Microsoft .NET Hosting Blog: Binding and IDataErrorInfo in Silverlight 4

Silverlight 3 introduced the idea of types throwing exceptions from their setters as a way of reporting errors back to the UI. That works well but there are other ways of achieving that kind of error status from a type and the IDataErrorInfo interface has been around for a while as a way of doing this ( introduced in Windows Forms and then added to WPF V3.5 at a later point ).

It’s a simple enough interface;

// Summary:
//     Defines properties that data entity classes can implement to provide custom
//     validation support.
public interface IDataErrorInfo
{
  // Summary:
  //     Gets a message that describes any validation errors for the object.
  //
  // Returns:
  //     The validation error on the object, or null or System.String.Empty if there
  //     are no errors present.
  string Error { get; }

  // Summary:
  //     Gets a message that describes any validation errors for the specified property
  //     or column name.
  //
  // Parameters:
  //   columnName:
  //     The name of the property or column to retrieve validation errors for.
  //
  // Returns:
  //     The validation error on the specified property, or null or System.String.Empty
  //     if there are no errors present.
  string this[string columnName] { get; }
}
whereby a caller can enquire about the general state of an object or about specific properties. Lots of people have implemented this in the past for Windows Forms applications and might;

- be very familiar with the interface so want to keep using it
- want to avoid the model of “have to throw an exception in a setter in order to validate the object”
- have a bunch of code that already uses IDataErrorInfo and want to keep using it

and so Silverlight 4 adds support for IDataErrorInfo. I can go and make myself a pretend class like this one;

public class Person : IDataErrorInfo
{
  public string FirstName
  {
    get
    {
      return (firstName);
    }
    set
    {
      firstName = value;
    }
  }
  public string LastName
  {
    get
    {
      return (lastName);
    }
    set
    {
      lastName = value;
    }
  }
  public int Age
  {
    get
    {
      return (age);
    }
    set
    {
      age = value;
    }
  }
  [Display(AutoGenerateField=false)]
  public string Error
  {
    get { return (null); }
  }
  [Display(AutoGenerateField = false)]
  public string this[string columnName]
  {
    get
    {
      string error = null;

      switch (columnName)
      {
        case "FirstName":
          if (string.IsNullOrEmpty(firstName))
          {
            error = "Provide a first name";
          }
          break;
        case "LastName":
          if (string.IsNullOrEmpty(lastName))
          {
            error = "Provide a last name";
          }
          break;
        case "Age":
          if ((age < 0) || (age > 120))
          {
            error = "Age out of range";
          }
          break;
      }
      return (error);
    }
  }
  string firstName;
  string lastName;
  int age;
}

then I can wrap it up into a UI by feeding it as the DataContext of a DataGrid as below;

public partial class MainPage : UserControl
{
  public MainPage()
  {
    InitializeComponent();

    this.Loaded += (s, e) =>
      {
        this.DataContext =
          new Person[] { new Person() { FirstName = "Fred", LastName = "Smith", Age = 22 } };
      };
  }
}
with the corresponding XAML file;

<UserControl x:Class="SilverlightApplication25.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:dg="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Data"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

and that gives me a UI that knows about its errors as in;

Naturally – this isn’t just a DataGrid thing, it applies to regular controls and DataForms and so on and should help in getting a bunch of code across to Silverlight 4. In terms of making use from regular controls, if we switch our UI to be something like;

note that I’ve set the ValidatesOnDataErrors property of these TextBoxes to be True which causes them to do the right thing around IDataErrorInfo. I’ve also set the NotifyOnValidationError but that’s a Silverlight 3 thing and I’m really using that to count errors as they occur and get fixed so that I can enable/disable my Submit button based on that.

The code behind this I changed to;

public partial class MainPage : UserControl, INotifyPropertyChanged
{
  public MainPage()
  {
    InitializeComponent();

this.person = new Person() { FirstName = "Fred", LastName = "Smith", Age = 22 };

      this.Loaded += (s, e) =>
     {
        this.DataContext = this;
      };
  }
public Person Person
  {
    get
    {
      return (person);
    }
  }
  public bool NoErrors
  {
    get
    {
      return (errorCount == 0);
    }
  }
  void FirePropertyChanged(string property)
  {
    if (PropertyChanged != null)
    {
      PropertyChanged(this, new PropertyChangedEventArgs(property));
    }
  }
  private void OnValidationError(object sender, ValidationErrorEventArgs e)
  {
    switch (e.Action)
    {
      case ValidationErrorEventAction.Added:
        errorCount++;
        break;
      case ValidationErrorEventAction.Removed:
        errorCount--;
        break;
      default:
        break;
    }
    FirePropertyChanged("NoErrors");
  }
  int errorCount;
  Person person;
  public event PropertyChangedEventHandler PropertyChanged;
}
and most of that is really around having a NoErrors property to enable/disable the button – the bits around IDataErrorInfo are as they were in the DataGrid example with the one exception of using the new NotifyOnValidationError on the binding directive and that gives me a UI made up of TextBoxes with the validation display being pretty much the same;

Microsoft .NET Hosting Blog

Tuesday, October 12, 2010

Binding and IDataErrorInfo in Silverlight 4

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