5.2.4CREATING THE PHOTOGRAPH CLASS

The Photograph class represents a photograph stored in a file. Earlier, we laid out this class as follows.

public class Photograph

{

//Create a new instance from a file name.

//Properties:

//- get the file name for the Photograph

//- get the Bitmap for the Photograph

//Methods:

//- see if two Photographs are equal

}

While we could implement this class within the PhotoAlbum.cs file, it makes more sense to separate these two classes into two separate files. In this section we create this new class file and add some initial properties for the class. The following steps create our Photograph.cs source file.

		ADD A PHOTOGRAPH CLASS FILE
	Action		Result
1	Open the dialog to add a		The Add New Item dialog opens with the Class template
	new class for the		selected.
	MyPhotoAlbum project.
	How-to
	a. In Solution Explorer, click
	the MyPhotoAlbum
	project.
	b. Open the Project menu.
	c. Select Add Class….
	Alternately
	Right-click on the
	MyPhotoAlbum project and
	select Add Class… from
	the Add submenu.
2	Create the new		A Photograph.cs file is added to the MyPhotoAlbum project,
	“Photograph.cs” file.		and the default code displays in the main window.
	How-to
	Enter the appropriate name
	and click the Open button.

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141

		ADD A PHOTOGRAPH CLASS FILE (continued)
	Action		Result
3	Add some class		. . .
	documentation.		/// <summary>
			/// The Photograph class represents a single
			/// photo and its properties.
			/// </summary>
			public class Photograph
			. . .

Once again, let’s modify the namespace to be Manning.MyPhotoAlbum.

		MODIFY THE NAMESPACE
	Action	Result
4	Modify the namespace to be	. . .
	Manning.MyPhotoAlbum.	namespace Manning
		{
		namespace MyPhotoAlbum
		{
		. . .
		}
		}

We now have a fully functional class as part of our library. Of course, it doesn’t do anything yet. Let’s start by tracking the file name and bitmap for the photograph.

DEFINE THE FILE AND BITMAP PROPERTIES

	Action	Result
5	Create private member	public class Photograph
	variables to track the file	{
	name and any Bitmap	private string _fileName;
		private Bitmap _bitmap;
	object.
		Note: Here and elsewhere in the book, we indicate that a
		variable is private and not available outside of the con-
		taining class by prefixing it with an underscore.
6	Create a constructor to	public Photograph(string fileName)
	initialize these members	{
	from a given file name.	_fileName = fileName;
		_bitmap = null;
		}
		Note: We allow a Photograph to be created with an
		invalid file name.
7	Create a FileName	public string FileName
	property to return the	{
	current file name.	get { return _fileName; }
		}

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SetFile-

DEFINE THE FILE AND BITMAP PROPERTIES (continued)

	Action	Result
8	Create an Image property	public Bitmap Image
	to return the corresponding	{
	Bitmap object.	get
		{
	Note: We intentionally	if (_bitmap == null)
	ignore any error here. We	{
		_bitmap = new Bitmap(_fileName);
	will fix this later in the
		}
	chapter.	return _bitmap;
		}
		}

This is the first time we’ve created our own properties, so it is worth a short discussion. A property in C# is created much like a method. You define an access level, a type, and a name for the property. By convention, property and method names begin with a capital letter. The lack of parentheses after the name informs the compiler that this is a property and not a method.

Inside the braces, the access methods for the property are defined. The access methods provide read access, via the get keyword, or write access, via the set keyword. The get access method must return the defined type, while the set access method uses the reserved word value to access the value provided on the right side of the equals sign ‘=’. For example, if we wanted users of our Photograph class to set the FileName property, we could code this as follows:

public string FileName

{

get { return _fileName; }

set { _fileName = value; } // example only, not in our code

}

Of course, in an actual implementation it might be good to verify that the value provided to the set call is a real file and does indeed represent a photograph. For our purposes, the Photograph class is tied to a specific file name, so we do not provide a set implementation here. In this case the FileName property is said to be read-only, since the value can be read but not written.

Practically, properties permit safe access to a class without the need to expose internal variables or other features. To duplicate the get and set functionality for a file name member in C++, programmers typically provide methods such as

Name and GetFileName for this purpose. Properties formalize this concept for C# so that all programs use a standard mechanism for this style access.

Since properties are invoked similar to methods, additional calculations can be performed as part of their definition. In the code for the Image property, for example, the Bitmap is created as required before returning it to the user.

public Bitmap Image

{

get

CLASS LIBRARIES

143

{

if (_bitmap == null)

{

_bitmap = new Bitmap(_fileName);

}

return _bitmap;

}

}

Astute readers will note here that the given file may or may not exist and may or may not be an actual image file. We will handle any exception that occurs as a result of such an error in a moment.

We now have enough to link our classes into the main application. One problem remains: the MyPhotoAlbum project will once again not compile. Now the error is something like this:

Error The type or namespace name 'Bitmap' could not be found (are

you missing a using directive or an assembly reference?)

This is because Bitmap is part of the System.Drawing namespace, which is referenced by our MyPhotos project, but not the MyPhotoAlbum project. Unlike System.Collections, this namespace is provided in a separate library, namely the System.Drawing.dll library. We need to reference this DLL and then use it in our class.

ADD SYSTEM.DRAWING REFERENCE

	Action	Result
9	Display the Add Reference
	dialog for the
	MyPhotoAlbum project.
10	Add System.Drawing.dll as	The System.Drawing assembly appears in the References list
	a reference.	for the MyPhotoAlbum project.
	How-to
	a. Click the .NET tab.
	b. Locate and click the Sys-
	tem.Drawing.dll item
	from the list.
	c. Click the Select button.
	d. Click the OK button.
11	Add a using directive for	The Photograph.cs file now contains two using directives:
	the System.Drawing	using System;
	namespace at the top of
		using System.Drawing;
	the file.

144	CHAPTER 5 REUSABLE LIBRARIES