OOP Design and Practices in Business Systems
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by Brian Mains
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Validation

When creating a business object, the object also has to be validated.  The data entered into a form needs to be checked for accuracy and correctness.  What means should we use?  Should the object be stored in the business layer if there is an error anyway?  Where should validation be performed and how?

In most instances, data is entered into a web or windows form and a button is clicked to add the information to the repository.  At this point, the object representing this record has not yet been created, so validation is a perfect time to occur here.  However, some applications require a different approach.  They are very dynamic in that they tie together multiple resources from many places.  For instance, there is a form that contains many fields; it is common to break this up into several forms, or to use a tab control of some sort.  However, some of the information related to this object may only be created after the primary record is saved to the database and the user has had time to review the information.  A better solution would be to save the object, even though in error, to the repository and come back to it later.

For instance, additional properties for the object may only be able to set up after the object is created.  This object would be saved, but it would not be considered correct, through the use of some sort of status property, and any additional changes could be made later.  In the windows world, this is easily taken care of; but in the web world persisting these objects to a data store has to occur often because the web is stateless and knows nothing about the objects created in the previous page lifecycle.  An external means, such as a database or cache, can be used to persist the object and must be weighed against how many people will use the application.  Caching and session mechanism only store the objects for so long, so error handling must be used to ensure that if the object doesn't exist, the application can still work.

Another consideration is the actual validation of the fields that are input.  For instance, you can make use of the built-in validation features within windows and web applications, or you can make use of the Enterprise Library 3 Validation block.  This new application block has the ability to validate business applications through attributes or configuration files established for a business class.  An example of a business object is shown below:

Listing 8

public class User
{
  [StringLengthValidator(5, RangeBoundaryType.Inclusive, 7,
    RangeBoundaryType.Inclusive,
    "The authorization code is outside the range of valid values", Ruleset =
    "primary"), AuthorizationCodeValidator(Ruleset = "primary")]
  public string AuthorizationCode
  {
    get
    {
      return _authorizationCode;
    }
    set
    {
      _authorizationCode = value;
    }
  }
 
  [StringLengthValidator(7, RangeBoundaryType.Inclusive, 150,
    RangeBoundaryType.Inclusive,
    "Email address must be between 7 and 150 characters", Ruleset = "primary"),
    ContainsCharactersValidator("@.", ContainsCharacters.All,
    "The email must have at least an @ and at least one decimal", Ruleset =
    "primary"), EmailDomainValidator(".com"".net"".edu", ".gov"".biz",
    ".tv", Ruleset = "primary")]
  public string Email
  {
    get
    {
      return _email;
    }
    set
    {
      _email = value;
    }
  }
}

The Enterprise Library Validation block also has integration features into ASP.NET with a new PropertyProxyValidator that performs server-side validation using the validation attributes/configuration.  It is also customizable to include additional custom validators for whatever your needs may be.  Below is a definition of that validator:

Listing 9

<el:PropertyProxyValidator ID="ppvName" runat="server" 
      SourceTypeName="Mains.Examples.User,App_Code"
      PropertyName="Name" RulesetName="primary" ControlToValidate="txtName">*</el:PropertyProxyValidator>

The repository or factory you build could validate a business object using the validation block, and if in error, store this error state in a flag within the object.  That way, any erroring objects are not persisted to the data store, and our new SaveChanges method (modified from above) is below:

Listing 10

public virtual int SaveChanges()
{
  int savedItems = 0;
 
  foreach (T item in this.Items)
  {
    if (item.IsDirty)
    {
      if (Validation.Validate < T > (item).IsValid)
      {
        this.SaveItem(item);
        item.ClearDirtyStatus();
      }
      else
        item.IsValid = false;
 
      savedItems++;
    }
  }
 
  return savedItems;
}

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