Home > Articles > Programming > C#

This chapter is from the book

This chapter is from the book

Introducing Flow Control

Later in this chapter is a code listing (Listing 3.43) that shows a simple way to view a number in its binary form. Even such a simple program, however, cannot be written without using control flow statements. Such statements control the execution path of the program. This section discusses how to change the order of statement execution based on conditional checks. Later on, you will learn how to execute statement groups repeatedly through loop constructs.

A summary of the control flow statements appears in Table 3.1. Note that the General Syntax Structure column indicates common statement use, not the complete lexical structure.

Table 3.1. Control Flow Statements


General Syntax Structure


if statement

if (input == "quit")
    "Game end");

if (input == "quit")
    "Game end");

while statement

while(count < total)
    "count = {0}", count);

do while statement

  embedded-statement while(boolean-expression);
    "Enter name:");
  input =
while(input != "exit")

for statement

for (int count = 1;
   count <= 10;
    "count = {0}", count);

foreach statement

foreach(type identifier in
foreach (char letter in email)
{  if(!insideDomain)
    if (letter == '@')
      insideDomain = true;

continue statement


switch statement

  case const-expression:
  case "exit":
  case "quit":
      "Exiting app....");
  case "restart":
    goto case "start";
  case "start":

break statement


goto statement

goto identifier;
goto case const-expression;
goto default;

An embedded-statement in Table 3.1 may be any statement other than a labeled statement or a declaration, but it is typically a block statement.

Each C# control flow statement in Table 3.1 appears in the tic-tac-toe3 program and is available in Appendix B and for download with the rest of the source code listings from the book. The program displays the tic-tac-toe board, prompts each player, and updates with each move.

The remainder of this chapter looks at each statement in more detail. After covering the if statement, it introduces code blocks, scope, Boolean expressions, and bitwise operators before continuing with the remaining control flow statements. Readers who find the table familiar because of C#’s similarities to other languages can jump ahead to the section titled C# Preprocessor Directives or skip to the Summary section at the end of the chapter.

The remainder of this chapter looks at each statement in more detail. After covering the if statement, it introduces code blocks, scope, Boolean expressions, and bitwise operators before continuing with the remaining control flow statements. Readers who find the table familiar because of C#’s similarities to other languages can jump ahead to the section titled C# Preprocessor Directives or skip to the Summary section at the end of the chapter.

if Statement

The if statement is one of the most common statements in C#. It evaluates a Boolean expression (an expression that results in either true or false) called the condition. If the condition is true, the consequence statement is executed. An if statement may optionally have an else clause that contains an alternative statement to be executed if the condition is false. The general form is as follows:

if (condition)

Listing 3.20. if/else Statement Example

class TicTacToe      // Declares the TicTacToe class.
  static void Main() // Declares the entry point of the program.
      string input;

      // Prompt the user to select a 1- or 2-player game.
      System.Console.Write (
          "1 – Play against the computer\n" +
          "2 – Play against another player.\n" +
      input = System.Console.ReadLine();

          // The user selected to play the computer.                                       
              "Play against computer selected.");                                          
          // Default to 2 players (even if user didn't enter 2).                           
              "Play against another player.");                                             

In Listing 3.20, if the user enters 1, the program displays "Play against computer selected.". Otherwise, it displays "Play against another player.".

Nested if

Sometimes code requires multiple if statements. The code in Listing 3.21 first determines whether the user has chosen to exit by entering a number less than or equal to 0; if not, it checks whether the user knows the maximum number of turns in tic-tac-toe.

Listing 3.21. Nested if Statements

1.  class TicTacToeTrivia
2.  {
3.    static void Main()
4.    {
5.        int input;    // Declare a variable to store the input.
7.        System.Console.Write(
8.            "What is the maximum number " +
9.            "of turns in tic-tac-toe?" +
10.           "(Enter 0 to exit.): ");
12.       // int.Parse() converts the ReadLine()
13.       // return to an int data type.
14.       input = int.Parse(System.Console.ReadLine());
16.       if (input <= 0)
17.               // Input is less than or equal to 0.
18.           System.Console.WriteLine("Exiting...");
19.       else
20.           if (input < 9)
21.               // Input is less than 9.
22.               System.Console.WriteLine(
23.                   "Tic-tac-toe has more than {0}" +
24.                   " maximum turns.", input);
25.           else
26.               if(input>9)
27.                   // Input is greater than 9.
28.                   System.Console.WriteLine(
29.                       "Tic-tac-toe has fewer than {0}" +
30.                       " maximum turns.", input);
31.               else
32.                   // Input equals 9.
33.                   System.Console.WriteLine(
34.                       "Correct, tic-tac-toe " +
35.                       "has a max. of 9 turns.");
36.   }
37. }

Output 3.13 shows the results of Listing 3.21.

Output 3.13.

What is the maximum number of turns in tic-tac-toe? (Enter 0 to exit.): 9
Correct, tic-tac-toe has a max. of 9 turns.

Assume the user enters 9 when prompted at line 14. Here is the execution path.

  1. Line 16: Check if input is less than 0. Since it is not, jump to line 20.
  2. Line 20: Check if input is less than 9. Since it is not, jump to line 26.
  3. Line 26: Check if input is greater than 9. Since it is not, jump to line 33.
  4. Line 33: Display that the answer was correct.

Listing 3.21 contains nested if statements. To clarify the nesting, the lines are indented. However, as you learned in Chapter 1, whitespace does not affect the execution path. Without indenting and without newlines, the execution would be the same. The code that appears in the nested if statement in Listing 3.22 is equivalent to Listing 3.21.

Listing 3.22. if/else Formatted Sequentially

if (input < 0)
else if (input < 9)
      "Tic-tac-toe has more than {0}" +
      " maximum turns.", input);
else if(input < 9)
      "Tic-tac-toe has less than {0}" +
      " maximum turns.", input);
      "Correct, tic-tac-toe has a maximum " +
      " of 9 turns.");

Although the latter format is more common, in each situation use the format that results in the clearest code.

Each if statement listing above omits the use of braces. However, as discussed next, this is not in accordance with the guidelines, which advocate the use of code blocks except, perhaps, in the simplest of single-line scenarios.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020