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Understanding Expression Trees
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The key to understanding LINQ is to understand the role of expression trees in the implementation of LINQ. Expression trees are representations of source code as data. Consider the following code passed to the Where() method in LINQ: var result = someCollection.Where( x => x.SomeVal == 42); The parameter provided to the Where() method is a lambda function a more compact syntax for an anonymous method. However, when Where() is invoked, the lambda function is converted into a data structure in the form of an expression tree that looks like Figure 14-3.
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CHAPTER 14 LINQ TO C SLA
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Figure 14-3. A simple expression tree Expression trees allow for code to be executed by alternative means that is, the code can be translated to other forms, such as SQL. For example, in LINQ to SQL, the previous expression will get converted into the following SQL query: SELECT * FROM someCollection WHERE SomeVal = 42 What the expression gets converted to in other LINQ providers will vary according to the provider. For example, LINQ to XML could convert the expression to an XPath expression that ordinary code in the System.Xml namespace could execute. In CSLA .NET, the key differences in how expressions are handled occur in two areas. First, the optimization of Where takes advantage of indexing (which I cover in detail later in this chapter). Second, CSLA .NET makes sure that LINQ queries against BusinessListBase return a LinqBindingList that synchronizes with the collection from which it was originally queried, rather than a Sequence that features no such synchronization. The indexing code analyzes the expression passed to the Where clause to determine if indexing is possible. The synchronization code analyzes the expression passed to the Select clause to determine if the projection is an identity projection, and thus, able to be synchronized.
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Digging into IQueryProvider
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The IQueryProvider interface contains, surprisingly, only four members, as shown in Table 14-3.
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Table 14-3. IQueryProvider Members
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Member
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CreateQuery() CreateQuery<TResult>() Execute() Execute<TResult>()
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Handles LINQ methods that return an IQueryable result CreateQuery(), but with a known return type Handles LINQ methods that return an IQueryable result Execute(), but with a known return type
CHAPTER 14 LINQ TO C SLA
Of course, within this somewhat simple interface lies a great deal of complexity. The parameter provided to CreateQuery() or Execute() is an object of type Expression, which we must evaluate in a manner that returns an expected result. To further complicate this, when you choose to implement your own IQueryProvider, you give up the default LINQ to Objects implementation and are required to implement the entire range of possible Expression objects that might be passed in.
Evaluating Expression for Synchronization Support
To support synchronization, you must first determine whether you re handling an identity projection. That test, which is in CreateQuery(), is surprisingly simple. This only applies to the generic overload of CreateQuery() (CreateQuery<TResult>()), because in that version, it is known what type of IQueryable needs to be returned. typeof(TElement) == typeof(C) If this test passes, then you have an identity projection, because you ll be returning a collection that is of the same type as the source collection. In this case, you can return a LinqBindingList<C> object that will have all the appropriate information to allow for further query execution. _filter = new LinqBindingList<C>(_parent, this, expression); _filter.BuildFilterIndex(); return (IQueryable<TElement>)_filter; Because there are corner cases in which the LinqBindingList needs to be referenced by the CslaQueryProvider object later on, you store it as a private field. The LinqBindingList needs to store three critical values, so the constructor takes three parameters: the original collection, the IQueryProvider instance, and the expression. The original collection is the _parent parameter. The IQueryProvider instance, which holds query execution state, is the this parameter. The expression, which allows the LinqBindingList to continue query execution when query results are requested, is the expression parameter.
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