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CHAPTER 14 LINQ TO C SLA
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child classes that are marked Indexable. Any code written the old way because it specified not only what but how would not be able to take advantage of that optimization without recoding. The latter example, by contrast, requires no updates when you decide to use indexes with your collection. I ll discuss indexes more in the next section.
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Overview of LINQ to CSLA .NET
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While LINQ provides some great opportunities, it also presents special challenges to the implementers of frameworks. One of the chief reasons for implementing a business object framework is the reduction of UI code you need to write to implement a solution. Therefore, the ability to bind collections that inherit from BusinessListBase is a critical component of a framework like CSLA .NET.
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Binding to Results from LINQ to Objects
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Just as collections that inherit from BusinessListBase should work properly with binding, so should the result of LINQ queries that are run over collections of BusinessListBase objects. Unfortunately, the result of a standard query from LINQ to Objects is a type called Sequence<U>. As shown in Figure 14-1, this new collection doesn t have awareness of the collection from which it is generated. This means that operations on the result of the query, such as change tracking and collection-level validation, will fail to work as expected, due to the fact that the new collection has no awareness of the old one.
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Figure 14-1. Binding from a projection The overall goal in the development of LINQ to CSLA .NET is to make it possible to use LINQ with CSLA .NET collections and have data binding behavior work as expected. Namely, the goal is to make it so that you can bind to the result of a LINQ query performed against a CSLA .NET collection and have all the behavior you would expect when binding to the entire collection itself. Later in this chapter, I ll provide more detail about how this works.
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Indexed LINQ Queries
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Unlike most standard collections in the .NET Framework, such as List<T> and Dictionary<K,V>, collections in CSLA .NET are aware of changes that occur in the objects they contain. Because of this,
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CHAPTER 14 LINQ TO C SLA
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my colleague Aaron Erickson and I have added the ability for business collections to optimize queries through the implementation of indexes. Let s say you re using LINQ to query against CSLA .NET collections under CSLA .NET 3.5 or higher. If you ve marked fields as indexable using the Indexable attribute, then the implementation of Where() the method that makes filtering work in the query will take advantage of indices internal to the collection.
LINQ and Projection
Running a query against any IEnumerable<T> and generating a Sequence<U> of results using LINQ to Objects is called projection. The execution of a simple LINQ to Objects query has two main parts: a where clause, which applies criteria (in the form of an anonymous delegate or lambda) to find items, and a select clause, which transforms each result into something else. Let s start with a typical LINQ query, such as this one that selects numbers equal to 42 from an array of numbers and returns the square of the result: var coolNumbers = from n in allNumbers where n == 42 select n * n; The LINQ query syntax is merely syntactic sugar that you can rewrite in more traditional form as follows: var coolNumbers = allNumbers.Where(n => n == 42).Select(n => n*n); This core mechanism filter and project is at the heart of every LINQ query. The filter (where) determines which items from the collection matter, and the projection (select) transforms each result from one thing into another thing.
Note
The term projection refers to the collection you can generate from the result of the Select() method.
Not all projections are equivalent. For example, let s say you call Select() like this: .Select(n => n*0). In this case, it is impossible to determine how a change in the projected object should be reflected in the original. Since each result of n*0 is always 0, there is no information in the resulting object that would allow you to know which object it maps back to in the source collection. Because of the lack of a definitive mapping, synchronization of any individual item is impossible. That said, in certain cases such as the projection of the same object type you started with in the original collection (an identity projection) where you re operating on exactly the same object type you started with you don t have to worry about trying to figure out how changes in the projection reflect back to changes in the original. You can map an object in the projected collection back to the source collection in a way that is deterministic. This fact makes it possible for CSLA .NET to allow for binding to this type of projection.
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