In this part of the C# tutorial, will talk about the flow control. We define several keywords that enable us to control the flow of a C# program.

In C# language there are several keywords that are used to alter the flow of the program. When the program is run, the statements are executed from the top of the source file to the bottom. One by one. This flow can be altered by specific keywords. Statements can be executed multiple times. Some statements are called conditional statements. They are executed only if a specific condition is met.

C# if statement

The if statement has the following general form:

if (expression)
{
    statement;
}

The if keyword is used to check if an expression is true. If it is true, a statement is then executed. The statement can be a single statement or a compound statement. A compound statement consists of multiple statements enclosed by the block. A block is code enclosed by curly brackets.

using System;

var r = new Random();
int n = r.Next(-5, 5);

Console.WriteLine(n);

if (n > 0)
{
    Console.WriteLine("The n variable is positive");
}

A random number is generated. If the number is greater than zero, we print a message to the terminal.

var r = new Random();
int n = r.Next(-5, 5);

These two lines generate a random integer between -5 .. 5.

if (n > 0)
{
    Console.WriteLine("The n variable is positive");
}

Using the if keyword, we check if the generated number is greater than zero. The if keyword is followed by a pair of round brackets. Inside the brackets, we place an expression. The expression results in a boolean value. If the boolean value is true, then the block enclosed by two curly brackets is executed. In our case, the string "The n variable is positive" is printed to the terminal. If the random value is negative, nothing is done. The curly brackets are optional if we have only one expression.

$ dotnet run
-3
$ dotnet run
-4
$ dotnet run
-1
$ dotnet run
1
The n variable is positive

When the condition is met, the message is written to the console.

C# else statement

We can use the else keyword to create a simple branch. If the expression inside the square brackets following the if keyword evaluates to false, the statement following the else keyword is automatically executed.

using System;

var r = new Random();
int n = r.Next(-5, 5);

Console.WriteLine(n);

if (n > 0)
{
    Console.WriteLine("The number is positive");
} else
{
    Console.WriteLine("The number is negative");
}

Either the block following the if keyword is executed, or the block following the else keyword.

if (n > 0)
{
    Console.WriteLine("The number is positive");
} else
{
    Console.WriteLine("The number is negative");
}

The else keyword follows the right curly bracket of the if block. It has its own block enclosed by a pair of curly brackets.

$ dotnet run
-3
The number is negative
$ dotnet run
-1
The number is negative
$ dotnet run
2
The number is positive

We execute the program three times.

C# else if

We can create multiple branches using the else if keyword. The else if keyword tests for another condition if and only if the previous condition was not met. Note that we can use multiple else if keywords in our tests.

using System;

var r = new Random();
int n = r.Next(-5, 5);

Console.WriteLine(n);

if (n < 0)
{
    Console.WriteLine("The n variable is negative");
} else if (n == 0)
{
    Console.WriteLine("The n variable is zero");

} else
{
    Console.WriteLine("The n variable is positive");
}

The previous program had a slight issue. Zero was given to negative values. The following program will fix this.

if (n < 0)
{
    Console.WriteLine("The n variable is negative");
} else if (n == 0)
{
    Console.WriteLine("The n variable is zero");
} else
{
    Console.WriteLine("The n variable is positive");
}

If the first condition evaluates to true, e.g. the entered value is less than zero, the first block is executed and the remaining two blocks are skipped. If the first condition is not met, then the second condition following the if else keywords is checked. If the second condition evaluates to true, the second block is executed. If not, the third block following the else keyword is executed. The else block is always executed if the previous conditions were not met.

$ dotnet run
-1
The n variable is negative
$ dotnet run
4
The n variable is positive
$ dotnet run
1
The n variable is positive
$ dotnet run
0
The n variable is zero

We execute the program three times. The 0 is correctly handled.

C# switch statement

The switch statement is a selection control flow statement. It allows the value of a variable or expression to control the flow of program execution via a multi-way branch. It creates multiple branches in a simpler way than using the combination of if/else if/else statements.

We have a variable or an expression. The switch keyword is used to test a value from the variable or the expression against a list of values. The list of values is presented with the case keyword. If the values match, the statement following the case is executed. There is an optional default statement. It is executed if no other match is found.

Since C# 7.0, the match expression can be any non-null expression.

The switch statement is quite complex.

Selecting day of week

In the following example, we select a day of week with switch statement.

using System;

var dayOfWeek = DateTime.Now.DayOfWeek;

switch (dayOfWeek)
{
    case DayOfWeek.Sunday:
        Console.WriteLine("dies Solis");
        break;

    case DayOfWeek.Monday:
        Console.WriteLine("dies Lunae");
        break;

    case DayOfWeek.Tuesday:
        Console.WriteLine("dies Martis");
        break;

    case DayOfWeek.Wednesday:
        Console.WriteLine("dies Mercurii");
        break;

    case DayOfWeek.Thursday:
        Console.WriteLine("dies Jovis");
        break;

    case DayOfWeek.Friday:
        Console.WriteLine("dies Veneris");
        break;

    case DayOfWeek.Saturday:
        Console.WriteLine("dies Saturni");
        break;
}

The example determines the current day of week and prints its Latin equivalent.

switch (dayOfWeek)
{
    ...
}

In the round brackets, the switch keyword takes a value from an expression, which is going to be tested. The body of the switch keyword is placed inside a pair or curly brackets. In the body, we can place multiple case options. Each option is ended with the break keyword.

case DayOfWeek.Sunday:
    Console.WriteLine("dies Solis");
    break;

With the case statement, we test the value of the matching expression. If it is equal to DayOfWeek.Sunday, we print the Latin dies Solis.

$ dotnet run
dies Solis

The program was run on Sunday.

Selecting domain

The user is requested to enter a domain name. The domain name is read and stored in a variable. The variable is tested with the switch keyword against a list of options.

using System;

Console.Write("Enter a domain name: ");

string domain = Console.ReadLine();

domain = domain.Trim().ToLower();

switch (domain)
{
    case "us":
        Console.WriteLine("United States");
    break;

    case "de":
        Console.WriteLine("Germany");
    break;

    case "sk":
        Console.WriteLine("Slovakia");
    break;

    case "hu":
        Console.WriteLine("Hungary");
    break;

    default:
        Console.WriteLine("Unknown");
    break;
}

In our program, we have a domain variable. We read a value for the variable from the command line. We use the case statement to test for the value of the variable. There are several options. If the value equals for example to "us" the "United States" string is printed to the console.

string domain = Console.ReadLine();

Input from the user is read from the console.

domain = domain.Trim().ToLower();

The Trim method strips the variable from potential leading and trailing white spaces. The ToLower converts the characters to lowercase. Now the "us", "US", "us " are viable options for the us domain name.

case "us":
    Console.WriteLine("United States");
break;

In this case option, we test if the domain variable is equal to "us" string. If true, we print a message to the console. The option is ended with the break keyword. If one of the options is successfully evaluated, the break keyword terminates the switch block.

default:
    Console.WriteLine("Unknown");
break;

The default keyword is optional. If none of the case options is evaluated, then the default section is executed.

$ dotnet run
Enter a domain name: us
United States

$ dotnet run
Enter a domain name: HU
Hungary

$ dotnet run
Enter a domain name: pl
Unknown

Using when statement

The case statement can be used with when statement to specify additional condition.

using System;

Console.Write("Enter your age: ");

var input = Console.ReadLine();

var age = Int32.Parse(input.Trim());

switch (age)
{
    case var myAge when myAge < 0:
        Console.WriteLine("Age cannot be a negative value");
    break;

    case var myAge when myAge > 130:
        Console.WriteLine("This is an unlikely high age");
    break;

    default:
        Console.WriteLine("The entered age is {0}", age);
    break;
}

In the program the user is asked to enter his age.

case var myAge when myAge < 0:
    Console.WriteLine("Age cannot be a negative value");
break;

With the help of the when expression, we test if the entered value is less than 0. With the var keyword, we create a temporary myAge variable.

Using enumeration as match expression

For the matching expression, we can use any type. In the following example, we use an enumeration.

using System;

var color = (Color) (new Random()).Next(0, 7);

switch (color)
{
    case Color.Red:
        Console.WriteLine("The color is red");
        break;

    case Color.Green:
        Console.WriteLine("The color is green");
        break;

    case Color.Blue:
        Console.WriteLine("The color is blue");
        break;

    case Color.Brown:
        Console.WriteLine("The color is brown");
        break;

    case Color.Yellow:
        Console.WriteLine("The color is yellow");
        break;

    case Color.Pink:
        Console.WriteLine("The color is pink");
        break;

    case Color.Orange:
        Console.WriteLine("The color is orange");
        break;

    default:
        Console.WriteLine("The color is unknown.");
        break;
}

enum Color { Red, Green, Blue, Brown, Yellow, Pink, Orange }

The example generates randomly a colour enumeration. The swith statement determines which colour value was generated.

$ dotnet run
The color is orange

$ dotnet run
The color is blue

$ dotnet run
The color is brown

We run the program.

Switch expressions

Switch expressions make the syntax of a switch statement more concise. They were introduced in C# 8.0.

using System;

Console.Write("Enter a domain name: ");

string domain = Console.ReadLine();

domain = domain.Trim().ToLower();

string result = domain switch
{
    "us" => "United States",
    "de" => "Germany",
    "sk" => "Slovakia",
    "hu" => "Hungary",

    _ => "Unknown"
};

Console.WriteLine(result);

The variable comes before the switch keyword. The case and : elements are replaced with =>. The default case is replaced with a _ discard. The bodies are expressions, not statements.

C# while statement

The while statement is a control flow statement that allows code to be executed repeatedly based on a given boolean condition.

This is the general form of the while loop:

while (expression)
{
    statement;
}

The while keyword executes the statements inside the block enclosed by the curly brackets. The statements are executed each time the expression is evaluated to true.

using System;

int i = 0;
int sum = 0;

while (i < 10)
{
    i++;
    sum += i;
}

Console.WriteLine(sum);

In the code example, we calculate the sum of values from a range of numbers.

The while loop has three parts. Initialization, testing and updating. Each execution of the statement is called a cycle.

int i = 0;

We initiate the i variable. It is used as a counter.

while (i < 10)
{
   ...
}

The expression inside the round brackets following the while keyword is the second phase, the testing. The statements in the body are executed until the expression is evaluated to false.

i++;

This is the last, third phase of the while loop, the updating. We increment the counter. Note that improper handling of the while loops may lead to endless cycles.

Do-while statement

It is possible to run the statement at least once. Even if the condition is not met. For this, we can use the do while keywords.

using System;

int count = 0;

do {
    Console.WriteLine(count);
} while (count != 0);

First the block is executed and then the truth expression is evaluated. In our case, the condition is not met and the do while statement terminates.

C# for statement

When the number of cycles is know before the loop is initiated, we can use the for statement. In this construct we declare a counter variable which is automatically increased or decreased in value during each repetition of the loop.

Simple for loop

A for loop has three phases: initialization, condition and code block execution, and incrementation.

using System;

for (int i=0; i<10; i++)
{
    Console.WriteLine(i);
}

In this example, we print numbers 0..9 to the console.

for (int i=0; i<9; i++)
{
    Console.WriteLine(i);
}

There are three phases. In the first phase, we initiate the counter i to zero. This phase is done only once. Next comes the condition. If the condition is met, the statement inside the for block is executed. In the third phase the counter is increased. Now we repeat the 2, 3 phases until the condition is not met and the for loop is left. In our case, when the counter i is equal to 10, the for loop stops executing.

For loop array traversal

A for loop can be used for traversal of an array. From the Length property of the array we know the size of the array.

using System;

string[] planets = { "Mercury", "Venus", "Earth",
    "Mars", "Jupiter", "Saturn", "Uranus", "Pluto" };

for (int i = 0; i < planets.Length; i++)
{
    Console.WriteLine(planets[i]);
}

Console.WriteLine("In reverse:");

for (int i = planets.Length - 1; i >= 0; i--)
{
    Console.WriteLine(planets[i]);
}

We have an array holding the names of planets in our Solar System. Using two for loops, we print the values in ascending and descending orders.

for (int i = 0; i < planets.Length; i++)
{
    Console.WriteLine(planets[i]);
}

The arrays are accessed by zero-based indexing. The first item has index 0. Therefore, the i variable is initialized to zero. The condition checks if the i variable is less than the length of the array. In the final phase, the i variable is incremented.

for (int i = planets.Length - 1; i >= 0; i--)
{
    Console.WriteLine(planets[i]);
}

This for loop prints the elements of the array in reverse order. The i counter is initialized to array size. Since the indexing is zero based, the last element has index array size-1. The condition ensures that the counter is greater or equal to zero. (Array indexes cannot be negative). In the third step, the i counter is decremented by one.

More expressions in for loop

More expressions can be placed in the initialization and iteration phase of the for loop.

using System;

var r = new Random();
var values = new int[10];

int sum = 0;
int num = 0;

for (int i = 0; i < 10; i++, sum += num)
{
    num = r.Next(10);
    values[i] = num;
}

Console.WriteLine(string.Join(",", values));
Console.WriteLine("The sum of the values is {0}", sum);

In our example, we create an array of ten random numbers. A sum of the numbers is calculated.

for (int i = 0; i < 10; i++, sum += num)
{
    num = r.Next(10);
    values[i] = num;
}

In the third part of the for loop, we have two expressions separated by a comma character. The i counter is incremented and the current number is added to the sum variable.

Console.WriteLine(string.Join(",", values));

Using the Join method of the System.String class, we print all the values of the array in one shot. They will be separated by a comma character.

$ dotnet run
9,3,1,7,9,8,5,6,3,3
The sum of the values is 54

Nested for loops

For statements can be nested; i.e. a for statement can be placed inside another for statement. All cycles of a nested for loops are executed for each cycle of the outer for loop.

using System;

var a1 = new string[] { "A", "B", "C" };
var a2 = new string[] { "A", "B", "C" };

for (int i=0; i<a1.Length; i++)
{
    for (int j=0; j<a2.Length; j++)
    {
        Console.WriteLine(a1[i] + a2[j]);
    }
}

In this example, we create a cartesian product of two arrays.

var a1 = new string[] { "A", "B", "C" };
var a2 = new string[] { "A", "B", "C" };

We have two arrays. Each of the arrays has tree letters. A caresian product is when each of the elements from one array is paired with all elements of the other array. To achieve this, we use a nested for loop.

for (int i=0; i<a1.Length; i++)
{
    for (int j=0; j<a2.Length; j++)
    {
        Console.WriteLine(a1[i] + a2[j]);
    }
}

There is a nested for loop inside another parent for loop. The nested for loop is executed fully for each of the cycles of the parent for loop.

$ dotnet run
AA
AB
AC
BA
BB
BC
CA
CB
CC

C# foreach statement

The foreach construct simplifies traversing over collections of data. It has no explicit counter. The foreach statement goes through the array or collection one by one and the current value is copied to a variable defined in the construct.

using System;

string[] planets = { "Mercury", "Venus", "Earth", "Mars", "Jupiter", 
    "Saturn", "Uranus", "Neptune" };

foreach (string planet in planets)
{
    Console.WriteLine(planet);
}

In this example, we use the foreach statement to go through an array of planets.

foreach (string planet in planets)
{
    Console.WriteLine(planet);
}

The usage of the foreach statement is straightforward. The planets is an array that we iterate through. The planet is a temporary variable that has the current value from the array. The foreach statement goes through all the planets and prints them to the console.

$ dotnet run
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune

C# break statement

The break statement can be used to terminate a block defined by while, for or switch statements.

using System;

var random = new Random();

while (true)
{
    int num = random.Next(1, 30);
    Console.Write("{0} ", num);

    if (num == 22)
    {
        break;
    }
}

Console.Write('\n');

We define an endless while loop. We use the break statement to get out of this loop. We choose a random value from 1 to 30. We print the value. If the value equals to 22, we finish the endless while loop.

$ dotnet run
18 3 21 26 12 27 23 25 2 21 15 4 18 12 24 13 7 19 10 26 5 22

C# continue statement

The continue statement is used to skip a part of the loop and continue with the next iteration of the loop. It can be used in combination with for and while statements.

In the following example, we print a list of numbers that cannot be divided by 2 without a remainder (odds).

using System;

int num = 0;

while (num < 1000)
{
    num++;

    if (num % 2 == 0)
    {
        continue;
    }

    Console.Write($"{num} ");
}

Console.Write('\n');

We iterate through numbers 1..999 with the while loop.

if (num % 2 == 0)
{
    continue;
}

If the expression num % 2 returns 0, the number in question can be divided by 2. The continue statement is executed and the rest of the cycle is skipped. In our case, the last statement of the loop is skipped and the number is not printed to the console. The next iteration is started.

In this part of the C# tutorial, we have covered C# control flow structures.