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CHAPTER 27 LINQ TO OBJECTS
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// group the results based on first letter IEnumerable<IGrouping<char, string>> groupResults = results.GroupBy(e => e[0]); // convert the results to a Dictionary<TKey, TVal> Dictionary<char, IGrouping<char, string>> resultDict = groupResults.ToDictionary(e => e.Key); // print out the contents of the dictionary foreach (char key in resultDict.Keys) { Console.WriteLine("Dictionary Key: {0}", key); // get the IGrouping associated with the key IEnumerable<string> groupEnum = resultDict[key]; foreach (string str in groupEnum) { Console.WriteLine("Dictionary entry: {0}", str); } } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } The results from the query in Listing 27-28 are converted into an array, a List<T>, and a Dictionary<TKey, TVal>. You can find out more about arrays in 13 and about the List<T> and Dictionary<TKey, TVal> classes in 19. Compiling and running Listing 27-28 produces the following results: Array result: plum Array result: pear Array result: persimmon Array result: lemon Array result: lime Array result: pineapple List result: plum List result: pear List result: persimmon List result: lemon List result: lime List result: pineapple Dictionary Key: p Dictionary entry: plum Dictionary entry: pear Dictionary entry: persimmon Dictionary entry: pineapple Dictionary Key: l Dictionary entry: lemon Dictionary entry: lime Press enter to finish
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CHAPTER 27 LINQ TO OBJECTS
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The legacy collection classes contained in the System.Collections namespace implement the nongeneric IEnumerable interface, rather than IEnumerable<T>. This means that LINQ can t infer the type of the range variable from the type of the data contained in the collection. LINQ provides three different approaches to addressing this, which is useful for when you want to use LINQ to query data created by legacy code.
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Explicitly Specifying the Range Variable Type
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The first approach you can take is to explicitly specify the type of the range variable in the LINQ query so that LINQ doesn t need to infer it for you. Listing 27-29 provides a demonstration using the legacy ArrayList class. Listing 27-29. Explicitly Specifying the Range Variable Type using using using using System; System.Collections; System.Collections.Generic; System.Linq;
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class Listing 29 { static void Main(string[] args) { ArrayList myFruitList = new ArrayList() { "apple", "plum", "cherry", "grape", "banana", "pear", "mango" , "persimmon", "lemon", "lime", "coconut", "pineapple", "orange"}; // define the first query IEnumerable<string> results = from string e in myFruitList select e; // enumerate the results foreach (string str in results) { Console.WriteLine("Result item: {0}", str); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } In Listing 27-29 I have created an ArrayList that contains the fruit names. In the LINQ query, I have told LINQ that the ArrayList contains string values by preceding the range variable with the string type. This works if you are sure of the contents of the collection, but if any of the items in the ArrayList are not strings, then a System.InvalidCastException will be thrown.
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CHAPTER 27 LINQ TO OBJECTS
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Using Cast and OfType Extension Methods
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The second approach is to convert the nongeneric collection class to a generic IEnumerable<T> using the Cast extension method, which can be applied to IEnumerable implementations. Listing 27-30 contains a demonstration. Listing 27-30. Using the Cast Method using using using using System; System.Collections; System.Collections.Generic; System.Linq;
class Listing 30 { static void Main(string[] args) { ArrayList myFruitList = new ArrayList() { "apple", "plum", "cherry", "grape", "banana", "pear", "mango" , "persimmon", "lemon", "lime", "coconut", "pineapple", "orange"}; // use the Cast extension method to create an IEnumerable<string> IEnumerable<string> myEnum = myFruitList.Cast<string>(); // define the query IEnumerable<string> results = from e in myEnum select e; // enumerate the results foreach (string str in results) { Console.WriteLine("Result item: {0}", str); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } Much like providing an explicit type for the range variable, using the Cast extension method requires that all the objects in the source collection can be converted to the type you have specified, string values in the example. An exception will be thrown in an object cannot be converted to the target type. Compiling and running Listing 27-30 produces the following results: Result Result Result Result Result Result item: item: item: item: item: item: apple plum cherry grape banana pear
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