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ADD A PHOTOGRAPH CLASS FILE (continued) Action 3 Add some class documentation. Result
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. . . /// <summary> /// The Photograph class represents a single /// photo and its properties. /// </summary> public class Photograph . . .
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Once again, let s modify the namespace to be Manning.MyPhotoAlbum.
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MODIFY THE NAMESPACE Action 4 Modify the namespace to be
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Manning.MyPhotoAlbum.
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. . . namespace Manning { namespace MyPhotoAlbum { . . . } }
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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.
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DEFINE THE FILE AND BITMAP PROPERTIES Action 5 Create private member variables to track the file name and any Bitmap object. Result
public class Photograph { private string _fileName; private Bitmap _bitmap;
Note: Here and elsewhere in the book, we indicate that a variable is private and not available outside of the containing class by prefixing it with an underscore. 6 Create a constructor to initialize these members from a given file name.
public Photograph(string fileName) { _fileName = fileName; _bitmap = null; }
Note: We allow a Photograph to be created with an invalid file name. 7 Create a FileName property to return the current file name.
public string FileName { get { return _fileName; } }
CHA PTE R 5
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DEFINE THE FILE AND BITMAP PROPERTIES (continued) Action 8 Create an Image property to return the corresponding Bitmap object. Note: We intentionally ignore any error here. We will fix this later in the chapter. Result
public Bitmap Image { get { if (_bitmap == null) { _bitmap = new Bitmap(_fileName); } 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 SetFileName 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
{ 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 9 Display the Add Reference dialog for the MyPhotoAlbum project. Add System.Drawing.dll as a reference.
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