Chapter02.dib

#!csharp

#!markdown

# Chapter 2 - Speaking C#

#!csharp

using static System.Console;

#!markdown

## Showing the compiler version

#!csharp

// #error version

#!markdown

## Help for writing correct code

In a .NET Interactive notebook, you do not need to end the last statement with a semi-colon.

#!csharp

// with two compile errors
Console.Writeline("Hello World!")

// fixed
Console.WriteLine("Hello World!")

#!markdown

## Revealing the extent of the C# vocabulary

When running this code inside a .NET Interactive notebook, it lists all possible assemblies, including some used by the extension like `Microsoft.DotNet.Interactive`.

#!csharp

using System.Linq;
using System.Reflection;

// loop through the assemblies that this app references
foreach (var r in Assembly.GetEntryAssembly()
  .GetReferencedAssemblies())
{
  // load the assembly so we can read its details
  var a = Assembly.Load(new AssemblyName(r.FullName));

  // declare a variable to count the number of methods
  int methodCount = 0;
  // loop through all the types in the assembly
  foreach (var t in a.DefinedTypes)
  {
    // add up the counts of methods
    methodCount += t.GetMethods().Count();
  }

  // output the count of types and their methods
  Console.WriteLine(
    "{0:N0} types with {1:N0} methods in {2} assembly.", 
    arg0: a.DefinedTypes.Count(),
    arg1: methodCount, 
    arg2: r.Name);
}

#!markdown

## Naming things and assigning values

#!csharp

// let the heightInMetres variable become equal to the value 1.88
double heightInMetres = 1.88;
Console.WriteLine($"The variable {nameof(heightInMetres)} has the value {heightInMetres}.");

#!markdown

## Storing text

#!csharp

char letter = 'A'; // assigning literal characters
char digit = '1'; 
char symbol = '$';
char userChoice = GetSomeKeystroke(); // assigning from a fictitious function

string firstName = "Bob"; // assigning literal strings
string lastName = "Smith";
string phoneNumber = "(215) 555-4256";

// assigning a string returned from a fictitious function
string address = GetAddressFromDatabase(id: 563);

#!markdown

## Understanding verbatim strings

In a .NET Interactive notebook, a brighter red color is used for escaped characters inside a string value to make them easier to see.

`\t` means tab. `\b` means backspace. `\s` does is not a valid escape character.

#!csharp

string fullNameWithTabSeparator = "Bob\tSmith";

string filePath = "C:\televisions\sony\bravia.txt";

#!csharp

string filePath = @"C:\televisions\sony\bravia.txt";

#!markdown

## Storing numbers

#!csharp

// unsigned integer means positive whole number or 0
uint naturalNumber = 23;

// integer means negative or positive whole number or 0
int integerNumber = -23;

// float means single-precision floating point
// F suffix makes it a float literal
float realNumber = 2.3F;

// double means double-precision floating point
double anotherRealNumber = 2.3; // double literal

#!markdown

## Storing whole numbers

#!csharp

// three variables that store the number 2 million
int decimalNotation = 2_000_000;
int binaryNotation = 0b_0001_1110_1000_0100_1000_0000; 
int hexadecimalNotation = 0x_001E_8480;

// check the three variables have the same value
// both statements output true 
Console.WriteLine($"{decimalNotation == binaryNotation}"); 
Console.WriteLine(
  $"{decimalNotation == hexadecimalNotation}");

#!markdown

## Writing code to explore number sizes

#!csharp

Console.WriteLine($"int uses {sizeof(int)} bytes and can store numbers in the range {int.MinValue:N0} to {int.MaxValue:N0}."); 
Console.WriteLine($"double uses {sizeof(double)} bytes and can store numbers in the range {double.MinValue:N0} to {double.MaxValue:N0}."); 
Console.WriteLine($"decimal uses {sizeof(decimal)} bytes and can store numbers in the range {decimal.MinValue:N0} to {decimal.MaxValue:N0}.");

#!markdown

## Comparing double and decimal

#!csharp

Console.WriteLine("Using doubles:"); 
double a = 0.1;
double b = 0.2;

if (a + b == 0.3)
{
  Console.WriteLine($"{a} + {b} equals 0.3");
}
else
{
  Console.WriteLine($"{a} + {b} does NOT equal 0.3");
}

#!csharp

Console.WriteLine("Using decimals:");
decimal c = 0.1M; // M suffix means a decimal literal value
decimal d = 0.2M;

if (c + d == 0.3M)
{
  Console.WriteLine($"{c} + {d} equals 0.3");
}
else
{
  Console.WriteLine($"{c} + {d} does NOT equal 0.3");
}

#!markdown

## Storing Booleans

#!csharp

bool happy = true; 
bool sad = false;

#!markdown

## Storing any type of object

#!csharp

object height = 1.88; // storing a double in an object 
object name = "Amir"; // storing a string in an object
Console.WriteLine($"{name} is {height} metres tall.");

//int length1 = name.Length; // gives compile error!
int length2 = ((string)name).Length; // tell compiler it is a string
Console.WriteLine($"{name} has {length2} characters.");

#!markdown

## Storing dynamic types

#!csharp

// storing a string in a dynamic object 
// string has a Length property 
dynamic anotherName = "Ahmed";

// int does not have a Length property 
anotherName = 12;

// an array of any type has a Length property
anotherName = new[] { 3, 5, 7 };

// this compiles but would throw an exception at run-time 
// if you later store a data type that does not have a 
// property named Length
Console.WriteLine($"Length is {anotherName.Length}");

#!markdown

## Specifying and inferring the type of a local variable

*Note*: click **Execute Code** in the following cell to import the namespaces for the subsequent code cell.

#!csharp

using System.IO;
using System.Xml;

#!csharp

var population = 66_000_000; // 66 million in UK
var weight = 1.88; // in kilograms
var price = 4.99M; // in pounds sterling
var fruit = "Apples"; // strings use double-quotes
var letter = 'Z'; // chars use single-quotes
var happy = true; // Booleans have value of true or false

// good use of var because it avoids the repeated type
// as shown in the more verbose second statement
var xml1 = new XmlDocument(); 
XmlDocument xml2 = new XmlDocument();

// bad use of var because we cannot tell the type, so we
// should use a specific type declaration as shown in
// the second statement
var file1 = File.CreateText(@"C:\something.txt"); 
StreamWriter file2 = File.CreateText(@"C:\something.txt");

XmlDocument xml3 = new(); // target-typed new in C# 9 or later

#!csharp

class Person
{
  public DateTime BirthDate;
}

Person kim = new();
kim.BirthDate = new(1967, 12, 26); // instead of: new DateTime(1967, 12, 26)

#!markdown

## Getting default values for types

The default value of a `string` is `null` which outputs as nothing.

#!csharp

Console.WriteLine($"default(int) = {default(int)}"); 
Console.WriteLine($"default(bool) = {default(bool)}"); 
Console.WriteLine(
  $"default(DateTime) = {default(DateTime)}"); 
Console.WriteLine(
  $"default(string) = {default(string)}");

#!csharp

int number = 13;
Console.WriteLine($"number has been set to: {number}");
number = default;
Console.WriteLine($"number has been reset to its default: {number}");

#!markdown

## Storing multiple values an array

#!csharp

string[] names; // can reference any array of strings

// allocating memory for four strings in an array
names = new string[4];

// storing items at index positions
names[0] = "Kate";
names[1] = "Jack"; 
names[2] = "Rebecca"; 
names[3] = "Tom";

// looping through the names
for (int i = 0; i < names.Length; i++)
{
  // output the item at index position i
  Console.WriteLine(names[i]);
}

#!markdown

## Making a value type nullable

#!csharp

int thisCannotBeNull = 4; 
// thisCannotBeNull = null; // compile error!

int? thisCouldBeNull = null; 
Console.WriteLine(thisCouldBeNull); 
Console.WriteLine(thisCouldBeNull.GetValueOrDefault());

thisCouldBeNull = 7; 
Console.WriteLine(thisCouldBeNull); 
Console.WriteLine(thisCouldBeNull.GetValueOrDefault());

#!markdown

## Checking for null

#!csharp

string authorName = null;

// the following throws a NullReferenceException 
// int x = authorName.Length;

// instead of throwing an exception, null is assigned to y 
int? y = authorName?.Length;

Console.WriteLine($"y is null: {y is null}");

// result will be 3 if authorName?.Length is null 
var result = authorName?.Length ?? 3; 
Console.WriteLine(result);

#!markdown

## Formatting using numbered positional arguments

#!csharp

using static System.Console;

#!csharp

int numberOfApples = 12; 
decimal pricePerApple = 0.35M;

WriteLine(
  format: "{0} apples costs {1:C}", 
  arg0: numberOfApples,
  arg1: pricePerApple * numberOfApples);

string formatted = string.Format(
  format: "{0} apples costs {1:C}",
  arg0: numberOfApples,
  arg1: pricePerApple * numberOfApples);

//WriteToFile(formatted); // writes the string into a file

#!markdown

## Formatting using interpolated strings

#!csharp

WriteLine($"{numberOfApples} apples costs {pricePerApple * numberOfApples:C}");

#!markdown

## Understanding format strings

#!csharp

string applesText = "Apples"; 
int applesCount = 1234;

string bananasText = "Bananas"; 
int bananasCount = 56789;

WriteLine(
  format: "{0,-8} {1,6:N0}",
  arg0: "Name",
  arg1: "Count");

WriteLine(
  format: "{0,-8} {1,6:N0}",
  arg0: applesText,
  arg1: applesCount);

WriteLine(
  format: "{0,-8} {1,6:N0}",
  arg0: bananasText,
  arg1: bananasCount);

#!markdown

## Getting text input from the user

.NET Interactive notebooks do not support `ReadLine()` so in the following code we must set literal string values for the two variables.

#!csharp

Write("Type your first name and press ENTER: "); 
string firstName = "Gary"; // cannot use Console.ReadLine()

Write("Type your age and press ENTER: "); 
string age = "34"; // cannot use Console.ReadLine()

WriteLine(
  $"Hello {firstName}, you look good for {age}.");

#!markdown

## Getting key input from the user

`Console.ReadKey()` does not work in a .NET notebook.

#!markdown

## Getting arguments

Arguments cannot be passed to a .NET notebook.

#!markdown

## Exercise 3 - Practice number sizes and ranges

#!csharp

WriteLine("--------------------------------------------------------------------------");
WriteLine("Type    Byte(s) of memory               Min                            Max");
WriteLine("--------------------------------------------------------------------------");
WriteLine($"sbyte   {sizeof(sbyte),-4} {sbyte.MinValue,30} {sbyte.MaxValue,30}");
WriteLine($"byte    {sizeof(byte),-4} {byte.MinValue,30} {byte.MaxValue,30}");
WriteLine($"short   {sizeof(short),-4} {short.MinValue,30} {short.MaxValue,30}");
WriteLine($"ushort  {sizeof(ushort),-4} {ushort.MinValue,30} {ushort.MaxValue,30}");
WriteLine($"int     {sizeof(int),-4} {int.MinValue,30} {int.MaxValue,30}");
WriteLine($"uint    {sizeof(uint),-4} {uint.MinValue,30} {uint.MaxValue,30}");
WriteLine($"long    {sizeof(long),-4} {long.MinValue,30} {long.MaxValue,30}");
WriteLine($"ulong   {sizeof(ulong),-4} {ulong.MinValue,30} {ulong.MaxValue,30}");
WriteLine($"float   {sizeof(float),-4} {float.MinValue,30} {float.MaxValue,30}");
WriteLine($"double  {sizeof(double),-4} {double.MinValue,30} {double.MaxValue,30}");
WriteLine($"decimal {sizeof(decimal),-4} {decimal.MinValue,30} {decimal.MaxValue,30}");
WriteLine("--------------------------------------------------------------------------");