Action

Our library is now ready; all we need to do is add code. One last task before we do this is to make certain we can use our library from within the MyPhotos application project. For this to work, the MyPhotos project must include a reference to the MyPhotoAlbum class. This corresponds to the /reference switch on the C# compiler (csc.exe) that we saw in chapter 1, and is a bit like linking a library into your program in C++. Since there are no header files in C#, a reference is all we need to start using classes from the library in our project.

REFERENCE MYPHOTOALBUM FROM THE MYPHOTOS PROJECT

REFERENCE MYPHOTOALBUM FROM THE MYPHOTOS PROJECT (continued)

It is important to realize that our new reference refers to the assembly produced by the MyPhotoAlbum project, and not the project itself. Visual Studio automatically uses the correct path when compiling the MyPhotos project to pick up the most recent MyPhotoAlbum library from the corresponding project.

If you are not using Visual Studio .NET to build your program, you will need to establish the correct library location manually. The command-line tools discussed in chapter 1 are used for this purpose. The next section provides a short discussion on this topic.

5.2.2USING THE COMMAND-LINE TOOLS

As we saw in chapter 1, you can build Windows Forms applications without using Visual Studio .NET. The interactive environment makes a number of tasks easier, but also uses memory and other system resources. On a computer with limited resources, this can present some problems. If you have a favorite editor and are comfortable working with makefiles, you can create the examples in this book without using Visual Studio .NET.

To create a class library such as MyPhotoAlbum.dll, create a MyPhotoAlbum directory for the library and place the required source files in it. In this case you would create a PhotoAlbum.cs file to hold the PhotoAlbum class source code, and create other files as required. You can create an AssemblyInfo.cs file as well, or simply include the version number and other assembly information at the top of your file as we did in chapter 1. The C# compiler (csc.exe) discussed in chapter 1 is used to produce both executables and libraries. The /target switch specifies the type of output file to produce.

C# compiler output options (/target switch)

The /out switch can be used to specify the output file name. Both /out and /target must appear before any source file names.

For example, the following line will create a library assembly called MyPhotoAlbum.dll using a single source file PhotoAlbum.cs.

>csc /target:library /out:MyPhotoAlbum.dll PhotoAlbum.cs /r:System.dll

To use this library with your MyPhotos application, you will need to include a /r reference when compiling the application. For example, if your library was in a directory called C:\MyProjects\MyPhotoAlbum, then you would use the following switch when compiling the MyPhotos application:

/r:C:\MyProjects\MyPhotoAlbum

5.2.3CREATING THE PHOTOALBUM CLASS

No matter how you compile your library, we are now ready to implement the PhotoAlbum class. These next two sections take us through the initial implementation of this and the Photograph class. If you find typing all this code a bit tedious (or are a really bad typist!), don’t be afraid to download the final code from the book’s web site and simply read the accompanying text. For the rest of us, let’s forge ahead.

IMPLEMENT PHOTOALBUM CLASS (continued)

You may notice here that the MyPhotoAlbum project does not compile. Try to do so and the compiler returns an error something like the following:

Error The type or namespace name 'CollectionBase' could not be

found (are you missing a using directive or an assembly ref-

erence?)

This is because CollectionBase is part of the System.Collections namespace. It turns out this namespace is part of the system library, so there is no need for another reference in our project. We could fix the error by declaring the class as follows:

public PhotoAlbum : System.Collections.CollectionBase

{

. . .

Since we may use other objects or names from the System.Collections namespace, we will instead simply indicate that our class will use this namespace at the top of the file.

USE SYSTEM.COLLECTIONS NAMESPACE

Now the project should compile with no errors. Before we implement any members for this class, let’s also take a look at the Photograph class.