Lesson 2: Using Common Reference Types in Visual Basic .NET

Printer QR Code JIS X 0510 in Visual Basic .NET Lesson 2: Using Common Reference Types

finally { tr.Close(); } } catch (System.IO.FileNotFoundException ex) { MessageBox.Show("Sorry, the file does not exist."); } catch (System.UnauthorizedAccessException ex) { MessageBox.Show("Sorry, you lack sufficient privileges."); } catch (Exception ex) { MessageBox.Show(ex.Message); }

5. Run your application again and verify that it displays your new error message when you provide an invalid filename.

Reference types directly contain the address of data rather than the actual data. When you assign an instance of a value type, a second copy of the value is created. When you assign an instance of a reference type, only the pointer is copied. Therefore, if you assign an instance of a reference type and then modify the data, the data referenced by both the original variable and the second variable are changed. The .NET Framework includes a large number of built-in reference types that you can use directly or use to build your own custom types. Strings are immutable; use the StringBuilder class to create a dynamic string. Use streams to read from and write to files, memory, and the network. Use the catch clause within try blocks to filter exceptions by type. Close and dispose of nonmemory resources in the finally clause of a try block.

You can use the following questions to test your knowledge of the information in Lesson 2, Using Common Reference Types. The questions are also available on the companion CD if you prefer to review them in electronic form.

Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.

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1. Which of the following are reference types (Choose all that apply.) A. Types declared Nullable B. String C. Exception D. All types derived from System.Object 2. What is the correct order for catch clauses when handling different exception types A. Most general to most specific B. Most likely to occur to least likely to occur C. Most specific to most general D. Least likely to occur to most likely to occur 3. Of the following scenarios, which would be a good reason to use the StringBuilder class instead of the String class A. When building a string from shorter strings B. When working with text data longer than 256 bytes C. When you want to search and replace the contents of a string D. When a string is a value type 4. Why should you close and dispose of resources in a finally block instead of a catch block A. It keeps you from having to repeat the operation in each catch. B. A finally block runs whether or not an exception occurs. C. The compiler throws an error if resources are not disposed of in the finally block. D. You cannot dispose of resources in a catch block. 5. You create an application with built-in exception handling. For some types of exceptions, you want to add an event to the Event Log that specifies the line of code that initiated the exception. Which Exception property should you use A. Message B. StackTrace C. Source D. Data

6. You pass a value-type variable into a procedure as an argument. The procedure changes the variable; however, when the procedure returns, the variable has not changed. What happened (Choose one.) A. The variable was not initialized before it was passed in. B. Passing a value type into a procedure creates a copy of the data. C. The variable was redeclared within the procedure level. D. The procedure handled the variable as a reference.

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In object-oriented languages, the bulk of the work should be performed within objects. All but the simplest applications require constructing one or more custom classes, each with multiple properties and methods used to perform tasks related to that object. This lesson discusses how to create custom classes.

After this lesson, you will be able to: Describe and use inheritance Describe and use interfaces Describe and use partial classes Create a generic type and use the built-in generic types Respond to and raise events Add attributes to describe assemblies and methods Move a type from one class library to another using type forwarding Estimated lesson time: 40 minutes