C# do-while Loop : A Tutorial

The do-while loop in C# is a control flow statement that executes a block of code at least once, and then repeats execution while a specified condition remains true.

It is useful when the loop must run at least one time, regardless of the condition.

This tutorial covers:

• Syntax of the do-while Loop
• How the do-while Loop Works
• Example Usage of do-while Loop
• Using break and continue in a do-while Loop
• Handling User Input with a do-while Loop
• Using a do-while Loop with Arrays and Lists
• Difference Between while and do-while Loops
• Best Practices for Using do-while Loops

1. Syntax of the do-while Loop

The do-while loop executes the loop body first, then checks the condition after each iteration.

Syntax

do
{
    // Code to be executed
} while (condition);

• The loop executes at least once because the condition is checked after execution.

2. How the do-while Loop Works

1. The loop body executes once, regardless of the condition.
2. The condition is checked after execution.
3. If true, the loop repeats.
4. If false, the loop stops.

Example: Basic do-while Loop

int count = 1;

do
{
    Console.WriteLine("Iteration " + count);
    count++; // Increment count
} while (count <= 5);

Output:

Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5

• count starts at 1 and increments by 1 each time.
• The loop stops when count > 5.

3. Example Usage of do-while Loop

Ensuring Code Runs at Least Once

int number = 10;

do
{
    Console.WriteLine("Number is: " + number);
} while (number < 5);

Output:

Number is: 10

• Even though number < 5 is false, the loop runs once before stopping.

4. Using break and continue in a do-while Loop

Using break to Exit the Loop

The break statement immediately exits the loop, regardless of the condition.

int num = 1;

do
{
    if (num == 5)
    {
        Console.WriteLine("Stopping loop at " + num);
        break; // Exit the loop
    }
    Console.WriteLine(num);
    num++;
} while (num <= 10);

Output:

1
2
3
4
Stopping loop at 5

• The loop stops when num == 5.

Using continue to Skip an Iteration

The continue statement skips the current iteration and moves to the next.

int num = 0;

do
{
    num++;

    if (num == 3)
    {
        Console.WriteLine("Skipping " + num);
        continue; // Skips iteration when num == 3
    }

    Console.WriteLine(num);
} while (num < 5);

Output:

1
2
Skipping 3
4
5

• The loop skips num = 3 but continues running.

5. Handling User Input with a do-while Loop

A do-while loop is often used to repeatedly ask for user input until a valid response is given.

Example: Asking for a Valid Number

int number;

do
{
    Console.Write("Enter a number greater than 0: ");
} while (!int.TryParse(Console.ReadLine(), out number) || number <= 0);

Console.WriteLine("You entered: " + number);

• TryParse() ensures only valid numbers are accepted.
• The loop repeats until valid input is given.

6. Using a do-while Loop with Arrays and Lists

Iterating Through an Array Using do-while

int[] numbers = { 10, 20, 30, 40, 50 };
int index = 0;

do
{
    Console.WriteLine(numbers[index]);
    index++; // Move to the next index
} while (index < numbers.Length);

Output:

10
20
30
40
50

• The loop runs until the end of the array.

Iterating Through a List Using do-while

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List<string> names = new List<string> { "Alice", "Bob", "Charlie" };
        int index = 0;

        do
        {
            Console.WriteLine(names[index]);
            index++;
        } while (index < names.Count);
    }
}

Output:

Alice
Bob
Charlie

• names.Count provides the list size.

7. Difference Between while and do-while Loops

Example: while vs do-while

int number = 10;

// while loop (does not execute because condition is false)
while (number < 5)
{
    Console.WriteLine("This will not print.");
}

// do-while loop (executes once before checking condition)
do
{
    Console.WriteLine("This will print once.");
} while (number < 5);

Output:

This will print once.

• The while loop does not execute because number < 5 is false.
• The do-while loop executes once before checking.

8. Best Practices for Using do-while Loops

Use a do-while Loop When the Code Must Run at Least Once

do
{
    Console.WriteLine("This will always run at least once.");
} while (false);

• Ensures execution at least once.

Use break to Exit the Loop If Needed

do
{
    string input = Console.ReadLine();
    if (input == "exit")
    {
        break; // Exit the loop when "exit" is entered
    }
} while (true);

• break is useful for user-controlled exits.

Use continue to Skip Unwanted Iterations

do
{
    int value = new Random().Next(1, 10);
    if (value % 2 == 0) continue; // Skip even numbers
    Console.WriteLine(value);
} while (true);

• continue helps skip unnecessary operations.

Use TryParse() for Safe User Input

do
{
    Console.Write("Enter a number: ");
} while (!int.TryParse(Console.ReadLine(), out int value));

• This prevents crashes from invalid input.

Conclusion

• The do-while loop executes code at least once before checking the condition.
• Use do-while when an action must run at least once.
• Use break to exit early and continue to skip an iteration.
• Ensure the loop condition eventually becomes false to avoid infinite loops.
• Use do-while loops for user input validation and processing lists/arrays.