002-Hello-CSharp

Hello C# World

This file is part of a multi-series workshop on learning C# on Linux available here.

Author: Martin Woodward

Before you start you want to make sure that you have .NET Core installed and configured in your path

 dotnet --version

Assuming the command above works, you are good to go! If not, please see Tutorial 1 - Getting Started with C# on Linux.

My First C# Program

Now you have .NET Core installed, let's create our first C# program. To begin we need to create a folder to store our C# code project in, i.e.

 mkdir -p ~/source/dotnet/002-hello
 cd ~/source/dotnet/002-hello

Then we want to create a new C# project in it

 dotnet new

This will create the following files in that directory

• Program.cs - the actual program, with a basic Hello World sample ready to run
• project.json - a meta-data file that describes the entire .NET project to the compiler and other tooling.
• NuGet.Config - the configuration of NuGet (the .NET package manager) for your project

Getting ready to run

Now we have an application, the next thing we have to do it tell NuGet to download the dependencies we need for our project.

NOTE - Temporary Hack for RHEL Bug

At the time of writing, the build for Fedora based distributions has a bug. To work-around this to be able to run your new application you need to do the following:

 dotnet restore --runtime "centos.7-x64"
 sed -ie 's:DNXCore,Version=v5.0/centos.7-x64:DNXCore,Version=v5.0/rhel.7.2-x64:g' project.lock.json

You'll be needing to do this a lot, so until the bug is fixed you might want to create an alias by adding the following to your .bashrc file. That way instead of doing a dotnet restore you can type dnr to execute the two commands.

alias dnr='dotnet restore --runtime "centos.7-x64" && sed -ie ''s:DNXCore,Version=v5.0/centos.7-x64:DNXCore,Version=v5.0/rhel.7.2-x64:g'' project.lock.json'

If you are not on RHEL / Fedora then you can download the dependencies from NuGet by simply doing

 dotnet restore

Compiling and running your project

Then to build and run your application, simply type

 dotnet run

That's all there is to it. Now let's take a look at Program.cs and make some changes.

The Structure of a C# program

Open Program.cs in your favorite editor and take a look at the contents. After you ran dotnet new the following file would have been created

using System;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}

Note that C# ignores whitespace so while the code above is indented for readability it is not required. Lines of code are terminated with a semi-colon (;) and blocks of code are grouped using curly braces ({ }). The .NET Core codebase itself uses use Allman style braces, where each brace begins on a new line. A single line statement block can go without braces but many developers enjoy debating if that is evil or not. (It is)

Using Directive

The first line you see is the using directive. using is similar to import in other languages like Java, Python or Go. It tells the C# compiler where to find the classes that you want to call. In the example above, the full name of Console is actually System.Console but the using System directive means that the compiler automatically looks for the Console class with-in the System namespace.

Namespaces

A namespace is a way of grouping classes together in a way that limits their scope and avoids name conflicts. This is similar to package in Java however in C# while it is convention to have a hirerarchy of namespaces represented as a tree structure in a directory, you don't have to. A single .cs file could declare multiple classes in multiple namespaces - though that would be a little bit evil. In the example code created, you are defining Program to live in the ConsoleApplication namespace, therefore the full name of the class would be ConsoleApplication.Program.

In .NET, code is organized into assemblies - a package of one or more namespaces of code in a shared library (a DLL) or an executable. Shared libraries (including the .NET Framework itself) are typically grouped into NuGet packages for distribution.

Main()

A C# program must contain a public static method called Main which controls the start and end of the process and there can only be one Main method per project. When the program is executed the .NET runtime will look for the class containing the 'Main' method and execute it. You can change the class name and namespace in your example and the Main method will still execute when you run the application.

You can declare the Main method in two ways:

    public static void Main(string[] args)
    {
        // Do Something
    }

Which would always return a 0 exit code, or if you want to have control of the exit codes returned by your application you can declare your exit code explicitly by indicating that your Main method returns an int and then returning an integer in your Main method.

    public static int Main(string[] args)
    {
        // Do Something
        return 0;
    }

You can run the code above and verify the exit code returned by typing

 dotnet run
 echo $?

Comments

Note in the example above, we are showing comments. In common with many languages C# supports C style comments, i.e

 int x = 1; // Single line comment
 /* You can always
  * spread you comments out over many lines
  */

Visibility

In the example above, the Main method was declared with a visibility of public so that is can be accessed by code in the same assembly of any assembly that references it. The other most common access modifier to control visibility is 'private' which means the code can be accessed by the same class or struct. 'protected' means that the code can be accessed by the class or anything that derrives from that class and 'internal' code can be accessed by any code in that same assembly but not from another assembly. Note that in C# the access modifier visibility is scoped at the assembly level not at the namespace level. For more information on all the access modifies in C# please see the reference documentation.

Exercises

1. Create a new .NET Project and run it.
2. Modify the program to change the message it returns and run it.
3. Modify the program to return an exit code of 1.

Conclusion

In this tutorial we learned how to create a new C# application, the structure of a basic C# program, how to run the application and how to change the exit code of the process.

Additional Information

• Using directive in C#
• Namespaces
• System.Console
• Main() and Command-Line Arguments
• Visibility and Access Modifies in C#
• NuGet
• project.lock.json

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• Next: Tutorial 3 - C# Language Basics
• Previous: Tutorial 1 - Getting Started with C# on Linux
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