s COMPLETING THE FRAMEWORK in Visual Basic .NET

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CHAPTER 5 s COMPLETING THE FRAMEWORK
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Dim t As Type = CslaDataSource.GetType(mTypeAssemblyName, mTypeName) If GetType(IEnumerable).IsAssignableFrom(t) Then ' this is a list so get the item type t = Utilities.GetChildItemType(t) End If At this point, the method has a Type object corresponding to the type of business object (either the object itself, or the child objects in a collection). The method then gets a collection of PropertyDescriptor objects, each one representing a property of the business object: Dim props As PropertyDescriptorCollection = _ TypeDescriptor.GetProperties(t) Each of the PropertyDescriptor methods is checked to see if the corresponding property is marked as <Browsable(False)>, in which case the IsBrowsable property will return False. Those properties are ignored. All other properties are used to create an instance of an ObjectFieldInfo object, which represents that property s schema information: For Each item As PropertyDescriptor In props If item.IsBrowsable Then result.Add(New ObjectFieldInfo(item)) End If In the end, an array of these ObjectFieldInfo objects is returned as a result: Return result.ToArray Each element in this array contains schema information about a business object property that is available for data binding.
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ObjectFieldInfo
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The ObjectFieldInfo class maintains schema information about a single property on a business object. One of these objects is created for each bindable property on a business object, and is .NET through the GetFields() method of ObjectViewSchema. returned to ASP The ObjectFieldInfo class implements the System.Web.UI.Design.IDataSourceFieldSchema interface as required by ASP.NET. This is a lengthy interface that defines a number of properties that provide information about the column/property. Many of these properties are designed specifically for columns from a DataTable, and have little meaning when applied to elements of a business object s interface, while others are valid in both cases. The ObjectFieldInfo class maintains a set of instance fields to hold information about the business object property: Public Class ObjectFieldInfo Implements System.Web.UI.Design.IDataSourceFieldSchema Private Private Private Private Private mField As PropertyDescriptor mPrimaryKey As Boolean mIsIdentity As Boolean mIsNullable As Boolean mLength As Integer
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Public Sub New(ByVal field As PropertyDescriptor) mField = field GetDataObjectAttribute() End Sub Most of these values come from a <DataObjectField()> attribute that the business object developer can apply to properties of a business object.
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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DataObjectField Attribute
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The <DataObjectField()> attribute comes from the System.ComponentModel namespace and can be used on a business object s property like this: <DataObjectField(True, True, False)> _ Public ReadOnly Property Id() As Guid Get CanReadProperty(True) return mId End Get End Property In this example, the attribute specifies that the Id property is a primary key, represents the identity of the object, and is not nullable. Table 5-9 lists the possible parameters for the <DataObjectField()> attribute. Table 5-9. Parameters for the DataObjectField() Attribute
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Parameter
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primaryKey isIdentity isNullable length
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Description
Indicates whether the property acts as a primary key value for the object Indicates whether the property uniquely identifies the object within a parent collection Indicates whether the property is nullable Provides the maximum length of the property in bytes
The GetDataObjectAttributes() helper method retrieves this attribute from the business object s property and sets the corresponding instance fields: Private Sub GetDataObjectAttribute() Dim attribute As DataObjectFieldAttribute = _ CType(mField.Attributes.Item(GetType(DataObjectFieldAttribute)), _ DataObjectFieldAttribute) If (Not attribute Is Nothing) Then With attribute mPrimaryKey = .PrimaryKey mIsIdentity = .IsIdentity mIsNullable = .IsNullable mLength = .Length End With End If End Sub Each of these values is exposed as a property through the IDataSourceFieldSchema interface. For instance, here s the PrimaryKey property: Public ReadOnly Property PrimaryKey() As Boolean _ Implements System.Web.UI.Design.IDataSourceFieldSchema.PrimaryKey Get Return mPrimaryKey End Get End Property Visual Studio can use these properties to assist the UI developer in building an appropriate interface for the business object s properties.
CHAPTER 5 s COMPLETING THE FRAMEWORK
The Nullable property is a bit more complex, however, because it is sometimes possible to detect that a property is nullable even without the use of the <DataObjectField()> attribute. This is due to the nullable support built into .NET 2.0: Public ReadOnly Property Nullable() As Boolean _ Implements System.Web.UI.Design.IDataSourceFieldSchema.Nullable Get Dim t As Type = Me.mField.PropertyType If Not t.IsValueType OrElse mIsNullable Then Return True End If If t.IsGenericType Then Return (t.GetGenericTypeDefinition Is GetType(Nullable)) End If Return False End Get End Property A property can be nullable if it is a reference type or if mIsNullable is True. It can also be nullable if the property is declared using the Nullable(Of T) generic type.
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