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Reading Books: Another Implementation
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In the previous chapter, you set the Text and Title properties after you initialized the class. In this chapter, you ll create two different constructors for initializing the class fields. The first, a default constructor, creates an instance exactly the way one was created in 1. The second constructor initializes the Title and Text properties. Also, instead of creating a Library class, you ll store the books in a simple array. You ll use the array as the data source of the ListBox control. Creating Constructors in the Book Class This exercise builds on the exercise in 1. You create a new project but bring in the form and Book class you created in 1. You then modify the Book class by adding two constructors and a ToString method. Create the project To create this project, follow this procedure: Create a new Microsoft Windows application project, naming it ReadMoreBooks. 1. In the Solution Explorer, right-click Form1, and click Delete on the shortcut menu. Click OK to confirm the deletion. 2. In the Solution Explorer, right-click ReadMoreBooks, point to Add, and then click Add Existing Item on the shortcut menu. 3. In the Add Existing Item dialog box, navigate to the Form1.vb or Form1.cs file from 1, click it, and click Open. A new copy of Form1 is added to the ReadMoreBooks project. 4. In the Solution Explorer, right-click ReadMoreBooks, point to Add, and then click Add Existing Item on the shortcut menu. 5. In the Add Existing Item dialog box, navigate to the Book file from 1, click it, and then click Open. A new copy of the Book class is added to the ReadMoreBooks project. Convert the text from a field to a property By creating a constructor, you will be able to determine whether the Text field or Title field of your Book class is blank. To further extend the validity of the text, convert it to a property. (You won t see many public fields in the remainder of this book.) 1. In the Solution Explorer, double-click Book.vb or Book.cs, depending on the language you re using, to open the file in the code editor. 2. Modify the Text field so that it s a private field. Remember that by convention private fields have the m_ prefix. Private m_text As String = "" Visual Basic private string m_text = ""; // Visual C# 3. Add a Text property definition to the class. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. } Visual Basic Public Property Text() As String Get Return m_text End Get Set(ByVal Value As String) m_text = Value End SetEnd Property // Visual C#public string Text { get { return m_text; } set { m_text = value; }
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Formatting C# Code C# uses the semicolon to delimit statements, which gives you more control over formatting your code than Visual Basic offers. In the case of the preceding C# code, a more compact format for a property definition is shown. Never sacrifice readability for compactness.
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Add a constructor without parameters 1. If you re using Visual C#, you should rename the namespace for Book.cs and Form1.cs to ReadMoreBooks. This step will keep all your class declarations in the same project namespace. It will make declaring class variables easier. Change the namespace declaration at the top of the file so that it looks like this: 2. // Visual C# only 3. // Change in Book.cs and Form1.cs namespace ReadMoreBooks 4. If needed, in Visual Basic, click Book (ReadMoreBooks) in the Class Name list. In Visual C#, click ReadMoreBooks.Book in the Class Name list. 5. In the Method Name drop-down list, click New if you re using Visual Basic. Click Book if you re using Visual C#. The following code is added to the Book class: 6. 7. 8. 9. 10. 11. 12. } Your next task is to create a constructor that has parameters. You can create multiple constructors in one class, as long as they can be distinguished by their parameter lists. Create a constructor with parameters When creating constructors with parameters, remember that any code in a constructor is executed after the field initializations. You can therefore override any initializations of the fields. 1. Add the following code after the field declarations to declare a constructor with two parameters: title and text. Don t modify the exising constructor created in the preceding section. You re creating a second constructor. 2. Visual Basic 3. Public Sub New(ByVal title As String, ByVal text As String) 4. End Sub 5. // Visual C# 6. public Book(string title, string text) { Public Sub New() End Sub public Book() { // // TODO: Add constructor logic here // // Visual C# Visual Basic
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Add the following code to your new constructor to ensure that neither the title nor the text is blank. If either is blank, raise an exception. An exception stops execution of the program and prevents the class from being instantiated. 8. 9. 11. 12. 14. 15. // Visual C# if ((title != "") && (text != "") && (title != null) && (text != null)) { 16. 17. 18. m_title = title; m_text = text;}else { throw new System.Exception("Title or text is an empty string."); Visual Basic If (title <> "") And (text <> "") And _ m_title = title m_text = text Throw New Exception("Title or text is an empty string.") End If
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