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When marking a child class using the Indexable attribute, you can further specify the behavior of the index. In some cases, you might think it is desirable to force the creation of an index for a child class. For example, if you know for sure that you are going to use the index and you don t want to take the performance hit the first time you use it, you would use the IndexModeAlways option on the attribute. This way, when items are added, they re indexed immediately rather than the first time a query needs it. On the other hand, if it is important to optimize for load time on the collection and if it is
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CHAPTER 14 LINQ TO C SLA
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acceptable to take the hit on index creation the first time you do a query that may take advantage of the index, you would use IndexModeOnDemand. The IndexModeNever option is provided to mark cases in which you want to ensure that indexes are never created on a field. Table 14-1 summarizes the parameters of the Indexable attribute.
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Table 14-1. Index Modes
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Parameter
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IndexModeEnum.IndexModeOnDemand IndexModeEnum.IndexModeAlways IndexModeEnum.IndexModeNever
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Load an index only when needed. Always load and maintain an index for this property. Never create an index for this property.
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The following example illustrates the use of the Indexable attribute on a child class: Class Student Inherits BusinessBase Private _photo As Byte() <Indexable(IndexModeEnum.IndexModeNever)> _ Public Property Photo() As Byte() 'we never search by photo Get Return _photo End Get Set(ByVal value As Byte()) _photo = value End Set End Property Private _ssn As String <Indexable(IndexModeEnum.IndexModeAlways)> _ Public Property SSN() As String 'we search by SSN all the time Get Return _ssn End Get Set(ByVal value As String) _ssn = value End Set End Property Private _firstName As String <Indexable(IndexModeEnum.IndexModeOnDemand)> _ Public Property FirstName() As String 'not as common of a search Get Return _firstName End Get Set(ByVal value As String) _firstName = value End Set End Property Private _lastName As String <Indexable()> _
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CHAPTER 14 LINQ TO C SLA
Public Property LastName() As String Get Return _lastName End Get Set(ByVal value As String) _lastName = value End Set End Property End Class
' default is IndexModeOnDemand
The IQueryable Implementation for CSLA .NET
When fully implemented, the IQueryable interface is the mechanism by which anyone can implement his own implementation of LINQ. LINQ to XML, LINQ to SQL, and dozens of other LINQ implementations all implement this interface. Thus, in order to implement the expected behavior when you run a LINQ query against BusinessListBase, IQueryable has been implemented for BusinessListBase. The IQueryable interface mandates only three members be implemented. Table 14-2 shows which members are required for an IQueryable implementation.
Table 14-2. IQueryable Members
Member
ElementType Expression Provider
Description
Provides the source type for the implementation The most recent expression tree associated with the collection The IQueryProvider implementation used by IQueryable
ElementType and Expression both have reasonably simple implementations. In CSLA .NET, ElementType simply returns an instance of the child type as follows: Public ReadOnly Property ElementType() As Type Get Return GetType(C) End Get End Property The Expression property is also fairly simple. It returns the expression tree associated with the collection (I ll cover expression trees in the next section). In BusinessListBase, this represents the entire collection, because you use LinqBindingList rather than BusinessListBase to represent the result of a query. In BusinessListBase, Expression returns a private backing field where the current expression is held. Public ReadOnly Property Expression() As Expression Get If _expression Is Nothing Then _expression = Expression.Constant(Me) End If Return _expression End Get End Property
CHAPTER 14 LINQ TO C SLA
The last member in any IQueryable implementation is the Provider property. The Provider is where the core of any IQueryable implementation really occurs. The property itself is fairly simple, returning a new instance of the CslaQueryProvider: Public ReadOnly Property Provider() As IQueryProvider Get Return New Linq.CslaQueryProvider(Of T, C)(Me) End Get End Property CslaQueryProvider is the CSLA .NET implementation of the IQueryProvider interface, which defines a custom implementation of LINQ. Of course, the devil is in the details, and understanding how an IQueryProvider works is key to understanding what LINQ to CSLA .NET is doing behind the scenes.
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