Control Flow in C#

If we just write lines of code one after another, we would have no way to make decisions and take different paths in our code. To allow us this flexibility we have the idea of flow control statements.

The first and more simple of these is the if statement.

If

The if statement has this basic structure:

if (someBooleanCondition) {
  // Code if the condition is true
}

We can also handle the case where the condition is false:

if (someBooleanCondition) {
  // Code if the condition is true
}
else {
  // Code if the condition is false
}

The part someBooleanCondition can be a boolean variable or a condition that evaluates to a boolean value.

Boolean Conditions

Here are some examples of statements that result in a boolean value. As you can see we can compare strings to other strings, and numbers to other numbers.

Examples:

if (name == "Paul") {
  Console.WriteLine("Greetings!");
} else {
  Console.WriteLine("Wait, who are you?");
}

Boolean Or, Boolean And

We can combine two conditions using a boolean or and a boolean and. In C# we write an or as || with and being &&

So if we wanted a condition that is true if the score is lower than 20 OR greater than 90:

if (score < 20 || score > 90) {
  // Some code here
}

What if we wanted a condition that is true if the person is named Paul AND the score is more than 85:

if (name == "Paul" && score > 85) {
  // Some code here
}

As you can see we can have conditions on different types together. This is because each part of the boolean statement results in a boolean and then these are combined.

The following truth tables show the various combinations of true and false values and how they combine for && (and) as well as || (or):

A && B

A || B

Chaining if statements

A series of if/else statements can be chained.

if (name == "Paul") {
  Console.WriteLine("Here");
} else if (name == "Dorothy") {
  Console.WriteLine("Also here");
} else if (name == "Sam") {
  Console.WriteLine("Here again");
} else {
  Console.WriteLine("Didn't find anything");
}

Cleaning up a long sequence of if/else

When we find ourselves comparing the same variable to a sequence of values we can use another kind of control flow known as a switch statement.

switch (name)
{
    case "Paul":
        Console.WriteLine("Here");
        break;
    case "Dorothy":
        Console.WriteLine("Also Here");
        break;
    case "Sam":
        Console.WriteLine("Here Again");
        break;
    default:
        Console.WriteLine("Didn't find anything");
        break;
}

The switch statement is a cleaner way of writing this type of control flow. It also comes with some additional neat features.

For instance we can handle multiple values by repeating the case statement:

switch (name)
{
    case "Paul":
    case "Peter":
    case "Mary":
        Console.WriteLine("Here");
        break;
    case "Dorothy":
        Console.WriteLine("Also Here");
        break;
    case "Sam":
        Console.WriteLine("Here Again");
        break;
    default:
        Console.WriteLine("Didn't find anything");
        break;
}

In this code we will see the message Here for name if it is either Paul OR Peter OR Mary

Let's say we are working with an int variable named score and we wanted to print a grade associated to a score.

We can use the ability of the case statement to allow us to use conditionals to compare the score to a value. Here we are using both < and >= in these comparisons.

var score = 95;
switch (score)
{
    case < 65:
        Console.WriteLine("F");
        break;
    case < 70:
        Console.WriteLine("D");
        break;
    case < 80:
        Console.WriteLine("C");
        break;
    case < 90:
        Console.WriteLine("B");
        break;
    case >= 90:
        Console.WriteLine("A");
        break;
    default:
        Console.WriteLine("Hmmm, I don't recognize this score");
        break;
}

Repeating code

So far all of the control flow we've seen keeps our code processing line-by-line and proceeding to the end of the code. What if we want to repeat some code?

This is where a loop style control flow helps us. The first we will see is the while statement.

The while statement repeats the code inside the { } braces as long as the condition supplied remains true.

Let's say we want to ask the user their name and greet them until the user enters the text quit. The code would look like this:

Console.Write("What is your name? ");
var name = Console.ReadLine();
while (name != "quit") {
  Console.WriteLine($"Hello {name}");
  Console.Write("What is your name? ");
  name = Console.ReadLine();
}