Validation Rules in VB.NET

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Validation Rules
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As discussed in 2, most business objects will be validating data based on various business rules. The actual implementation to manage an object s validation rules and maintain a list of broken business rules will be discussed later, in the Csla.Validation Namespace section. However, the BusinessBase class encapsulates that behavior and exposes it in an easy-to-use manner.
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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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The validation rules and broken rules will be managed by a ValidationRules object, and BusinessBase will collaborate with this object to manage all validation rule behaviors. A reference to this object is kept by BusinessBase, and is exposed through a property: Private mValidationRules As Validation.ValidationRules Protected ReadOnly Property ValidationRules() _ As Validation.ValidationRules Get If mValidationRules Is Nothing Then mValidationRules = New Validation.ValidationRules(Me) End If Return mValidationRules End Get End Property The property implements a lazy loading approach, so the ValidationRules object is created only on first use. This is ideal, since an object that doesn t use any of the validation rules functionality won t even incur the overhead of creating the object. The ValidationRules object maintains a list of validation rules for each property on the object. These rules are configured by the business developer in an AddBusinessRules() method, defined in BusinessBase, and overridden in the business class: Protected Overridable Sub AddBusinessRules() End Sub This method is called when the object is created through the constructor in the BusinessBase class: Protected Sub New() AddBusinessRules() AddAuthorizationRules() End Sub An AddAuthorizationRules() method is also called, and will be discussed shortly in the Authorization Rules section. AddBusinessRules() must also be called when the business object is deserialized. This will happen after a clone operation or when the object moves across the network via the data portal. It is not efficient to try to maintain the list of rule delegates for each property during serialization and deserialization. Instead, when the object is deserialized, it can simply call AddBusinessRules() to reestablish the rule references: <OnDeserialized()> _ Private Sub OnDeserializedHandler(ByVal context As StreamingContext) ValidationRules.SetTarget(Me) AddBusinessRules() OnDeserialized(context) End Sub <EditorBrowsable(EditorBrowsableState.Advanced)> _ Protected Overridable Sub OnDeserialized( _ ByVal context As StreamingContext) ' do nothing - this is here so a subclass ' could override if needed End Sub
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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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The <OnDeserialized()> attribute is used to tell the .NET serialization infrastructure to call this method once deserialization is complete. This attribute comes from the System.Runtime. Serialization namespace, and is one of a set of attributes you can use to decorate methods that are to be called by the .NET Framework during the serialization and deserialization of an object. Inside this method, the AddBusinessRules() method is called. Before that, however, the ValidationRules object needs to be given a reference to the business object so it can properly apply the validation rules to the properties. Finally, an Overridable OnDeserialized method is invoked so that the business developer can respond to the deserialization operation if desired. The ValidationRules object maintains a list of currently broken rules. This was used earlier in the implementation of the IsValid property, but there s value in exposing the collection itself: <Browsable(False)> _ <EditorBrowsable(EditorBrowsableState.Advanced)> _ Public Overridable ReadOnly Property BrokenRulesCollection() _ As Validation.BrokenRulesCollection Get Return ValidationRules.GetBrokenRules End Get End Property Within ValidationRules, this collection is implemented to be read-only. Even though the collection is exposed as a Public property, it can t be changed by the UI. However, the UI can display the list of broken rules to the user if so desired.
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System.ComponentModel.IDataErrorInfo
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Windows Forms data binding uses the IDataErrorInfo interface to interrogate a data source for validation errors. This interface allows a data source, such as a business object, to provide humanreadable descriptions of errors at the object and property levels. This information is used by grid controls and the ErrorProvider control to display error icons and tooltip descriptions. The ValidationRules object will provide a list of broken rules for each property on the object, making it relatively easy to implement IDataErrorInfo: Private ReadOnly Property [Error]() As String _ Implements System.ComponentModel.IDataErrorInfo.Error Get If Not IsValid Then Return ValidationRules.GetBrokenRules.ToString Else Return "" End If End Get End Property Private ReadOnly Property Item(ByVal columnName As String) As String _ Implements System.ComponentModel.IDataErrorInfo.Item Get Dim result As String = "" If Not IsValid Then Dim rule As Validation.BrokenRule = _ ValidationRules.GetBrokenRules.GetFirstBrokenRule(columnName)
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