, I will show you two implementations of the IValidationMessage.
   * They both do the same thing, but are Typed differently so that you can segregate your messages by Type.
   * This gives more flexibility that using an Enum.
   * 
   * First is the Error validation message.*/
  public class ValidationErrorMessage : IValidationMessage
  {
      public ValidationErrorMessage() : this(string.Empty)
      { }
  
      public ValidationErrorMessage(string message)
      {
          this.Message = message;
      }
  
      public string Message { get; private set; }
  }
  </code></pre>
  
  <p>Now every time I run the code like this code in the example shown at Sullinger Blog I get a Exception:</p>
  
  <blockquote>
    <p><strong>System.Reflection.TargetInvocationException:</strong> 'The text associated with this error code could not be found.</p>
  </blockquote>
  
  <p>I'm using VS2017 I'm trying to apply MVVM pattern to Validation in UWP, of course I could do validation on my ViewModel for every Field but it implies that I'll have to write Validation for every view I created and as far as I see in this example, this could save me tons of code.</p>
  
  <p>Does anyone understand what is wrong with this code?</p>
  
  <p>I don't want to use tools like MVVM Light or MVVM Cross or Prism this is purely Custom MVVM on UWP.</p>