Note: This document is a work in progress. You can help improve it.

Intro to C#

To get started

  • Download the CLI and verify its running by running dotnet --version

Your first program

  • Create a new console application. A console application runs in the terminal. Create a new app using the CLI dotnet new console
  • Our code will go in the Main() method in Program.cs
  • To run the app, use the command dotnet run. This command builds and runs our project.
  • C# is a compiled language. That means that the code we write is translated in machine code before anything runs. Compiling our codes allows for our to be (probably) faster and eliminate an entire class of bugs.
  • Every line of code needs a ; or a { at the end

Declaring Variables

  • Variables are strongly typed.
  • Can be declared like:
string name = "Mark";
int score = 95;
  • Better to use var
  • var is used to type a variable implicitly
  • type is inferred from the value on the right of the expression
var name = "Mark"; // typed to string
var score = 95; // typed to int

Primitive types

  • Primitives are the basic built in types in C#

  • The common primitives are:

    • int : a whole number from -2,147,483,648 to 2,147,483,64
    • long : a whole number from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
    • double : a 64 bit floating point value that has an approximate precision of ~15-17 digits
    • float : a 32 bit floating poit value that has an approximate precision of 6-9 digits
    • decimal : a more precise way to store decimal numbers, but has a smaller range
    • char : represents a unicode character
    • bool : true or false
    • byte : represents a raw chunk of data
  • Primitives can not be null

  • NOTE: do not feel the need to memorize the difference in types, know that they exist. Most of the time developers will just var and not think about the types until it matters, for example, in the finance apps, where rounding matters, a developer should be aware of the precision of the numbers.

References Types

  • Reference Types are everything else in C#. Reference types include String, Object and any custom objects that you create.
  • Reference Types can be null

Variable Examples

  • integers var score = 42;

  • floats var total = 10f;

  • decimal

  • var total = 10m;

  • double

  • var total = 10.0;

  • var total = 10d;

  • char are respresented by single quotes ' var piratesFavoriteLetter = 'c';

  • Strings are treated as arrays of characters var name = "Mark"; var x = name.Length;

  • Strings are indexed based var firstLetter = name[0];

  • Crashes when trying to access an index that does not exist var oops = name[-1];

  • String interpolation var new_string = $"My score is {41 + 1}";


  • Arrays are more restrictive on functionality are rarely used

  • Arrays still have 0 based indexes

  • Once an array is created, it's size cannot change

  • Crashes when trying to access a index that does not exist var scores = new int[] { 100, 98, 42, 65 }; var second = scores[1]; var numberOfScores = scores.Length;

  • Arrays can only store data of the same type var arrayOfSize10 = new int[10]; arrayOfSize10[4] = "hello"; // Can't do this since arrayOfSize10 is a an array of int, not strings


  • Lists are a more flexible, commonly used arrays

  • Lists are most like JavaScript arrays

  • to create a list of doubles var scores = new List<double>();

  • to add to a list scores.Add(100);

  • Lists are still accessed as a 0 based list scores[0] = 100;


  • A Dictionary is a strongly typed key:value lookup table
  • A dictionary in C# acts like an actual dictionary, a thing that has words and those words each have a definition. In this case, the word is the key, and the definition is value.
  • When creating a dictionary, the first type is the type of the Key and the second type is the type of the Value. var oxford = new Dictionary<string, string>(); This statement creates a dictionary like we are used to, with a string as the key (the word) and a string as value (the definition).
  • The following creates a dictionary with a string as the key (the player name) and an int as the value (the player's score) var playerScores = new Dictionary<string, int>(); ` playerScores.Add("Robbie Lakeman", 1247700 )
  • NOTE: Your program crashes if you try to access a key that doesn't exist

Interacting with users

  • Console.WriteLine() outputs to the console.
  • Console.WriteLine() will convert what is passed into it to a string
var hello = "Hello, World!";
  • Console.ReadLine() will wait for user input. Console.ReadLine() continues to accept input until the user hits enter.
Console.WriteLine("What is your name?");
var input = Console.ReadLine();
Console.WriteLine($"Sorry {name}, I'm afraid I can do that");

Control Flow

  • if statements
if (song == "Harpua")
Console.WriteLine("Oom Pah Pah");
else if (song == "Run Like an Antelope")
Console.WriteLine("Out of Control")
Console.WriteLine("Jam on");


  • For loops
for(var i = 0; i < 10; i++){
  • Foreach loops
  • works on arrays, lists, dictionaries, or any IEnumerable
for (var score in scores)


  • Comments start with a //.
  • Block Comments (multi-line) are denoted by /* */
  • # can be used to compile time commands and events, don't worry about using these.


  • Organizing code

  • Placing a name on a set of steps or a way of doing something

  • Methods have what's called a signature. Signatures are composed of the methods name and the params

  • Signatures have to be unique in their scoping.

  • Generic Method declaration

public ReturnType Name(Param1Type Param1, Param2Type Param2)
// method body
// This method has no inputs or return
public void PrintHello()
// This method has input but no returns
public void PrintName(string name)
Console.WriteLine($"Hello, {name}");
// This method has inputs and a return
public string MakeSentence(string name, int score)
return $"{name} has a score of {score}";
// Method Expressions, if the body of the Method is only 1 line, we can use the lambda syntax
// this method is the same as one above
public string MakeSentence(string name, int score) => $"{name} has a score of {score}";

More control flow

  • Conditionals
    • Are two things true:
      • if (score > 10 && score < 80)
    • Are either of two things true
      • if (score > 90 || name == "Gavin")
  • What if we are looking for a few options we have the switch statement
int caseSwitch = new Random().Next(0,2);
switch (caseSwitch)
case 1:
Console.WriteLine("Case 1");
case 2:
Console.WriteLine("Case 2");
Console.WriteLine("Default case");