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a list of strings. These are two types in their own right, built by passing different type arguments to List<T>. Plugging in type arguments to form a new type is called instantiating the generic type. Generics were added in C# 2.0 mainly to support collection classes such as List<T>. Before this, we had to use the ArrayList class (which you should no longer use; it s not present in Silverlight, and may eventually be deprecated in the full .NET Framework). ArrayList was also a resizable array, but it represented all items as object. This meant it could hold anything, but every time you read an element, you were obliged to cast to the type you were expecting, which was messy. With generics, we can write code that has one or more placeholder type names the T in List<T>, for example. We call these type parameters. (The distinction between parameters and arguments is the same here as it is for methods: a parameter is a named placeholder, whereas an argument is a specific value or type provided for that parameter at the point at which you use the code.) So you can write code like this:

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public class Wrapper<T> { public Wrapper(T v) { Value = v; } public T Value { get; private set; } }

This code doesn t need to know what type T is and in fact T can be any type. If we want a wrapper for an int, we can write Wrapper<int>, and that generates a class exactly like the example, except with the T replaced by int throughout. Some classes take multiple type parameters. Dictionary collections (which are described in 9) require both a key and a value type, so you would specify, say, Dictionary<string, MyClass>. An instantiated generic type is a type in its own right, so you can use one as an argument for another generic type, for example, Diction ary<string, List<int>>. You can also specify a type parameter list for a method. For example, .NET defines an extension method for all collections called OfType<TResult>. If you have a List<object> that happens to contain a mixture of different kinds of objects, you can retrieve just the items that are of type string by calling myList.OfType<string>().

You may be wondering why .NET offers arrays when List<T> appears to be more useful. The answer is that it wouldn t be possible for List<T> to exist if there were no arrays: List<T> uses an array internally to hold its elements. As you add elements, it allocates new, larger arrays as necessary, copying the old contents over. It employs various tricks to minimize how often it needs to do this.

To try out the hash function implemented in Listing 1-23, create a hash list and put a couple of items in it before trying to look up both existing and non-existing items. This is shown in Listing 1-24. The comment after each qDebug line shows the expected result. Listing 1-24. Hashing the Person class QHash<Person, int> hash; hash[ Person( "Anders", "8447070" ) ] = 10; hash[ Person( "Micke", "7728433" ) ] = 20; qDebug() qDebug() qDebug() qDebug() << << << << hash.value( hash.value( hash.value( hash.value( Person( Person( Person( Person( "Anders", "8447070" ) ); // 10 "Anders", "8447071" ) ); // 0 "Micke", "7728433" ) ); // 20 "Michael", "7728433" ) ); // 0

multiple pieces of information, as programs often do, it s very common to need some flexibility around the amount of information fixed-size lists are the exception rather than the rule. (An individual s calendar tends to change over time, for example.) So have we just wasted your time with the first half of this chapter Not at all not only do arrays crop up a lot in APIs, but List<T> collections are very similar in use to arrays. We could migrate most of the examples seen so far in this chapter from arrays to lists. Returning to our earlier, nonrectangular example, we would need to modify only the first line of Example 7-10, which creates an array of CalendarEvent objects. That line currently reads:

It is followed by the list of objects to add to the array, contained within a pair of braces. If you change that line to this:

the initializer list can remain the same. Notice that besides changing the variable declaration to use the List<T> type (with the generic type argument T set to the element type CalendarEvent, of course) we also need an explicit call to the constructor. (Normally, you d expect parentheses after the type name when invoking a constructor, but those are optional when using an initializer list.) As you saw earlier, the use of new is optional when assigning a value to a newly declared array, but C# does not extend that courtesy to other collection types. While we can initialize the list in much the same way as we would an array, the difference is that we are free to add and remove elements later. To add a new element, we can use the Add method:

Returns a string containing the column name Returns a Sys.Type object containing the data type Returns the default value for this column