The while statement evaluates a Boolean expression and continues to loop while it is true.

Type the following code inside the Main method:

int x = 0; 
while (x < 10) 
{ 
   WriteLine(x); 
   x++; 
} 

Run the console application and view the output:

0123456789

The do statement is like while, except the Boolean expression is checked at the bottom of the block instead of the top, which means that it always executes at least once.

Type the following code at the end of the Main method and run it:

string password = string.Empty; 
do 
{ 
   Write("Enter your password: "); 
   password = ReadLine(); 
} while (password != "secret"); 
WriteLine("Correct!"); 

You will be prompted to enter your password repeatedly until you enter it correctly, as shown in the following output:

Enter your password: password
Enter your password: 12345678
Enter your password: ninja
Enter your password: asdfghjkl
Enter your password: secret
Correct!

As an optional exercise, add statements so that the user can only make ten attempts before an error message is displayed.

The for statement is like while, except that it is more succinct. It combines an initializer statement that executes once at the start of the loop, a Boolean expression to check whether the loop should continue, and an incrementer that executes at the bottom of the loop. The for statement is commonly used with an integer counter, as shown in the following code:

for (int y = 1; y <= 10; y++)
{
   WriteLine(y);
}

Run the console application and view the output, which should be the numbers 1 to 10.

The foreach statement is a bit different from the other three iteration statements. It is used to perform a block of statements on each item in a sequence, for example, an array or collection. Each item is read-only, and if the sequence is modified during iteration, for example, by adding or removing an item, then an exception will be thrown.

Type the following code inside the Main method, which creates an array of string variables and then outputs the length of each of them:

string[] names = { "Adam", "Barry", "Charlie" };
foreach (string name in names)
{
   WriteLine($"{name} has {name.Length} characters.");
}

Run the console application and view the output:

Adam has 4 characters.
Barry has 5 characters.
Charlie has 7 characters.

Technically, the foreach statement will work on any type that implements an interface called IEnumerable. An interface is a contract and you will learn more about them in Chapter 4,Implementing Interfaces and Inheriting Classes.

The compiler turns the foreach statement in the preceding code into something like this:

IEnumerator e = names.GetEnumerator();
while (e.MoveNext())
{ 
   string name = (string)e.Current; // Current is read-only!
   WriteLine($"{name} has {name.Length} characters.");
}