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C# for Java Developers

At first glance, Java developers might not get particularly excited about C# code, because of the

syntactical similarity between it and Java. However, look more closely and you will see subtle yet

significant differences: features such as operator overloading, indexers, delegates, properties, and

type safe enumerations in C#.

This appendix focuses on applying much-loved Java programming tricks to C# code, highlighting

features that C# adds to the picture, and pointing out tricks that C# cannot do (although you won’t

find many of those). Of course, we assume that as a reader of this appendix, you are a professional

Java developer; so we will not go into too much detail when describing the Java language.

Starting Out

Let’s take a look at the infamous “Hello World!” example in Java:

public class Hello {

public static void main(String args []) {

System.out.println(“Hello world! This is Java Code!”);

}

The corresponding C# code for this is as follows:

using System;

public class Hello

{

public static void Main(string [] args)

{

System.Console.WriteLine(“Hello world! This is C# code!”);

}

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The first thing that you’ll notice is that the two appear to be very similar syntactically and both lan-

guages are case-sensitive. C# is object-oriented like Java, and all functionality must be placed inside a

class (declared by the keyword

class). These classes can contain methods, constructors, and fields just

as Java classes can, and a C# class can inherit methods and fields from another class or interface as in

Java. The implementation of classes and methods is similar in both languages.

C# code blocks are enclosed by braces just as in Java. The entry point to a C# application is the static

Main() method, as required by the compiler (similar to Java but note the uppercase “M”). Also note that

only one class in the application can have a

Main() method. Similar to Java, the static keyword allows

for the method to be called without creating an instance of the class first. For the

Main() method in C#

you have the choice of either a

void or int return type. void specifies that the method does not return a

value and

int specifies that it returns an integer type.

The

using keyword in C# corresponds to the import keyword in Java. Therefore, in the C# code above,

we are essentially importing the C# equivalent of a class package called

System. In C#, a class package is

called a namespace, and we will look more closely at these in the next section.

Note that although we have written it with a lowercase s here, in C# the string type can also be written

with a capital S as String. You will also notice that the array rank specifier (

[]) has been shifted from in

front of the

args variable in the Java example, to between the string type and args variable in the C#

sample. In fact, this specifier can occur before or after the variable in Java. However, in C#, the array

rank specifier must appear before the variable name because an array is actually a type of its own indi-

cated by type

[]. We’ll discuss arrays in more depth a bit later.

Finally, as you might expect, the names of methods tend to differ between the languages. For example,

in Java we would use

System.out.println() to display text in the command console. The equivalent

to this method in C# is

System.Console.WriteLine().

Compiling and Running C# Code

In Chapter 2, we noted that like Java code, C# source code is compiled in two stages: first to

Intermediate Language (IL), and then to native code. To run the previous C# code, you need to save it

with an appropriate filename (for example, HelloWorld) and file extension .cs, and then compile it to IL

using the csc command:

csc HelloWorld.cs

The next step is to compile the IL to native code and run the example. To do this, just type the name of

the file, without the extension (as we would with Java code):

HelloWorld

Hello world! This is C# code!

Namespaces

While Java classes reside in logical divisions referred to as packages, C# (and other managed) classes are

grouped together into namespaces. Packages and namespaces differ significantly in their implementation.

A Java class that you want to make part of the

com.samples package, for example, must have package

com.samples; as the first line of code in the file. This is, of course, excluding any comments. Any code

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557599 AppC_BC03.qxd 4/28/04 10:49 AM Page 1226

within that file automatically becomes a part of the specified package. Also, a Java package name is asso-

ciated with the folder containing the class file in that they must have the same name. The

com.samples

package must therefore be in class files that exist in the com/samples folder. Let’s take a look at some

examples of how packages work in Java:

package java2csharp.javasamples;

public class Hello {

public static void main(String args []) {

System.out.println(“Hello world! This is Java Code!”);

}

The following list provides examples of how the previous code could be referenced or executed. This list

assumes that the class file has been made available to the JRE:

❑ From the command line:

java java2csharp.javasamples.Hello

❑ As a direct reference in the code:

public class Referencer {

java2csharp.javasamples.Hello myHello = new java2csharp.samples.Hello();

❑ By utilizing the import directive one could omit fully qualified package names, so Referencer

could also be written as:

import java2csharp.javasamples.*;

public class Referencer {

Hello myHello = new Hello();

}

Wrapping a class in a namespace is achieved in C# by using the namespace keyword with an identifier,

and enveloping the target class in brackets. Here is an example:

namespace java2csharp.csharpsamples

{

using System;

public class Hello

{

public static void Main(string [] args)

{

System.Console.WriteLine(“Hello world! This is C# code!”);

}

As you can see, we delimit layers of namespaces using the . operator, as in Java. Note that C# does not

require an asterisk (*) needed in C#—applying the

using directive implicitly imports all elements of the

specified namespace. You will also have noticed the major difference from Java here: the use of name-

space parentheses in which we place classes associated with the namespace. The advantage of using the

parentheses like this is that we then disassociate package names from directory structures: feasibly we

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C# for Java Developers

557599 AppC_BC03.qxd 4/28/04 10:49 AM Page 1227

could place a file containing this namespace anywhere within the directory as long as the CLR recog-

nizes it. Therefore, it also enables us to call the file containing these classes anything we wish (it doesn’t

have to be the same name as the class as in Java); we can have more than one public class defined per

file; and we can split the classes defined in this namespace into different files in different parts of the

directory structure, as long as the namespace declaration appears in each of the files.

We can also introduce multiple namespaces in the same file with no restriction. We could, for example,

add the definition of a new class and place it in a new namespace in the same file and still not be outside

the bounds of the language:

namespace java2csharp.csharpsamples

{

using System;

public class Hello

{

public static void Main(string [] args)

{

System.Console.WriteLine(“Hello world! This is C# code!”);

}

namespace java2csharp.morecsharpsamples

{

using System;

public class AnotherHello

{

public static void Main(string [] args)

{

System.Console.WriteLine(“Hello again! This is more C# code!”);

}

As we pointed out in the previous section, classes from a particular namespace can be imported into

another namespace with the

using keyword. We can see that we import classes from the System name-

space (the top level .NET Base Class namespace) into both namespaces above. We can also import classes

from other namespaces directly into our classes by referring to the imported class using its full name

(namespace included), in a similar way to using direct referencing of classes in Java code.

Namespaces may also be defined within other namespaces. This type of flexibility is impossible in Java

without having to create a subdirectory. We could change the previous example so that the

AnotherHello

class is in the java2csharp.csharpsamples.hellosamples namespace:

namespace java2csharp.csharpsamples

{

namespace hellosamples

{

using System;

public class AnotherHello

{

public static void Main(string [] args)

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Appendix C

557599 AppC_BC03.qxd 4/28/04 10:49 AM Page 1228

{

System.Console.WriteLine(“Hello again! This is more C# code!”);

}

Java classes are part of a package; all classes created are part of the default package. C# mimics this func-

tionality. Even if you do not declare one, a default namespace is created for you. It is present in every file

and available for use in named namespaces. Just as in Java you cannot change package information, in

C# namespaces cannot be modified either. Packages can span multiple files in the same folder; name-

spaces can span multiple files in any number of folders, and even multiple assemblies (the name given

to code libraries in .NET), as discussed in Chapter 13.

Note that the default accessibility for types inside a namespace is internal. You must specify types as

public if you want them available without full qualification; however, we strongly advise against this

practice. No other access modifiers are allowed. In Java, internal package types may also be marked as

final or abstract or not marked at all (this default access makes them available only to consumers

inside the package). Access modifiers are discussed later in this appendix.

One final feature of namespaces not available to Java packages is that they may be given a

using alias.

using aliases make it very easy to qualify an identifier to a namespace or class. The syntax is simple.

Suppose you had a namespace

Very.Very.Long.NameSpace.Name. You could define and use a using

alias (here VVLNN) for the namespace as follows:

using VVLNN = Very.Very.Long.Namespace.Name;

Declaring Variables

C# follows a similar scheme of variable declaration to Java, where the declaration consists of a datatype

keyword and followed by the name of the variable to hold that datatype. For example, to declare an

integer (

int) variable called myInt, we would use the following code:

int myInt;

Identifiers are the names we give to classes, objects, class members, and variables. Raw keywords, dis-

cussed in the next section, can neither be Java nor C# identifiers; however, in C# you can use keywords

as variable names by prefixing the name with

@. Note that this exception is only with keywords and

does not allow the breaking of any other rules. Although identifiers may have letters and numbers, the

first letter of the identifier in both C# and Java must not be a number. Here are some valid and invalid

examples of variable declaration:

int 7x; //invalid, number cannot start identifier

int x7; //valid, number may be part of identifier

int x; //valid

int x$; //invalid, no symbols allowed

int @class; //valid, prefix @ allows it to be used as an identifier

int @7k; //invalid, prefix @ only works for keywords

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C# for Java Developers

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