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Compile() and DynamicInvoke() are expensive operations to conduct. You don t want to be doing these operations on a per-item basis as you search through a data structure.
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CHAPTER 14 LINQ TO C SLA
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Determining whether you have an index is much simpler: Private Function HasIndexFor(ByVal [property] As String) As Boolean _ Implements IIndexSet(Of T).HasIndexFor Return _internalIndexSet.ContainsKey([property]) End Function Private Function HasIndexablePropertyOnLeft( _ ByVal leftSide As Expression) As Boolean If leftSide.NodeType = ExpressionType.MemberAccess Then Return TryCast(Me, IIndexSet(Of T)).HasIndexFor(DirectCast(leftSide, _ MemberExpression).Member.Name) Else Return False End If End Function Determining whether you have an index for a given property specified by a string is relatively simple, because you have an internal field _internalIndexSet that is really just a Dictionary of indices. You can call ContainsKey() on the _internalIndexSet to determine whether you have the index. The examination of the expression that represents the left side of the total expression is slightly more complex. You need for this left-side expression to be a MemberExpression in order to make any sense of it. Thankfully, this is common, given that it rarely makes sense to have anything other than a MemberExpression on the left side. Note that this may be possible technically (in other words, Function(x) 1=1 can be passed, which, in theory, would return all results). The first thing you do is test the NodeType to ensure that it s a MemberAccess expression. If that is in fact the case, you know you can cast it to a MemberExpression, which then gives you access to the name of the property you re looking at (such as the name SomeProp on the left side, which is in an expression such as Function(x) x.SomeProp = 42). With this information, you have everything you need to determine whether you have an index on the property. Once you have established that you have a property that is Indexable and a value to check against the index, you can test the operation between both sides of the expression to determine which index operation to perform: Dim exprCompiled As Func(Of T, Boolean) = expr.Compile() Dim binExp = DirectCast(expr.Body, BinaryExpression) Dim val As Object = GetRightValue(binExp.Right) Dim rangedIndex As IRangeTestableIndex(Of T) If TypeOf _internalIndexSet([property]) Is IRangeTestableIndex(Of T) Then rangedIndex = DirectCast(_internalIndexSet([property]), _ IRangeTestableIndex(Of T)) Select Case binExp.NodeType Case ExpressionType.Equal Return _internalIndexSet([property]).WhereEqual(val, exprCompiled) Case ExpressionType.LessThan Return _internalIndexSet([property]). WhereLessThan(val, exprCompiled) 'and so forth, for other operations... End Select End If Based on the NodeType of the BinaryExpression object (shown as binExp), you can determine whether you have been passed an equality test, a less-than operation, or any other logical test. Of course, you have to make sure the index is IRangeTestableIndex before you try to use it in the context of a ranged logical operator such as less-than or greater-than. However, once you know that, you can handle each operation appropriately. If you can t handle the operation, pass it back to LINQ to Objects to handle in the default manner.
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CHAPTER 14 LINQ TO C SLA
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IIndex represents the expected methods and behavior that an index should provide. Most implementations of IIndexSet contain one or more IIndex objects per property that is being indexed on T. IIndex derives from ICollection and adds the members shown in Table 14-5.
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Table 14-5. IIndex Members
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IndexField WhereEqual() ReIndex() Loaded InvalidateIndex() LoadComplete()
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Description
Returns the PropertyInfo for the property that this index is indexing Returns the items that match the value passed to it Removes and adds back the given item from the index Returns whether the index has been established Sets the index as not loaded anymore Sets the index as loaded
Writing an IIndex implementation involves implementing all the methods you typically would for ICollection and then adding implementation for the members shown in Table 14-5. A typical implementation wraps an appropriate data structure, such as a red-black tree or dictionary, or possibly even derives from it, and then adds the implementation of the methods, which make indexed operations able to work with an IIndexSet. The only interesting part of building an IIndex implementation is the handling of WhereEqual(). Typically, WhereEqual() takes the object passed to it which represents a value from a property of the child object and uses some mechanism based on that value to find the item in a manner somehow faster than looking through every single one in the index. Exactly how this is done depends on the index structure. In a Dictionary-based structure, a hash code is generated from the property, and then the Where test is performed on each item with a matching hash code. On other structures, such as a red-black tree, the normal search mechanism for such a tree would be used to locate the appropriate key matches.
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