GENERICS in Visual C#.NET

Drawer Data Matrix ECC200 in Visual C#.NET GENERICS

Enumerators and Enumerable Types Using the IEnumerator Interface The IEnumerable Interface The Non-Interface Enumerator The Generic Enumeration Interfaces The IEnumerator<T> Interface The IEnumerable<T> Interface Iterators Common Iterator Patterns Producing Enumerables and Enumerators Producing Multiple Enumerables Producing Multiple Enumerators Behind the Scenes with Iterators

In 14, you saw that you can use a foreach statement to cycle through the elements of an array. In this chapter, you ll take a closer look at arrays and see why they can be processed by foreach statements. You ll also look at how you can add this capability to your own userdefined classes. Later in the chapter, I ll discuss the use of iterators.

When you use the foreach statement with an array, the statement presents you with each element in the array, one by one, allowing you to read its value. For example, the following code declares an array with four elements, and then uses a foreach loop to print out the values of the items: int[] arr1 = { 10, 11, 12, 13 }; foreach (int item in arr1) Console.WriteLine("Item value: // Define the array. // Enumerate the elements. {0}", item);

This code produces the following output: Item Item Item Item value: value: value: value: 10 11 12 13

Why does this work, apparently magically, with arrays The reason is that an array can produce, upon request, an object called an enumerator. The enumerator can return the elements of the array, one by one, in order, as they are requested. The enumerator knows the order of the items, and keeps track of where it is in the sequence. It then returns the current item when it is requested. For types that have enumerators, there must be a way of retrieving them. The standard way of retrieving an object s enumerator in .NET is to call the object s GetEnumerator method. Types that implement a GetEnumerator method are called enumerable types, or just enumerables. Arrays are enumerables. Figure 20-1 illustrates the relationship between enumerables and enumerators.

Figure 20-1. Overview of enumerators and enumerables The foreach construct is designed to work with enumerables. As long as the object it is given to iterate over is an enumerable type, such as an array, it will perform the following actions: Getting the object s enumerator by calling the GetEnumerator method Requesting each item from the enumerator and making it available to your code as the iteration variable, which your code can read, but not change Must be enumerable foreach( Type VarName in EnumerableObject ) { ... }

There are three variations on enumerators. They all work essentially the same way, with only slight differences. I will discuss all three types. You can implement enumerators using The IEnumerator/IEnumerable interfaces called the non-generic interface form The IEnumerator<T>/IEnumerable<T> interfaces called the generic interface form The form that uses no interfaces

This section will start by looking at the first in the preceding list: the non-generic interface form. This form of enumerator is a class that implements the IEnumerator interface. It is called non-generic because it does not use C# generics. The IEnumerator interface contains three function members: Current, MoveNext, and Reset. Current is a property that returns the item at the current position in the sequence. It is a read-only property. It returns a reference of type object, so an object of any type can be returned. MoveNext is a method that advances the enumerator s position to the next item in the collection. It also returns a Boolean value, indicating whether the new position is a valid position or is beyond the end of the sequence. If the new position is valid, the method returns true. If the new position is not valid (i.e., it s at the end), the method returns false. The initial position of the enumerator is before the first item in the sequence. MoveNext must be called before the first access of Current, or the CLR will raise an InvalidOperationException exception. Reset is a method that resets the position to the initial state. Figure 20-2 illustrates a collection of three items, which is shown on the left of the figure, and its enumerator, which is shown on the right. In the figure, the enumerator is an instance of a class called ArrEnumerator.