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Lab: Safely Performing Conversions
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The following exercises show how to avoid problems with implicit conversions so that your programs function predictably. If you encounter a problem completing an exercise, the completed projects are available on the companion CD in the Code folder.
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Exercise 1: Examine Implicit Conversion
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In this exercise, you will examine conversion to determine which number types allow implicit conversion. 1. Create a new console application in Visual Studio. 2. Declare instances of three value types: Int16, Int32, and double. The following code sample demonstrates this:
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' VB Dim i16 As Int16 = 1 Dim i32 As Int32 = 1 Dim db As Decimal = 1 // C# Int16 i16 = 1; Int32 i32 = 1; double db = 1;
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3. Attempt to assign each variable to all the others, as the following code sample demonstrates.
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1
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Framework Fundamentals
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' VB i16 = i32 i16 = db i32 = i16 i32 = db db = i16 db = i32 // C# i16 = i32; i16 = db; i32 = i16; i32 = db; db = i16; db = i32;
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4. Attempt to build your project. Which implicit conversions did the compiler allow, and why
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Exercise 2: Enable Option Strict (Visual Basic Only)
In this exercise, which is only for developers using Visual Basic, you will modify the compiler s options and then rebuild the project you created in Exercise 1. 1. In Visual Studio, open the project you created in Exercise 1. 2. Click the Project menu, and then click ProjectName Properties. 3. Click the Compile tab. For Implicit Conversion, change the Notification type to Error. 4. Attempt to build your project. Which implicit conversions did the compiler allow, and why
Lesson Summary
The .NET Framework can automatically convert between built-in types. Widening conversions occur implicitly in both Visual Basic and C#. Narrowing conversions require explicit conversion in C#, while Visual Basic allows narrowing conversions by default. Boxing allows any type to be treated as a reference type. You must specifically implement conversion operators to enable conversion in custom types.
Lesson 4: Converting Between Types
Lesson Review
You can use the following questions to test your knowledge of the information in Lesson 4, Converting Between Types. The questions are also available on the companion CD if you prefer to review them in electronic form.
NOTE Answers
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.
1. Why should boxing be avoided (Choose one.) A. It adds overhead. B. Users must have administrative privileges to run the application. C. It makes code less readable. 2. Structures inherit ToString from System.Object. Why would someone override that method within a structure (Choose as many correct answers as apply.) A. To avoid boxing. B. To return something other than the type name. C. The compiler requires structures to override the ToString method. D. To avoid run-time errors caused by invalid string conversions. 3. If there is no valid conversion between two types, what should you do when implementing the IConvertible interface A. Delete the ToType member that performs the conversion. B. Throw an InvalidCastException. C. Throw a new custom exception reporting the error. D. Leave the member body empty. 4. With strict conversions enabled, which of the following would allow an implicit conversion (Choose all that apply.) A. Int16 to Int32 B. Int32 to Int16 C. Int16 to Double D. Double to Int16
1 Review
Review
To further practice and reinforce the skills you learned in this chapter, you can perform the following tasks:
Review the chapter summary. Review the list of key terms introduced in this chapter. Complete the case scenarios. These scenarios set up real-world situations involving the topics of this chapter and ask you to create a solution. Complete the suggested practices. Take a practice test.
Summary
Value types are small variables that store data directly rather than storing a pointer to a second memory location that contains the data. Assignment between value types copies the data from one variable to the other, creating a separate instance of the data. You can make value types nullable using the Nullable generic type, and you can create structures that combine multiple value types. Reference types contain the address of data rather than the actual data. The .NET Framework includes thousands of reference types to perform almost any task you could require. The most commonly used reference type is the String class. Because the String class is immutable, it behaves differently from other reference types: when you copy a string, a unique instance of the data is created. When you copy most reference classes, only the pointer is copied, which means changes made to one instance are also reflected in the other instance. When an unexpected event occurs, the .NET Framework throws an exception. You can handle these exceptions by creating Try/Catch blocks in your code. Classes in .NET Framework languages are custom types that can include value types, reference types, methods, attributes, and properties. To enable consistency between classes, you can use inheritance (where you derive a new class from an existing class) or an interface (where you are required to implement specified interfaces). Generics enable you to create a class or method that works with a variety of types. To enable applications to respond to planned events, you can raise and respond to events.
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