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Figure 6-12. Constructor declaration For example, the following class uses its constructor to initialize its fields. In this case, it has a field called TimeOfInstantiation that is initialized with the current date and time. class MyClass { DateTime TimeOfInstantiation; ... public MyClass() { TimeOfInstantiation = DateTime.Now; } ... }

names you like for your namespaces there s no requirement that the namespace name match the program name.

The C# compiler will even let you put your own code into namespaces whose names begin with System, but you should not do this (at least, not unless you work for Microsoft and are adding types to some future version of .NET s class library). You re likely to cause confusion if you break the convention that System namespaces contain .NET Framework types.

Notice that the namespace is followed by an open brace ({). C# uses braces to denote containment here, everything inside these braces will be in our HelloWorld namespace. Since namespaces contain types, it should come as no great surprise that the next line in the file defines a type. Specifically, it defines a class. The .NET Framework class library isn t the only thing that gets to define classes in fact, if you want to write any code at all in C# you must provide a type to contain that code. Some languages (such as C++) do not impose this constraint, but C# is an objectoriented (OO) programming language. We ll explore OO concepts in the next chapter, but the main impact on our Hello, world example is that every bit of C# code must have a type that it calls home. There are a few different ways to define types in C#, which we ll get to in the next few chapters, but for the present simple example, the distinctions are not yet relevant. So we use the most common, a class:

class Program { ... }

Note Having finished the section on static properties, take a closer look at the line that initializes

One of the advantages of JSON is that it s easy to parse. Many JSON parsers, written for numerous languages, have been developed to automate the process of generating and parsing JSON. (A list is available at the official JSON site, http:// json.org.) In JavaScript, the parsing process is immediate: All you have to do is pass the JSON string to the JavaScript eval function. If you have a jsonString variable that contains the JSON data, the following code parses it and returns the corresponding JavaScript object:

Again, note the braces as with the namespace contents, the class s contents are delineated by a pair of braces. We re still not quite at the code yet code lives inside a class, but more specifically, it must live inside a particular method inside a class. A method is a named block of code, which may optionally return some data. The class in our example defines a method called Main, and once again we use a pair of braces to show where it starts and ends:

static void Main(string[] args) { ... }

The first keyword here, static, tells C# that it s not necessary to create a Program object (Program being the class that contains this method, remember) in order to use this method. As you ll see in the next chapter, a lot of methods require an object, but our simple example doesn t need one.

The next keyword is void This tells the compiler that our method doesn t return any data it just does some work Many methods return information For example, the SystemMath class s Cos method calculates the cosine of its input, and since it doesn t know what you want to do with that result, it provides it as a return value the output of the method But the code in this example is rather more proactive than that it decides to show a message on the screen, so there s nothing for it to return On methods that return data, you d write the type of data being returned here, but since there s nothing to return in this case, the nothingness is denoted by the void keyword The next part, Main, is the name of the method.

TimeOfInstantiation. The DateTime class is from the BCL, and Now is a static property of the DateTime class. The Now property creates a new instance of the DateTime class, initializes it with the current date and time from the system clock, and returns a reference to the new DateTime instance.

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