Dan Wahlin is a Microsoft MVP and Silverlight consultant. You can contact him here.

Data validation -- the process of guaranteeing to an application that every data value is accurate -- is a crucial part of any application. But for developers using Microsoft Silverlight, this has been a problem: There wasn't a good way to ensure that data, such as a user ID or email address, is entered correctly. That's changed with Silverlight 4, thankfully.

Before now, the typical way of validating data was to throw exceptions within property-setter blocks when data was invalid. Controls bound to a property could be notified of a data validation exception by setting ValidatesOnExceptions to True in the data binding. The typical technique used for validating data was to throw exceptions within property setter blocks as data was found to be invalid. Controls bound to a property could be notified of a data validation exception by setting ValidatesOnExceptions to True in the data binding. Although this technique certainly works, there are some new options available in Silverlight 4 that can be used without resorting to throwing exceptions.

With the release of Silverlight 4 two new interfaces are now available including IDataErrorInfo and INotifyDataErrorInfo. In this article I'll walk through the process of using the IDataErrorInfo interface which provides a more simplistic way to validate data and provide end users with notifications. Let's start by taking a look at the interface's members:

public interface IDataErrorInfo
{
        string this[string columnName] { get; }
        string Error { get; }
 }

The IDataErrorInfo interface only contains two members including Error which can return error information about the overall object and a default indexer property that can be used to write validation logic for different properties within an object. You'll implement the IDataErrorInfo interface on a client-side entity class in your Silverlight project such as the Person class shown in Listing 1.

public class Person : INotifyPropertyChanged, IDataErrorInfo
{
    public string this[string propertyName]
    {
        get
        {
               ….
        }
    }
    public string Error
    {
        get { …. }
    }
}

In situations where the Error property isn't used (which is often the case), you can simply return null:

public string Error
{
    get { return null; }
}

However, it can be used to return an error message detailing what's wrong with the overall object as well:

string _Errors;
const string _ErrorsText = "Error in Person data.";

public string Error
{
    get { return _Errors; }
}