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into a restaurant without going through the front door. Once the entry point is identified, a thread is created and associated with the process. The thread is started, executing the code located at the entry point. From that point on the thread follows the series of instructions. This first thread is referred to as the main thread of the process. Listing 1.1 contains the listing of a console application that satisfies the obligatory Hello World example.
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Listing 1.1 An example of a single-threaded application (VB.NET)
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Module ModuleHelloWorld Sub Main() Console.Write("Hello") Console.Write(" World") End Sub End Module
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As a console application, all input and output pass through the command-line environment. Visual Basic console applications utilize the concept of a module. A module is a Visual Basic construct that is identical in functionality to a C# class having all static members. This means that the method can be invoked without an instance of the class having been created. I ve found it very beneficial, when dealing with .NET, to examine the Microsoft Intermediate Language (MSIL) the compiler produces. MSIL is an assembly-like language produced by compilers targeting the .NET environment. MSIL is translated to machine instructions by the runtime. MSIL is similar to Java s bytecode. Listing 1.2 contains the MSIL that corresponds to the Main subroutine in listing 1.1.
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Listing 1.2 The MSIL produced by the Hello World example (MSIL)
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.method public static void Main() cil managed { .entrypoint .custom instance void [mscorlib]System.STAThreadAttribute::.ctor() = ( 01 00 00 00 ) // Code size 25 (0x19) .maxstack 8 IL_0000: nop IL_0001: ldstr "Hello" IL_0006: call void [mscorlib]System.Console::Write(string) IL_000b: nop IL_000c: ldstr " World" IL_0011: call void [mscorlib]System.Console::Write(string) IL_0016: nop IL_0017: nop IL_0018: ret } // end of method ModuleHelloWorld::Main
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Notice the static keyword. This lets the runtime know that this is a static method. Since the method is defined within a module, it is implicitly shared/static. Console applications require a static method be the entry point. A common approach is to have the console application contain a static Main that creates an instance of a class and invokes a method on that instance. The .entrypoint directive indicates that this method is the entry point for the application. This tells the framework that this method should be invoked after the assembly is loaded into memory. This example contains a single thread of execution that starts by entering the Main method and terminates when the ret, return, instruction executes. In this example the thread does not contain branching or looping. This makes it easy to see the path the thread will take. Let s examine listing 1.1 in detail. Figure 1.1 shows the path the main thread of the process takes.
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Figure 1.1 The execution path the main thread in the Hello World example follows
The arrows show the path the thread takes during execution of the Hello World program. We re covering this in such depth because, when doing multithreaded development, it is critical to understand the execution path that a thread follows. When there is more than one path, the complexity increases. Each conditional statement introduces another possible path through the program. When there are a large number of paths, management can become extremely difficult. When the path a thread takes contains branching and looping, following that path often becomes more difficult. As a review, branching occurs when a conditional instruction is encountered. Looping is accomplished by having a branching statement target an instruction that has previously been executed. Listing 1.3 contains a slightly more complex version of the Hello World example.
Listing 1.3 Hello World with a loop (C#)
using System; namespace HelloWorldAgain { class ClassHelloWorldAgain { [STAThread] static void Main(string[] args) {
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for (int i=0;i<2;i++) { Console.Write("Hello"); Console.Write(" World"); } } } }
It s easier to annotate the execution path by using the MSIL. Figure 1.2 contains the generated MSIL from listing 1.3 with numbered arrows indicating execution path. This example demonstrates that code that is relatively simple can produce an execution path that is somewhat complex. The interesting part of this example is the jump that occurs at step 4. The reason for this jump is that the for loop tests to see if the test condition is true before the loop executes. The important thing to take away from this is that the main thread will execute steps 1 through 10. Those steps are the path the thread will take through the code.
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