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UNDERSTANDING TYPES AND ASSEMBLIES
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This version of the Person class has two display methods, DisplayFirstName and DisplayLastName. It also has a method called ShowAll that displays both the first and last names. The Reflect.Main method retrieves the type of the Person class and uses Activator.CreateInstance(typ, args) to invoke its constructor and pass the required arguments. The arguments are passed as an array of objects. The program then loops through the type s methods using a regular expression to identify and call all methods whose names begin with the string "Display". The regular expression is created by:
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Regex r = new Regex("^display", RegexOptions.IgnoreCase);
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This expression will match any method names beginning with the string "Display" while ignoring case.
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Note that nowhere in our Main method do we declare a Person object. Instead we dynamically retrieve the Person type. Then we use reflection to discover its methods, and we use late binding to create a run time instance of the type and call its methods. When we save the file as reflect.cs, compile it, and run it, we get the result shown in figure 2.18.
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Dynamically creating a Person object using reflection
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We use the static Type.GetType method to retrieve the Person type. We can also search external assemblies by file name. For example, we could have used the following code:
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Assembly asm = Assembly.Load("reflect"); Type typ = asm.GetType("Person");
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The System.Reflection.Emit namespace System.Reflection.Emit is another important namespace in the reflection class library. The classes in this namespace enable IL to be generated and emitted on the fly thereby creating a dynamic assembly at run time. Such features are typically used by compilers, interpreters, script engines, and servers. For example the ASP.NET engine uses this facility to compile and execute code in a Web page when a client requests the page. In the next section, we use the Reflection.Emit classes to build a compiler for a simple programming language.
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TYPES, ASSEMBLIES, AND REFLECTION
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BUILDING A SIMPLE COMPILER
So far, our assemblies have been created for us by the C# compiler. Our final example in this chapter is a little more ambitious. We will use our knowledge of assemblies, types, and IL to create a compiler for a simple arithmetic language, we ll call AL, which adds, subtracts, and displays integers. We use the classes in the System.Reflection.Emit namespace to generate the executable code. These classes provide a fully object-oriented framework for code generation. The ability to generate IL can be useful in the development of certain types of applications such as simple search languages, or rule-based systems, or in the implementation of our own .NET scripting language. We can create a compiler for our simple language in fewer than 200 lines of code. Doing so clarifies the relationship between the language compiler, JIT compiler, and CLR. In particular, we ll see that we don t have to concern ourselves with native code generation, since .NET automatically takes care of that difficult task.
The AL language Listing 2.14 presents a sample AL program.
Listing 2.14 A Sample AL Program
- file : example1.al - description : a sample al program - compile : alc example1.al add 3 add 47 add 390 print add 12 print reset print sub 7 add -4 print add 11 print
- displays 440 - displays 452 - displays 0
- displays -11 - displays 0
Like IL, AL is a simple stack-based language. It consists of just four statements as illustrated in table 2.3. AL allows just one statement per line and the program executes from top to bottom with no variation in control flow. In spite of its simplicity, however, it provides a good vehicle for experimenting with code generation and the System.Reflection.Emit classes.
UNDERSTANDING TYPES AND ASSEMBLIES
Table 2.3
The AL programming language
Statement Description add i sub i reset print Adds integer i to value on top of stack, and leaves the result on the stack in its place Subtracts integer i from value on top of stack, and leaves the result on the stack in its place Sets value on top of stack to zero Writes value on top of stack to the console
In listing 2.14, we use the hyphen (-) to delimit comments. In fact, our compiler will simply ignore lines that don t start with one of the four statements in the language. Also, it will ignore any text after a statement and its argument, if present. So we can insert comments directly after statements on the same line without using the comment delimiter, if we like. The output from the compiler, alc.exe, will be a PE format Windows executable. If the input file is called progname.al, the output will be written to progname.exe. 2.10.2 Translating AL to IL Let s explore how we translate AL to IL. The IL program will need somewhere to store the result of AL s arithmetic operations so we ll use a single local integer variable that we ll initialize to zero at start up. Therefore, we need to emit the following IL to start:
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