While

Most of the time, a foreach-loop is the clearest in C# code. But sometimes we need an infinite loop, or one that continues until a condition is met.

This loop continues until its expression evaluates to false. We can use a while-true loop for an infinite loop—or one with complex exit conditions.

Example

This program shows a while-loop. The while-keyword is followed by an expression, and the expression must evaluate to a boolean value (true or false).

using System;

// Continue in while-loop until index is equal to 10.
int i = 0;
while (i < 10)
{
    Console.Write("While statement ");
    // Write the index to the screen.
    Console.WriteLine(i);
    // Increment the variable.
    i++;
}
While statement 0
While statement 1
While statement 2
While statement 3
While statement 4
While statement 5
While statement 6
While statement 7
While statement 8
While statement 9

While true

We can use "true" in the expression of a while-loop. This program uses a while-true loop. An empty while-loop with this condition is (by definition) an infinite loop.

using System;

int index = 0;
// Continue looping infinitely until internal condition is met.
while (true)
{
    int value = ++index;
    // Check to see if limit is hit.
    if (value > 5)
    {
        Console.WriteLine("While-loop break");
        break;
    }
    // You can add another condition.
    if (value > 100)
    {
        throw new Exception("Never hit");
    }
    // Write to the screen on each iteration.
    Console.WriteLine("While-loop statement");
}
While-loop statement
While-loop statement
While-loop statement
While-loop statement
While-loop statement
While-loop break

Variable assignment

Here we assign a variable in a while-loop's expression. We can alias a variable for use in the loop body. This is useful if the expression is slow or complex.

using System;

class Program
{
    static void Main()
    {
        int value = 4;
        int i;
        // You can assign a variable in the while-loop condition statement.
        while ((i = value) >= 0)
        {
            // In the while-loop body, both "i" and value are equal.
            Console.WriteLine("While {0} {1}", i, value);
            value--;
        }
    }
}
While 4 4
While 3 3
While 2 2
While 1 1
While 0 0

Implicitly convert type error

The condition of a while-loop must evaluate to true or false. In C# an int never evaluates to true or false—we must use an equality test.

class Program
{
    static void Main()
    {
        int number = 10;
        while (number)
        {
        }
    }
}
Error CS0029
Cannot implicitly convert type 'int' to 'bool'
class Program
{
    static void Main()
    {
        int number = 10;
        while (number > 0)
        {
            number--;
        }
    }
}

Do While

This is a less common loop in C#. It is an inverted version of the while-loop. It executes the loop statements unconditionally the first time.

using System;

class Program
{
    static void Main()
    {
        int number = 0;
        // Begin do-while loop.
        // ... Terminates when number equals 2.
        do
        {
            Console.WriteLine(number);
            // Add one to number.
            number++;
        } while (number <= 2);
    }
}
0
1
2

Empty statement

Suppose we want a while-loop with no statements in its body. We can use an empty statement. An empty block would also work.

using System;

class Program
{
    static void Main()
    {
        // Use an empty statement as the body of the while-loop.
        while (Method())
            ;
    }

    static int _number;

    static bool Method()
    {
        // Write the number, increment, and then return a bool.
        Console.WriteLine(_number);
        _number++;
        return _number < 5;
    }
}
0
1
2
3
4

Jump statements

In the C# language we can use break, return and goto to change control flow in a while-loop. We can think of control flow (how statements are encountered) like a river.

using System;

class Program
{
    static void ReturnIf5()
    {
        // Keep looping until a condition is reached, and return then.
        int index = 0;
        while (true)
        {
            if (index == 5)
            {
                Console.WriteLine("5 REACHED");
                return;
            }
            Console.WriteLine("NOT FOUND");
            index++;
        }
    }

    static void Main()
    {
        ReturnIf5();
    }
}
NOT FOUND
NOT FOUND
NOT FOUND
NOT FOUND
NOT FOUND
5 REACHED

A while-loop continues until its condition expression is false. This can make looping logic easier to reason about—which is a significant benefit.