Following is the list that needs to be added in order to perform validation:

1. After create model class, for example: SomeModel.cs, add EnableValidation property to SomeModel model.
   Example:

   public bool EnableValidation { get; set; }

2. Add the validation condition for the properties of SomeModel model.
   Example:

   public double Price
   {
       get
       {
           return _price;
       }
       set
       {
           if (_price != value)
           {
               _price = value;
               ClearError("Price");
   
               if (EnableValidation && double.IsNaN(this.Price) || this.Price == 0.0)
                   SetError("Price", "Price is required");
   
               OnPropertyChanged("Price");
           }
       }
   }

3. After create the view, for example: SomeView.xaml, create ValidationViewModelBase class that inherits from ViewModelBase and implements IDataErrorInfo interface.

   using System.Collections.Generic;
   using System.ComponentModel;
   using System.Linq;
   
   namespace ValidationOnTextInput.ViewModels
   {
       public class ValidationViewModelBase : ViewModelBase, IDataErrorInfo
       {
           #region Properties
   
           private Dictionary<string, string> _errors = new Dictionary<string, string>();
   
           public virtual bool HasErrors
           {
               get
               {
                   return _errors.Count > 0;
               }
           }
   
           #endregion
   
           #region Public
   
           public void SetError(string propertyName, string errorMessage)
           {
               _errors[propertyName] = errorMessage;
               this.OnPropertyChanged(propertyName);
           }
   
           #endregion
   
           #region Protected
   
           protected void ClearError(string propertyName)
           {
               this._errors.Remove(propertyName);
           }
   
           protected void ClearAllErrors()
           {
               List<string> properties = new List<string>();
   
               foreach (KeyValuePair<string, string> error in this._errors)
                   properties.Add(error.Key);
   
               this._errors.Clear();
   
               foreach (string property in properties)
                   this.OnPropertyChanged(property);
           }
   
           #endregion
   
           #region IDataErrorInfo Members
   
           public string Error
           {
               get
               {
                   if (this.HasErrors)
                       return this._errors.First().Value;
   
                   return null;
               }
           }
   
           public string this[string columnName]
           {
               get
               {
                   if (this._errors.ContainsKey(columnName))
                   {
                       return this._errors[columnName];
                   }
                   return string.Empty;
               }
           }
   
           #endregion
       }
   }

4. Create view model, for example: SomeViewModel.cs, which inherits from ValidationViewModelBase class.
   Example:

   using [ProjectName].Models;
   
   namespace [ProjectName].ViewModels
   {
       public class SomeViewModel : ValidationViewModelBase
       {
           // Constructor
           public SomeViewModel()
           {
               
           }
       }
   }

5. Binding the View to ViewModel, in this case is SomeView to SomeViewModel. This step includes: set ValidationOnDataErrors to True in the binding definition; set Intersoft:DataBinding.ClearErrorOnTextInput to True in order to have the error alert to disappear directly when user type into the textbox.
   Example:

   <Intersoft:UXTextBox HorizontalAlignment="Center" Width="60"
                           Text="{Binding Book.Price, Mode=TwoWay, ValidatesOnDataErrors=True, ValidatesOnExceptions=True, StringFormat=\{0:c\}}"
                           Intersoft:DataBinding.ClearErrorOnTextInput="True"/>

I enclosed a working simple sample of Model; ViewModel; and View for reference as attachment.

Hope this helps.

Yudi,

I have seen the code you're referring to in one of your samples.

However, it's largely irrelevant - I am using the Entity Framework for persistence as I've indiciated in my original post.

The true example of how to do validation is in the ClientUI SL Business App with RIA SP1 template.

I finally figured out that it wasn't working because I hadn't assigned an error string in my validation attribute - it should have a default one but well, it doesn't.

Georgios

Glad to hear the good news.

Should you need further assistance or run into any problems regarding our controls, feel free to post it into our forum. We would be happy to assist you again.