15: LINQ with Query Expressions in Visual C#

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on the target (that is, Keywords) that take delegates as parameters Delegates are objects that save information about what code to execute when the delegate is called, and since delegates contain only the data regarding what to execute, they can be stored until a later time when they are executed In the case of collections that implement IQueryable<T> (LINQ providers), the lambda expressions are translated into expression trees An expression tree is a hierarchical data structure broken down recursively into subexpressions Each subexpression represents a portion of the lambda expression that is further broken down until each part is the most fundamental unit that can no longer be broken down Frequently, expression trees are then enumerated and reconstructed as the original expression tree is translated into another language, such as SQL
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Query Expressions as Method Invocations
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In spite of the power and relative simplicity associated with query expressions, the CLR and IL do not require any query expression implementation Rather, the C# compiler translates query expressions into method calls Consider, for example, the query expression from Listing 151, a portion of which appears in Listing 1514
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Listing 1514: Simple Query Expression
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private static void ShowContextualKeywords1() { IEnumerable<string> selection = from word in Keywords where wordContains('*') select word; // } //
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After compilation, the expression from Listing 1514 is converted to an IEnumerable<T> extension method call from SystemLinqEnumerable, as shown in Listing 1515
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Listing 1515: Query Expression Translated to Standard Query Operator Syntax
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private static void ShowContextualKeywords3() {
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IEnumerable<string> selection = KeywordsWhere(word => wordContains('*')); // } //
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Furthermore, as discussed in 14, the lambda expression is translated to IL corresponding to a delegate invocation Moreover, the combination of extension methods and lambda expressions provides a superset of the functionality available through query expressions For example, there is no query expression equivalent for the extension method TakeWhile<T>(Func<T, bool> predicate), which repeatedly returns items from the collection as long as the predicate returns true Not all method invocations can be translated to query expressions, but the reverse, translating from query expressions to method expressions, is always possible Regardless, where translation is possible in either direction, it is not consistently more understandable Some queries are better suited for query expressions whereas others are more readable as method invocations I find the general rule is to use query expressions where possible, but to rely on method invocations otherwise Regardless, it is frequently helpful to refactor a complex query into multiple statements or even methods
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This chapter introduced a new syntax, that of query expressions Readers familiar with SQL will immediately see the similarities between query expressions and SQL However, query expressions also introduce additional functionality, such as grouping into a hierarchical set of new objects, which was unavailable with SQL All of the functionality of query expressions was already available via standard query operators, but query expressions frequently provide a simpler syntax for expressing such a query Whether through standard query operators or query expression syntax, however, the end result is a significant improvement in the way developers are able to code against collection APIs, an improvement that
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15: LINQ with Query Expressions
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ultimately provides a paradigm shift in the way object-oriented languages are able to interface with relational databases In the next chapter, we continue our discussion of collections: investigating some of the NET Framework collection types as well as how to define custom collections
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Building Custom Collections
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14 COVERED standard query operators, a set of extension methods on IEnumerable<T> that added a common set of methods to all collections However, this did not make all collections the same There is still a strong need for different collection types Some collections are better suited to searching by key, whereas others are better suited to index
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Iterators
More Collection Interfaces
IList<T> IDictionary<TKey, TValue> IComparable<T> ICollection<T>
Returning null or an Empty Collection
Collections
List<T> Primary Collection Dictionary<TKey, TValue> SortedDictionary<TKey, TValue> Classes and SortedList<T> Stack<T> Queue<T> LinkedList<T>