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Functional Style and Composition
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LINQ operators all share a common characteristic: they do not modify the data they work on. For example, you can get LINQ to sort the results of a query, but unlike Array.Sort or List<T>.Sort, which both modify the order of an existing collection, sorting in LINQ works by producing a new IEnumerable<T> which returns objects in the specified order. The original collection is not modified. This is similar in style to .NET s string type. The string class provides various methods that look like they will modify the string, such as Trim, ToUpper, and Replace. But strings are immutable, so all of these methods work by building a new string you get a modified copy, leaving the original intact. LINQ never tries to modify sources, so it s able to work with immutable sources. LINQ to Objects relies on IEnumerable<T>, which does not provide any mechanism for modifying the contents or order of the underlying collection.
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Of course, LINQ does not require sources to be immutable. IEnumera ble<T> can be implemented by modifiable and immutable classes alike. The point is that LINQ will never attempt to modify its source collections.
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This approach is sometimes described as a functional style. Functional programming languages such as F# tend to have this characteristic just as mathematical functions such as addition, multiplication, and trigonometric functions do not modify their inputs, neither does purely functional code. Instead, it generates new information based on its inputs new enumerations layered on top of input enumerations in the case of LINQ. C# is not a purely functional language it s possible and indeed common to write code that modifies things but that doesn t stop you from using a functional style, as LINQ shows. Functional code is often highly composable it tends to lead to APIs whose features can easily be combined in all sorts of different ways. This in turn can lead to more maintainable code small, simple features are easier to design, develop, and test than
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complex, monolithic chunks of code, but you can still tackle complex problems by combining smaller features. Since LINQ works by passing a sequence to a method that transforms its input into a new sequence, you can plug together as many LINQ operators as you like. The fact that these operators never modify their inputs simplifies things. If multiple pieces of code are all vying to modify some data, it can become difficult to ensure that your program behaves correctly. But with a functional style, once data is produced it never changes new calculations yield new data instead of modifying existing data. If you can be sure that some piece of data will never change, it becomes much easier to understand your code s behavior, and you ll have a better chance of making it work. This is especially important with multithreaded code.
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Deferred Execution
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7 introduced the idea of lazy enumeration (or deferred execution, as it s also sometimes called). As we saw, iterating over an enumeration such as the one returned by GetAllFilesInDirectory does the necessary work one element at a time, rather than processing everything up front. The query in Example 8-2 preserves this characteristic if you run the code, you won t have to wait for GetAllFilesInDirec tory to finish before you see any results; it will start printing filenames immediately. (Well, almost immediately it depends on how far it has to look before finding a file large enough to get through the where clause.) And in general, LINQ queries will defer work as much as possible merely having executed the code that defines the query doesn t actually do anything. So in our example, this code:
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var bigFiles = from file in GetAllFilesInDirectory(@"c:\") where new FileInfo(file).Length > 10000000 select file;
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does nothing more than describe the query. No work is done until we start to enumerate the bigFiles result with a foreach loop. And at each iteration of that loop, it does the minimum work required to get the next item this might involve retrieving multiple results from the underlying collection, because the where clause will keep fetching items until it either runs out or finds one that matches the condition. But even so, it does no more work than necessary. The picture may change a little as you use some of the more advanced features described later in this chapter for example, you can tell a LINQ query to sort your data, in which case it will probably have to look at all the results before it can work out the correct order. (Although even that s not a given it s possible to write a source that knows all about ordering, and if you have special knowledge about your data source, it may be possible to write a source that delivers data in order while still fetching items lazily. We ll see providers that do this when we look at how to use LINQ with databases in a later chapter.)
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Although deferred execution is almost always a good thing, there s one gotcha to bear in mind. Because the query doesn t run up front, it will run every time you evaluate it. LINQ doesn t keep a copy of the results when you execute the query, and there are good reasons you wouldn t want it to it could consume a lot of memory, and would prevent you from using the technique in Example 8-9. But it does mean that relatively innocuous-looking code can turn out to be quite expensive, particularly if you re using a LINQ provider for a database. Inadvertently evaluating the query multiple times could cause multiple trips to the database server.
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