The Daily Parker

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G is for Generics

Blogging A to ZFor day 7 of the Blogging A-to-Z challenge, I'm going totally generic.

A generic in C# allows your code to "defer the specification of one or more types until the class or method is declared and instantiated by client code." In other words, you can declare a class that takes a type to be named later.

Imagine you have a program that represents a house. Your house has rooms, and the rooms have windows, doors, and in some cases, fireplaces. They also have furniture. And sometimes headless corpses. (Don't ask.)

If you were using C# 1.0, you'd need to design structures to hold all of these things, and you'd probably wind up using arrays, like this:

#region Copyright ©2002 Inner Drive Technology

using System.Collections;

#endregion

namespace InnerDrive.DailyParkerAtoZ.WeekOne
{
	public class Window
	{
		public void Close() { }
		public void Open() { }
	}


	public class RoomV1
	{
		public void Add(Window window)
		{
			_windows.Add(window);
		}

		public IEnumerable Windows
		{
			get
			{
				return new ArrayList(_windows);
			}
		}

		private readonly ArrayList _windows = new ArrayList();
	}
}

The only problem is...you don't really know for sure whether the things going into or coming out of the _windows array list are actually windows. That's because the System.Collections.ArrayList class only stores objects, not specific kinds of objects.

Generics allow you to impose type safety on the collection of windows through the use of a type parameter. Now you can do this:

public class Room
{
	public void Add(Window window)
	{
		_windows.Add(window);
	}

	public IEnumerable<Window> Windows => new List<Window>(_windows);

	private readonly List<Window> _windows = new	List<Window>();
}

(Notice also the nifty shorthand on line 8 that eliminates the get method.)

This works because the List<T> class takes any object type for T, leaving it up to the compiler to work out what type it should contain. The prototype for the class looks something like this:

public class List<T>
{
	public void Add(T input) { }
}

The type parameter T can be anything. (You can restrict what T can contain, which I will leave as an exercise for the reader.)

Generics simplify a lot of programming tasks. But just wait until Tuesday when I show you how to make object-oriented designs really fly in C#.

Housekeeping note: The A-to-Z Challenge takes Sundays off. The next post in this series will be on Monday. Enjoy your weekend!

Comments (2) -

  • David Harper

    4/7/2018 2:53:46 PM +00:00 |

    It's worth pointing out that Java introduced generics in Java 5 in 2004, a year before they appeared in C# 2.0.  And C++ had templates much earlier than both of them: my second edition (1991) of Stroustrup has an entire chapter on them.

  • The Daily Parker

    4/8/2018 1:37:01 PM +00:00 |

    David, you're correct in the timing. Until version 4, C# wasn't terribly innovative—but it was really good at absorbing the best parts of other languages. Microsoft is the Borg, you know.

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