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Lesson 3: Constructing Classes
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C. Inheritance derives a type from a base type. D. Interfaces derive a type from a base type. 2. Which of the following are examples of built-in generic types (Choose all that apply.) A. Nullable B. Boolean C. EventHandler D. System.Drawing.Point 3. You are creating a generic class, and you need to dispose of the generic objects. How can you do this A. Call the Object.Dispose method. B. Implement the IDisposable interface. C. Derive the generic class from the IDisposable class. D. Use constraints to require the generic type to implement the IDisposable interface. 4. You ve implemented an event delegate from a class, but when you try to attach an event procedure you get a compiler error that there is no overload that matches the delegate. What happened A. The signature of the event procedure doesn t match that defined by the delegate. B. The event procedure is declared Shared/static, but it should be an instance member instead. C. You mistyped the event procedure name when attaching it to the delegate. D. The class was created in a different language.
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Framework Fundamentals
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Lesson 4: Converting Between Types
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Often, you need to convert between two different types. For example, you might need to determine whether an Integer is greater or less than a Double. You might need to pass a Double to a method that requires an Integer as a parameter. Or you might need to display a number as text. This lesson describes how to convert between types in both Visual Basic and C#. Type conversion is one of the few areas where Visual Basic and C# differ considerably.
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After this lesson, you will be able to:
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Convert between types. Explain boxing and why it should be avoided. Implement conversion operators.
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Estimated lesson time: 20 minutes
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Conversion in Visual Basic and C#
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By default, Visual Basic allows implicit conversions between types, while C# prohibits implicit conversions that lose precision. To turn off implicit conversions in Visual Basic, add Option Strict On to the top of each code file, or (in Visual Studio) select Project, choose Properties, select Compile, and select Option Strict On for the entire project. Both Visual Basic and C# allow implicit conversion if the destination type can accommodate all possible values from the source type. That is called a widening conversion, and it is illustrated by the following example:
' VB Dim i As Integer = 1 Dim d As Double = 1.0001 d = i ' Conversion allowed // C# int i = 1; double d = 1.0001; d = i; // Conversion allowed.
If the range or precision of the source type exceeds that of the destination type, the operation is called a narrowing conversion, which usually requires explicit conversion. Table 1-7 lists the ways to perform explicit conversions.
Lesson 4: Converting Between Types
Table 1-7
Methods for Explicit Conversion
System Type System.Convert
Visual Basic
Converts Between types that implement the System.IConvertible interface.
CType type.ToString, type.Parse type.TryParse, type.TryParseExact CBool, CInt, CStr, etc.
(type) cast operator
Between types that define conversion operators. Between string and base types; throws exception if the conversion is not possible. From string to a base type; returns false if the conversion is not possible. Between base Visual Basic types; compiled inline for better performance. (Visual Basic only.) Between types. DirectCast throws an exception if the types are not related through inheritance or if they do not share a common interface; TryCast returns Nothing in those situations. (Visual Basic only.)
DirectCast, TryCast
NOTE .NET 2.0
TryParse, TryParseExact, and TryCast are new in .NET 2.0. Previously, you had to attempt a parsing or conversion and then catch the exception if it failed.
Narrowing conversions fail if the source value exceeds the destination type s range or if a conversion between the types is not defined, so you should enclose a narrowing conversion in Try blocks or use TryCast or TryParse and check the return value.
1
Framework Fundamentals
What Is Boxing and Unboxing
Boxing converts a value type to a reference type, and unboxing converts a reference type to a value type. The following example demonstrates boxing by converting an Integer (a value type) to an Object (a reference type):
' VB Dim i As Integer = 123 Dim o As Object = CType(i, Object) // C# int i = 123; object o = (object) i;
Unboxing occurs if you assign a reference object to a value type. The following example demonstrates unboxing:
' VB Dim o As Object = 123 Dim i As Integer = CType(o, Integer) // C# object o = 123; int i = (int) o;
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