.NET/ASP.NET/C#/VB.NET PDF Document SDK

This example only half enters into the spirit of things it can accept any IEnumera ble<string>, but it stubbornly continues to return an array. This isn t necessarily a problem; after all, arrays implement IEnumerable<T>. However, our code is a little inelegant in the way that it creates a List<string> and then converts that into an array at the end. There s a better way C# makes it very easy to provide a sequence of objects directly as an IEnumerable<T>.

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A common version of the producer-consumer scenario is to have several producers serving a consumer with data For example, you can have several working threads providing data for the main thread The main thread is the only thread that can update the user interface, so it is logical to make it a consumer (it can also be a producer a thread can be both a producer and consumer at the same time) There are two issues that you need to deal with before you can use several TextProducer objects with the TextConsumer class presented in Listing 12-20 The first issue is the atEnd flag, which needs to be converted into a semaphore It will be released in the TextProducer constructor and acquired when the producer runs out of data in the run method In the consumer, the while loop cannot check for atEnd; atEndavailable() is used instead.

Before version 2 of C# (which shipped with Visual Studio 2005), writing your own enumerable types was tedious you had to write a class that implemented IEnumera tor, and that would usually be a separate class from the one that implemented IEnumerable, because multiple enumerators can be active simultaneously for any single collection. It wasn t hugely tricky, but it was enough of a hassle to put most people off. But C# 2 made it extremely easy to provide enumerations. Example 7-32 shows yet another reworking of the AddNumbers method.

Gets or sets the CSS class for this control Gets or sets the enablement of this control Reads the current CSS style for the control Gets or sets the position of this control in the tab index This is the position that it occupies when the user moves through the UI using the Tab key Gets or sets the visibility of the control If this is false, you will not see the control This can be set to Hide so the control is not seen or to Collapse so the control is minimized to its top label If the current record is the last one in the record set, then you cannot move to the next record, and this will be false; otherwise, it will be true.

static IEnumerable<string> AddNumbers(IEnumerable<string> names) { int i = 0; foreach (string currentName in names) { yield return string.Format("{0}: {1}", i, currentName); i += 1; } }

Instead of using the normal return statement, this method uses yield return. This special form of return statement can only be used inside a method that returns either an enumerable or an enumerator object you ll get a compiler error if you try to use it anywhere else. It works rather differently from a normal return. A normal return statement indicates that the method has finished, and would like to return control to the caller (returning a value, if the method s return type was not void). But yield return effectively says: I want to return this value as an item in the collection, but I might not be done yet I could have more values to return. The yield return in Example 7-32 is in the middle of a foreach loop. Whereas a normal return would break out of the loop, in this case the loop is still running, even though the method has returned a value. This leads to some slightly surprising flow of execution. Let s look at the order in which this code runs. Example 7-33 modifies the AddNumbers method from Example 7-32 by adding a few calls to Console.Writeline, so we can see exactly how the code runs. It also includes a Main method with a foreach loop iterating over the collection returned by AddNumbers, again with some Con sole.WriteLine calls to keep track of what s going on.

class Program { static IEnumerable<string> AddNumbers(IEnumerable<string> names) { Console.WriteLine("Starting AddNumbers"); int i = 0; foreach (string currentName in names) { Console.WriteLine("In AddNumbers: " + currentName); yield return string.Format("{0}: {1}", i, currentName); i += 1; } Console.WriteLine("Leaving AddNumbers"); } static void Main(string[] args) { string[] eventNames = { "Swing Dancing at the South Bank", "Saturday Night Swing", "Formula 1 German Grand Prix", "Swing Dance Picnic", "Stompin' at the 100 Club" }; Console.WriteLine("Calling AddNumbers"); IEnumerable<string> numberedNames = AddNumbers(eventNames); Console.WriteLine("Starting main loop"); foreach (string numberedName in numberedNames) { Console.WriteLine("In main loop: " + numberedName);