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private static void TestFeatureProgression() { // Example 1: // Olden days methodolgy for finding odd numbers in an array // back before even Generics were available ArrayList al = new ArrayList(){20, 1, 3, 4, 8, 9, 44, 47 }; int[] intList = new int[] {20, 1, 3, 4, 8, 9, 44, 47 }; ArrayList oddIntegers = new ArrayList(); foreach (int i in intList) { if ((i % 2) > 0) { oddIntegers.Add(i); } // if } // foreach DisplayNumbers(oddIntegers, "using an array search"); // or since we now have generics, we can jump ahead and // convert the arraylist to a generic list and pass it to // the method that displays a List<> DisplayNumbers(new List<int>(oddIntegers.ToArray(typeof(int)) as int[]), "using array search printed from generic list"); // Now, we bring in Generics and their power. Not only is a // List<int> more powerful than an ArrayList, but it is // strongly typed so that unlike an arraylist, you cannot add // a string to the List<int>. First we will create a // list of integers with automatic initialization just as we // previously did with the creation of an array. // which will be used by all three examples List<int> list = new List<int>() { 20, 1, 3, 4, 8, 9, 44, 47 }; // Example 2): // first, a Predicate delegate is a reference that returns a bool // note that IsOddNumber is a separate method shown below Predicate<int> callBack = new Predicate<int>(IsOddNumber); // now you can pass the Predicate delegate to the FindAll method of the generic List List<int> oddNumbers = list.FindAll(callBack); // at this point all even numbers are in the list evenNumbers DisplayNumbers(oddNumbers, "using Predicate delegate"); // Example 3): // now shorten code by using an anonymous method // note that we no longer need to declare and use the IsOddNumber method list = list.FindAll(delegate(int i) { return (i % 2) > 0; }); DisplayNumbers(oddNumbers, "using anonymous method"); // Example 4: // a lambda expression can even shorten it more // the => (lambda) symbol means that the parameter (list) on the left side // of the lambda symbol is to be processed by the expression on the right side // of the lambda symbol. The expression was originally a formally coded delegate // and in Example 2, it required a method to be the target of the Predicate. // In the second example we shortened the code by the use of an anonymous method // allowing us to do away with the Predicate and its target method (IsOddNumber). // In Example 4 shown below, the FindAll method of the list object will call // the implied anonymous method for each object in the List<int>. // To sum up the following line of code in english: // My list of paramaters "[int] i" will be processed by the expression "(i % 2) > 0" oddNumbers = list.FindAll(i => (i % 2) > 0); DisplayNumbers(oddNumbers, "using a lambda expression"); // you could } /// target for the Predicate<> delegate static bool IsOddNumber(int i) { // is it an even number? return (i % 2) > 0; } private static void DisplayNumbers(ArrayList oddNumbers, string s) { Console.WriteLine("Here are your odd numbers " + s); foreach (int oddNumber in oddNumbers) { Console.Write("{0}\t", oddNumber); } // foreach Console.WriteLine(Environment.NewLine + Environment.NewLine); } // method: DisplayNumbers private static void DisplayNumbers(List<int> oddNumbers, string s) { Console.WriteLine("Here are your odd numbers " + s); foreach (int oddNumber in oddNumbers) { Console.Write("{0}\t", oddNumber); } // foreach Console.WriteLine(Environment.NewLine + Environment.NewLine); }