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INTRODUCTION
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Understanding types and assemblies
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2.1 2.2 2.3 2.4 2.5 2.6 Introducing types 16 Value vs. reference types 18 Exploring System.Object 21 Understanding finalization 23 Introducing assemblies 26 Private vs. shared assemblies 29 2.7 Downloading assemblies 35 2.8 Programming in IL 37 2.9 Types, assemblies, and reflection 41 2.10 Building a simple compiler 44 2.11 Summary 54
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Creating a .NET application involves coding types, and packaging them into assemblies. .NET types are similar to data types in non-object-oriented languages except that they contain both data, in the form of fields, and behavior in the form of methods. .NET types are also language-neutral. An assembly containing types coded in one language can be used by an application coded in a different language. So types and assemblies are the basic building blocks and they are important concepts for .NET developers. In this chapter, we explore .NET types and assemblies. .NET provides both value and reference types and uses an elegant mechanism called boxing to convert between the two. Value types provide a lightweight, stack-based means of creating runtime objects, thus avoiding the overhead of garbage collection. As we ll see, most of the primitive .NET types are value types. 15
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We examine both private and shared assemblies. Private assemblies enable developers to ship DLLs while avoiding the potential problems associated with their public accessibility and versioning. A shared assembly, on the other hand, is shared by multiple applications. We also develop a small sample program that downloads and installs an assembly on demand. The .NET Framework provides reflection classes to enable applications to inspect assemblies and to discover and instantiate types at run time using late binding. The System.Reflection.Emit namespace provides classes that can be used to dynamically generate new types and assemblies at run time. We use these classes for our final example in this chapter when we develop a complete compiler for a simple programming language. If you are new to C#, now would be a good time to refer to appendix A. There, you ll find an introduction to the language that should equip you with the skills to work through the examples in this chapter, and in the remainder of the book.
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INTRODUCING TYPES
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Every object in a .NET program is an instance of a type that describes the structure of the object and the operations it supports. Table 2.1 lists the built-in types available in C#, Visual Basic .NET, and IL, and their relationship to the underlying .NET types.
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Table 2.1 .NET built-in types NET System.Boolean System.Byte System.Char System.DateTime System.Decimal System.Double System.Int16 System.Int32 System.Int64 System.Object System.SByte System.Single System.String System.UInt16 System.UInt32 System.UInt64 C# bool byte char decimal double short int long object sbyte float string ushort uint ulong VB.NET Boolean Byte Char Date Decimal Double Short Integer Long Object Single String IL bool unsigned int8 char float64 int16 int32 int64 object int8 float32 string unsigned int16 unsigned int32 unsigned int64 Value or Reference Value Value Value Value Value Value Value Value Value Reference Value Value Reference Value Value Value
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From the table, we can see that the built-in language types are just aliases for underlying .NET types. For example, .NET s System.Single, which represents a 32-bit
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UNDERSTANDING TYPES AND ASSEMBLIES
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floating-point number, is known as float in C#, Single in Visual Basic .NET, and float32 in IL. This cross-language type system is known as Common Type System (CTS), and it extends beyond the primitive types. The CTS defines how types are declared, used, and managed in the runtime, and it establishes the basis for crosslanguage integration. In fact, it provides an object-oriented model that supports the implementation of many modern programming languages, and it defines rules to ensure that objects written in different languages can interoperate. As we see in table 2.1, not all primitive types are directly supported in every language. Visual Basic .NET provides a built-in Date type that is not provided by C#. Likewise, C# s unsigned integer types are not supported by Visual Basic .NET, and IL does not have a decimal type. In such cases, you can always use the underlying .NET type directly:
// C# date... System.DateTime d = System.DateTime.Now; ' VB unsigned integer... Dim u As System.UInt32 // IL decimal... .locals (valuetype [mscorlib]System.Decimal d)
The CTS forms the foundation for .NET s cross-language abilities. Since new types are defined in terms of the built-in .NET types, mixed language application development presents no special difficulty for .NET developers. However, since not all types will be available in all languages, the Common Language Specification (CLS) specifies a subset which should be used when developing libraries for cross-language use. Such libraries are termed CLS-compliant. For example, System.SByte, System.UInt16, System.UInt32, and System.UInt64, are not CLS-compliant nor are they available as primitive types in Visual Basic .NET. Generally, non-CLS-compliant types may not be directly supported by some languages and should not be exposed as public members of programmerdefined types in libraries designed for cross-language use. If you annotate your code with the CLSCompliant attribute, the C# compiler will warn you about noncompliance. (See appendix A for a discussion of attributes.) Table 2.1 also includes the IL names of the .NET types. You can program directly in IL and, later in this chapter, we ll develop a skeleton program that can be used as a starter template for your own IL programs. The low-level nature of IL makes it an unsuitable choice for general-purpose application development. However, if you intend to use the System.Reflection.Emit classes to generate your own assemblies, an understanding of IL will be essential. We can explicitly create a new type by defining a class, as in listing 2.1.
Listing 2.1 A Person class in C#
// file : person.cs public class Person { public Person(string firstName, string lastName, int age) {
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