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The if statement had to be added because the main function might start to shut down while the thread was blocking on the lock call If it were not there, the blocked thread would output its text one time too many before realizing that stopThreads is true Listing 12-5 The new run method with a mutex for ordering QMutex mutex; void TextThread::run() { while( !stopThreads ) { mutexlock(); if( stopThreads ){ mutexunlock(); return; } qDebug() << m_text; sleep( 1 ); mutexunlock(); } } Running this example again, you ll see that "Foo" or "Bar" are printed once every second and always in the same order This halves the pace of the original application, in which both "Foo" and "Bar" were printed every second Which text is printed first is not guaranteed bar could initialize quicker than foo even if start is called first for foo.

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As with arrays, a List<T> will throw an IndexOutOfRangeException if you use too high an index, or a negative index. This applies for writes as well as reads a List<T> will not automatically grow if you write to an index that does not yet exist.

There is a subtle difference between array element access and list element access that can cause problems with custom value types (structs). You may recall that 3 warned that when writing a custom value type, it s best to make it immutable if you plan to use it in a collection. To understand why, you need to know how List<T> makes the square bracket syntax for element access work.

Arrays are an integral part of the .NET type system, so C# knows exactly what to do when you access an array element using the square bracket syntax. However, as List<T> demonstrates, it s also possible to use this same syntax with some objects that are not arrays. For this to work, the object s type needs to help C# out by defining the behavior for this syntax. This takes the form of a slightly unusual-looking property, as shown in Example 7-24.

The order is not guaranteed, either By increasing the workload of the system executing the threads or shortening the.

class Indexable { public string this[int index] { get { return "Item " + index; } set { Console.WriteLine("You set item " + index + " to " + value); } } }

This control allows you to query the specifics of a particular row. It supports three properties (as shown in Table 8-10). Table 8-10. DataRow Properties

This has the get and set parts we d expect in a normal property, but the definition line is a little unusual: it starts with the accessibility and type as normal, but where we d expect to see the property name we instead have this[int index]. The this keyword signifies that this property won t be accessed by any name. It is followed by a parameter list enclosed in square brackets, signifying that this is an indexer property, defining what should happen if we use the square bracket element access syntax with objects of this type. For example, look at the code in Example 7-25.

Indexable ix = new Indexable(); Console.WriteLine(ix[10]); ix[42] = "Xyzzy";

After constructing the object, the next line uses the same element access syntax you d use to read an element from an array. But this is not an array, so the C# compiler will look for a property of the kind shown in Example 7-24. If you try this on a type that doesn t provide an indexer, you ll get a compiler error, but since this type has one, that ix[10] expression ends up calling the indexer s get accessor. Similarly, the third line has the element access syntax on the lefthand side of an assignment, so C# will use the indexer s set accessor.

sleep time, the order can change. It works because the thread unlocking the mutex needs less than one second to reach the lock call and block.

If you want to support the multidimensional rectangular array style of index (e.g., ix[10, 20]), you can specify multiple parameters between the square brackets in your indexer. Note that the List<T> class does not do this while it covers most of the same ground as the built-in array types, it does not offer rectangular multidimensional behavior. You re free to create a jagged list of lists, though. For example, List<List<int>> is a list of lists of integers, and is similar in use to an int[][].

The indexer in Example 7-24 doesn t really contain any elements at all it just makes up a value in the get, and prints out the value passed into set without storing it anywhere. So if you run this code, you ll see this output:

It may seem a bit odd to provide array-like syntax but to discard whatever values are written, but this is allowed there s no rule that says that indexers are required to behave in an array-like fashion. In practice, most do the reason C# supports indexers is to make it possible to write classes such as List<T> that feel like arrays without necessarily having to be arrays. So while Example 7-24 illustrates that you re free to do whatever you like in a custom indexer, it s not a paragon of good coding style. What does any of this have to do with value types and immutability, though Look at Example 7-26. It has a public field with an array and also an indexer that provides access to the array.