C# Keywords and User Input

Programming is an act of solving problems through writing code. And code is just a set of instructions for a computer to execute.

I would like to invite you to a programming journey where you will take your first steps using C#.

What is C#?

C# is a multi-purpose high level programming language. That means that you don't need to manage low level things (like memory assignment or machine instructions). Those things are abstracted away from you through a managed environment.

With C# you can create:

• Webservices (ASP.NET Core)
• Desktop apps (Winforms, UWP, WPF)
• Mobile apps (Xamarin)
• Games (Unity)
• And more!

You can create pretty much anything you want using C#!

Most commonly it is used for building enterprise business applications. It's a language heavily supported by Microsoft, loved by the community and has a good future ahead of it. The latest available version (as of time of writing) is C# 9.

.NET

.NET is an ecosystem which adds all sorts of features built on top of C#. For example: reading a file, getting data from a URL, sending an email, connecting to a database, etc. .NET also includes other languages: Visual Basic and F#, however we will not be covering those during the boot camp.

Visual Studio

A writer needs a pen and paper; a programmer needs an IDE (integrated development environment). An IDE provides a programmer with all the needed tools to do their job: code editor, debugger, code runner, compiler, etc.

Visual Studio is an IDE for .NET.

In order to set Visual Studio up, refer to this video.

Your First Program

New Project

In order to start making something yourself, you will need to open Visual Studio and select Create a new project:

The most simple application you can create is a console application. In the next window, type console and select Console App (.NET Core). Make sure you select the one with C# tag on it:

In the next window, name your program RecipeApp (or anything you want) and hit Create.

Run, programmer, run!

A new window opens with your very first project! Well, what are you waiting for? Run it! You can do this by either hitting F5 or by clicking a run button at the top of the window:

On behalf of all the programmers of the world, welcome! Hello World! is how even the greatest programming heroes of today have started their journey. What you can see is a console application - a window without any user interface- just plain text on a black background. You can close the console window now to focus back on Visual Studio.

What just happened?

We just ran our first code- Program.cs. We compiled the code file- converted code into machine instructions- and our computer has executed them. To be precise, this code has been executed:

using System;

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

Let's try to understand it!

The file starts with a using System; statement. using is needed, when we want to refer to code which is in a different namespace. A namespace is just a container for multiple code files- usually a namespace is named after a physical directory path which holds the code. The System namespace contains code for basic interactions with a system, including - Console - therefore we had to import it. The System namespace is provided by .NET. The word statement- means a single instruction of code to execute and in C# a statement needs to end with ;.

Next is a namespace RecipeApp. Code must belong in some namespace and the first one in our code is usually named after the project it belongs to.

Next- a {. This refers to a scope- an access boundary of code. Scope has a start { and an end }. This scope is the scope of the namespace. Code outside of a scope (namespace RecipeApp in our case) is not accessible by default (unless we import it with a using statement). If there was no such thing as a scope, it would be really hard to pick what we want, because everything would be accessible to us.

After that follows a class. It's like a container for many code blocks. Program is a class which contains logic to start our application. The name itself can be anything, but the name we have is the default for a startup class. A class has its own scope too {}.

static void Main(string[] args) is an entry point of our application. It is a function- a container for a single logical block of code. Every function should tell 1 thing for a computer to do. In our case- it runs the application which prints Hello World. A function has its own scope as well {}. We will talk about static in future lessons.

Console.WriteLine("Hello World!"); - is the code which makes a call to print text on the console screen. Most of the code will be written inside functions. This function is part of the System namespace and is provided by .NET.

Where (scope) you write code matters. For now, all the code we write will be inside the Main function.

Solution Structure

The new project contains a bit more than just the Program.cs file. Focus your attention on the right side of

On the right side (by default) you can see a Solution Explorer. In .NET, a solution (marked as 1) is a container for all files needed that your program is made of. Essentially, treat a solution as a container of projects (marked as 2). Project is like a module (or a layer) for a full program, which provides functionality for some (or all) of it. Projects are made of different kinds of files, but for now, we will focus on .cs files. For code to be compiled as C# code, it needs to be put in .cs files (marked as 3).

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Variables

Variables are essentially data. For a variable to be meaningful, it needs to have 3 things:

• Type- what kind of data can a variable hold? There are 3 primitive variable categories: numbers, text and logical.
• Name- how will we refer to a variable? Can only be made of [aA-zZ], _, @, [0-9].
• Value- what data does the variable hold?

You can declare a variable without a value, but the C# compiler won't let you use it, because it has no value (is not initialized). An example of such an error can be seen in the image below.

If instead we assign a value, for example a = 5;, we will no longer see that error.

By convention, variable names are camelCase - first word starts with non-capital letter and then each new word starts with a capital letter.

Numbers

Number types are divided into two groups. Whole and floating point.

Whole numbers

Type	Bits	Min value	Max value :
byte	8	0	255
short	16	-32 768	32,767
int	32	-2 147 483 648	2 147 483 648
long	64	-9x1018	9x1018

Default is int.

Most whole number types have unsigned versions of them, which double their maximum size. For example, short is 32 767 max value, but ushort is 65 535, however, min value of ushort is 0 (no negative).

Floating point numbers (numbers with fraction)

Type	Bits	Value	symbol :
float	32	±1.5 x 10−45 to ±3.4 x 1038	f
double	64	±1.5 x 10−45 to ±3.4 x 1038	d
decimal	128	±1.5 x 10−45 to ±3.4 x 1038	m

Default is double.

If you want to declare a variable as float or decimal, you will need to explicitly say so by either:
A) using type symbol float f = 2.0f;
B) casting float f = (float) 2.0;

Important: for money or scientific calculations you should use decimal.

Text

char- is a single character. To store a char use ''.

char myChar = 'a';

string- is many characters. To store a string use "".

string myStr = "Joe#2020";

You cannot store an empty character (''- won't compile), but you can store an empty string.

Logical

bool is a logical type. It can be either true or false. Logical variables drive logical statements, but more about that in later lessons.

Arithmetic

Addition or Subtraction Results

Type1	Type2	Result type
byte	int	int
char	char	int
char	int	int
string	string	string
string	char	string
string	long	string

Adding or subtracting two values of different types will store the result in the larger type.

Division & Multiplication

Both are trivial and applicable only to number types.
While dividing whole numbers, you should pay attention because if the dividend number is less than the divisor number, the result will be 0.
For example: 1/2=0.
If we want the result to be 0.5, all we need to do is to add a fraction to either of the numbers when dividing.
Like so: 1/2.0=0.5.

Like in maths, division by 0 is impossible. Also, division and multiplication have priority over addition and subtraction.

Comments

In C#, comments are written using //. The purpose of comments is to add a note for yourself about the code below. Comments can be made from multiple lines as well using /**/. For example:

// This is a.
int a = 1;

/*
This is b.
b is equal to a.
*/
int b = a;

Function

A function is a named block of code.

First Look

We already know how to call the Console.WriteLine("Hello World") function. WriteLine is a function name, "Hello World" is the input. We pass input within the () after the function name.

Your First Function

A function, just like a variable, needs to have a (return) type and a name. Optionally, it can also take arguments (each which should have a type and a name). All that a function does is defined within its scope {}. Not all functions must return something, such functions will have a return type of void.

For example, a function which takes no arguments and does not return anything:

void WriteHello()
{
    Console.WriteLine("Hello");
}

Another example, a function which takes arguments and returns a value:

int Sum(int a, int b)
{
    return a + b;
}

By convention, function names are PascalCased - in which every word is capitalized.

Console

Write

To write text to console, all you have to do is Console.Write("My text");. If you want text to be printed in a new line, you can Console.WriteLine("My text");. The printed text will appear without "" because the quotation marks are used to distinguish between code and text, but such distinction is no longer needed when printing text.

If you type cw in the code editor and press tab, it will write Console.WriteLine(); to code file.

Read

We can read user input in many different ways, but for now, we will focus on the most simple one- Console.ReadLine();. This will try to read a line of text that a user typed and pressed enter. For example string name = Console.ReadLine();.

Parse

Parse is a term for converting text (string) into something more usable (e.g. a number). Usually, a string is converted to another type through a Parse or TryParse method.

Use Parse when you are sure that the given text is convertable to the type:

string to int:

string text = "123";
int number = int.Parse(text);

And when the text can be anything, use TryParse:

string text = "123a";
// Returns false and sets number to 0 (default value).
bool isNumber = int.TryParse(text, out int number);

anything to string:

string text = number.ToString();

String Concatination

If you add two strings together, you get a string. Therefore:

string piece1 = "Hello";
string piece2 = "World!";
// Hello World!
string full = piece1 + " " + piece2;

Adding strings like this might be quite a burden for a reader. It's easier to simplify this using string interpolation. It gives us a way to combine strings more naturally using a $ literal:

// Hello World!
string full = $"{piece1} {piece2}";

String interpolation also allows us to format a string. For example, if we have a double number = 0.0111111 printing it using Console.WriteLine(number); will print 0.0111111. What if we want only 2 digits after .? We can solve this problem by formatting the number string: Console.WriteLine($"{number:F2}");. F2 here means a number which will have two numbers after the decimal separator.

Prefer to use string interpolation whenever you combine strings.

var keyword

You don't have to explicitly declare a variable with a type. Sometimes it might even get tedious, especially when type names get more complex and long. Sometimes we either don't care about a type or it's extremely obvious what the type is. By convention, in most cases, we use var to define a type. For example: var name = "Kaisinel";. The type of name, in this case, will be string. The compiler is smart enough to know the type in most cases and when it doesn't, you will have to hint it with explicit types or casting.

var doesn't make C# determine type on the fly. If you hover over var in IDE, you will still see the actual type.

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Homework

1. Read name, surname, age, weight (in kg) and height (in cm) from console.
2. Print a message to the screen based on the information given in 1. (an example message is shown below):

Tom Jefferson is 19 years old, his weight is 50 kg and his height is 156.5 cm. 

3. Calculate and print body-mass index (BMI)
4. Do 1 and 2 for another person.

Do You Understand the Topic?

• Which data type would you use for storing:
• • age?
• • count of people?
• • weight and height?
• • first letter of your name?
• • the answer to the question: "Did you understand primitive types?"
• What is a variable?
• Every variable has what 3 parts?
• What arithmetic operations can be performed with number types?
• What happens when you divide a smaller integer from a bigger integer?
• What happens when you divide by zero?
• When do you need to cast explicitly?
• Do you need to cast implicitly?
• How can you read input from a Console?
• How can you output something to a Console?
• How to create a function which prompts a user to input something and then prints what was inputted?