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Getting Field Values
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The FieldDataManager class exposes a GetFieldData() method that other framework classes can use to retrieve field values. Friend Function GetFieldData(ByVal prop As IPropertyInfo) As IFieldData Try Return _fieldData(prop.Index) Catch ex As IndexOutOfRangeException Throw New InvalidOperationException(My.Resources.PropertyNotRegistered, ex) End Try End Function This method simply uses the index from the IPropertyInfo parameter to find and return the IFieldData object from the array. The interesting part of this method is the exception handling. Notice how any IndexOutOfRangeException is converted into the more useful InvalidOperationException, with the default message text of One or more properties are not registered for this type. The most common issue people face when using managed fields is that they register the property incorrectly. The normal result would be an unintuitive IndexOutOfRangeException, so this code ensures that the business developer will get a more useful exception and message. A field value is retrieved because BusinessBase or ReadOnlyBase needs the value. This means that GetFieldData() is invoked from a GetProperty() or ReadProperty() method in one of those classes. For example, here s the ReadProperty() method in BusinessBase, with the call to GetFieldData() and related code highlighted in bold: Protected Function ReadProperty(Of P)(ByVal propertyInfo As PropertyInfo(Of P)) As P Dim result As P = Nothing Dim data = FieldManager.GetFieldData(propertyInfo) If data IsNot Nothing Then Dim fd = TryCast(data, FieldManager.IFieldData(Of P)) If fd IsNot Nothing Then result = fd.Value Else result = CType(data.Value, P) End If
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Else result = propertyInfo.DefaultValue FieldManager.LoadFieldData(Of P)(propertyInfo, result) End If Return result End Function The GetFieldData() method is called to get the IFieldData object from the field manager. Assuming there is a corresponding field, the code then attempts to cast the result to an IFieldData(Of P) to use the strongly typed interface: Dim fd = TryCast(data, FieldManager.IFieldData(Of P)) If fd IsNot Nothing Then result = fd.Value This is always the case when using the default FieldData(Of T) type provided by CSLA .NET, so the field value will be returned without boxing or unboxing. If a person has implemented his own IFieldData that doesn t use the generic interface, then the boxed value must be converted to the right type and returned: else result = (P)data.Value; BusinessBase and ReadOnlyBase use this technique when they need to retrieve managed field values.
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FieldDataManager has two SetFieldData() methods: one generic and one not. The default is to use the generic overload, which looks like this: Friend Sub SetFieldData(Of P)(ByVal prop As IPropertyInfo, ByVal value As P) Dim field = GetOrCreateFieldData(prop) Dim fd = TryCast(field, IFieldData(Of P)) If fd IsNot Nothing Then fd.Value = value Else field.Value = value End If End Sub This code retrieves (or creates) the IFieldData object from the array and then attempts to cast it to an IFieldData(Of T). Normally this will succeed, because the storage object is a FieldData(Of T). It s then possible to store the field value without boxing or unboxing. The other overload is loosely typed, so it would incur boxing and unboxing costs. Even more, it will do type coercion if needed. Friend Sub SetFieldData(ByVal prop As IPropertyInfo, ByVal value As Object) Dim valueType As Type If value IsNot Nothing Then valueType = value.GetType Else valueType = prop.Type End If value = Utilities.CoerceValue(prop.Type, valueType, Nothing, value) Dim field = GetOrCreateFieldData(prop) field.Value = value End Sub
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This overload accepts the input value and then uses the CoerceValue() method to convert the value to the type expected by the field. Normally, you would expect that the inbound value is the same type (in which case CoerceValue() would do no work), but it is possible for a business object author to provide a value of some other type. This helps ensure that the value is converted to the right type before it is stored. Only BusinessBase supports read-write properties, so only BusinessBase calls these methods. For example, here s the LoadProperty() method from BusinessBase: Protected Sub LoadProperty(Of P)( _ ByVal propertyInfo As PropertyInfo(Of P), ByVal newValue As P) Try Dim oldValue As P = Nothing Dim fieldData = FieldManager.GetFieldData(propertyInfo) If fieldData Is Nothing Then oldValue = propertyInfo.DefaultValue fieldData = FieldManager.LoadFieldData(Of P)(propertyInfo, oldValue) Else Dim fd = TryCast(fieldData, FieldManager.IFieldData(Of P)) If fd IsNot Nothing Then oldValue = fd.Value Else oldValue = DirectCast(fieldData.Value, P) End If End If LoadPropertyValue(Of P)(propertyInfo, oldValue, newValue, False) Catch ex As Exception Throw New PropertyLoadException(String.Format( _ My.Resources.PropertyLoadException, propertyInfo.Name, ex.Message)) End Try End Sub The majority of this method is centered around retrieving the value from the field manager. It calls GetFieldData() to get the IFieldData object, and then it sets the oldValue field either to some default value or to the value retrieved from the field manager. All that work is in preparation for a call to LoadPropertyValue(), which does the real work. The complex LoadPropertyValue() method implements the flowchart shown earlier in Figure 7-2. You can look at the full code from the Download area of the Apress website (www.apress.com). The key thing to recognize is that it ultimately calls the SetFieldData() method from the field manager: FieldManager.SetFieldData(Of P)(propertyInfo, newValue) Throughout the entire process, the input value is strongly typed, including during the call to the field manager and storage in the FieldData(Of T) object. You may be wondering then where that non-generic SetFieldData() overload is invoked. It is invoked from a non-generic overload of SetProperty() implemented in BusinessBase. Protected Sub SetProperty(ByVal propertyInfo As IPropertyInfo, _ ByVal newValue As Object) FieldManager.SetFieldData(propertyInfo, newValue) End Sub This overload of SetProperty() exists to support scenarios in which the business developer is loading the fields of the object from an external data source, such as an XML file or database. In that case, the developer might be looping through all the registered properties, loading the field data for each one, and would be unable to use a generic method in that case. Additionally, the input values
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