Home > Articles

This chapter is from the book

Item 14: Minimize Duplicate Initialization Logic

Writing constructors is often a repetitive task. Many developers write the first constructor and then copy and paste the code into other constructors to satisfy the multiple overrides defined in the class interface. Ideally, you’re not one of those. If you are, stop it. Veteran C++ programmers would factor the common algorithms into a private helper method. Stop that, too. When you find that multiple constructors contain the same logic, factor that logic into a common constructor instead. You’ll get the benefits of avoiding code duplication, and constructor initializers generate much more efficient object code. The C# compiler recognizes the constructor initializer as special syntax and removes the duplicated variable initializers and the duplicated base class constructor calls. The result is that your final object executes the minimum amount of code to properly initialize the object. You also write the least amount of code by delegating responsibilities to a common constructor.

Constructor initializers allow one constructor to call another constructor. This example shows a simple usage:

public class MyClass
{
    // collection of data
    private List<ImportantData> coll;
    // Name of the instance:
    private string name;

    public MyClass() :
        this(0, "")
    {
    }

    public MyClass(int initialCount) :
        this(initialCount, string.Empty)
    {
    }

    public MyClass(int initialCount, string name)
    {
        coll = (initialCount > 0) ?
        new List<ImportantData>(initialCount) :
        new List<ImportantData>();
        this.name = name;
    }
}

C# 4.0 added default parameters, which you can use to minimize the duplicated code in constructors. You could replace all the different constructors for MyClass with one constructor that specifies default values for all or many of the values:

public class MyClass
{
    // collection of data
    private List<ImportantData> coll;

    // Name of the instance:
    private string name;

    // Needed to satisfy the new() constraint.
    public MyClass() :
        this(0, string.Empty)
    {
    }

    public MyClass(int initialCount = 0, string name = "")
    {
        coll = (initialCount > 0) ?
        new List<ImportantData>(initialCount) :
        new List<ImportantData>();
        this.name = name;
    }
}

There are tradeoffs in choosing default parameters over using multiple overloads. Default parameters create more options for your users. This version of MyClass specifies the default value for both parameters. Users could specify different values for either or both parameters. Producing all the permutations using overloaded constructors would require four different constructor overloads: a parameterless constructor, one that asks for the initial count, one that asks for the name, and one that asks for both parameters. Add more members to your class, and the number of potential overloads grows as the number of permutations of all the parameters grows. That complexity makes default parameters a very powerful mechanism to minimize the number of potential overloads that you need to create.

Defining default values for all parameters to your type’s constructor means that user code will be valid when you call the new MyClass(). When you intend to support this concept, you should create an explicit parameterless constructor in that type, as shown in the example code above. While most code would default all parameters, generic classes that use the new() constraint will not accept a constructor with parameters that have default values. To satisfy the new() constraint, a class must have an explicit parameterless constructor. Therefore, you should create one so that clients can use your type in generic classes or methods that enforce the new() constraint. That’s not to say that every type needs a parameterless constructor. However, if you support one, make sure to add the code so that the parameterless constructor works in all cases, even when called from a generic class with a new() constraint.

You’ll note that the second constructor specifies “” for the default value on the name parameter, rather than the more customary string.Empty. That’s because string.Empty is not a compile-time constant. It is a static property defined in the string class. Because it is not a compile-time constant, you cannot use it for the default value for a parameter.

However, using default parameters instead of overloads creates tighter coupling between your class and all the clients that use it. In particular, the formal parameter name becomes part of the public interface, as does the current default value. Changing parameter values requires a recompile of all client code in order to pick up those changes. That makes overloaded constructors more resilient in the face of potential future changes. You can add new constructors, or change the default behavior for those constructors that don’t specify values, without breaking client code.

Default parameters are the preferred solution to this problem. However, some APIs use reflection to create objects and rely on a parameterless constructor. A constructor with defaults supplied for all arguments is not the same as a parameterless constructor. You may need to write separate constructors that you support as a separate function. With constructors, that can mean a lot of duplicated code. Use constructor chaining, by having one constructor invoke another constructor declared in the same class, instead of creating a common utility routine. Several inefficiencies are present in this alternative method of factoring out common constructor logic:

public class MyClass
{
    private List<ImportantData> coll;
    private string name;

    public MyClass()
    {
        commonConstructor(0, "");
    }


    public MyClass(int initialCount)
    {
        commonConstructor(initialCount, "");
    }

    public MyClass(int initialCount, string Name)
    {
        commonConstructor(initialCount, Name);
    }

    private void commonConstructor(int count,
    string name)
    {
        coll = (count > 0) ?
        new List<ImportantData>(count) :
        new List<ImportantData>();
        this.name = name;
    }
}

That version looks the same, but it generates far-less-efficient object code. The compiler adds code to perform several functions on your behalf in constructors. It adds statements for all variable initializers (see Item 12 earlier in this chapter). It calls the base class constructor. When you write your own common utility function, the compiler cannot factor out this duplicated code. The IL for the second version is the same as if you’d written this:

public class MyClass
{
    private List<ImportantData> coll;
    private string name;

    public MyClass()
    {
        // Instance Initializers would go here.
        object(); // Not legal, illustrative only.
        commonConstructor(0, "");
    }


    public MyClass(int initialCount)
    {
        // Instance Initializers would go here.
        object(); // Not legal, illustrative only.
        commonConstructor(initialCount, "");
    }

    public MyClass(int initialCount, string Name)
    {
        // Instance Initializers would go here.
        object(); // Not legal, illustrative only.
        commonConstructor(initialCount, Name);
    }

    private void commonConstructor(int count,
    string name)
    {
        coll = (count > 0) ?
        new List<ImportantData>(count) :
        new List<ImportantData>();
        this.name = name;
    }
}

If you could write the construction code for the first version the way the compiler sees it, you’d write this:

// Not legal, illustrates IL generated:
public class MyClass
{
    private List<ImportantData> coll;
    private string name;

    public MyClass()
    {
        // No variable initializers here.
        // Call the third constructor, shown below.
        this(0, ""); // Not legal, illustrative only.
    }


    public MyClass(int initialCount)
    {
        // No variable initializers here.
        // Call the third constructor, shown below.
        this(initialCount, "");
    }

    public MyClass(int initialCount, string Name)
    {
        // Instance Initializers would go here.
        //object(); // Not legal, illustrative only.
        coll = (initialCount > 0) ?
        new List<ImportantData>(initialCount) :
        new List<ImportantData>();
        name = Name;
    }
}

The difference is that the compiler does not generate multiple calls to the base class constructor, nor does it copy the instance variable initializers into each constructor body. The fact that the base class constructor is called only from the last constructor is also significant: You cannot include more than one constructor initializer in a constructor definition. You can delegate to another constructor in this class using this(), or you can call a base class constructor using base(). You cannot do both.

Still don’t buy the case for constructor initializers? Then think about read-only constants. In this example, the name of the object should not change during its lifetime. This means that you should make it read-only. That causes the common utility function to generate compiler errors:

public class MyClass
{
    // collection of data
    private List<ImportantData> coll;
    // Number for this instance
    private int counter;
    // Name of the instance:
    private readonly string name;


    public MyClass()
    {
        commonConstructor(0, string.Empty);
    }

    public MyClass(int initialCount)
    {
        commonConstructor(initialCount, string.Empty);
    }

    public MyClass(int initialCount, string Name)
    {
        commonConstructor(initialCount, Name);
    }

    private void commonConstructor(int count,
    string name)
    {
        coll = (count > 0) ?
        new List<ImportantData>(count) :
        new List<ImportantData>();
        // ERROR changing the name outside of a constructor.
        //this.name = name;
    }
}

The compiler enforces the read-only nature of this.name and will not allow any code not in a constructor to modify it. C#’s constructor initializers provide the alternative. All but the most trivial classes contain more than one constructor. Their job is to initialize all the members of an object. By their very nature, these functions have similar or, ideally, shared logic. Use the C# constructor initializer to factor out those common algorithms so that you write them once and they execute once.

Both default parameters and overloads have their place. In general, you should prefer default values to overloaded constructors. After all, if you are letting client developers specify parameter values at all, your constructor must be capable of handling any values that users specify. Your original default values should always be reasonable and shouldn’t generate exceptions. Therefore, even though changing the default parameter values is technically a breaking change, it shouldn’t be observable to your clients. Their code will still use the original values, and those original values should still produce reasonable behavior. That minimizes the potential hazards of using default values.

This is the last item about object initialization in C#. That makes it a good time to review the entire sequence of events for constructing an instance of a type. You should understand both the order of operations and the default initialization of an object. You should strive to initialize every member variable exactly once during construction. The best way for you to accomplish this is to initialize values as early as possible. Here is the order of operations for constructing the first instance of a type:

  1. Static variable storage is set to 0.

  2. Static variable initializers execute.

  3. Static constructors for the base class execute.

  4. The static constructor executes.

  5. Instance variable storage is set to 0.

  6. Instance variable initializers execute.

  7. The appropriate base class instance constructor executes.

  8. The instance constructor executes.

Subsequent instances of the same type start at step 5 because the class initializers execute only once. Also, steps 6 and 7 are optimized so that constructor initializers cause the compiler to remove duplicate instructions.

The C# language compiler guarantees that everything gets initialized in some way when an object is created. At a minimum, you are guaranteed that all memory your object uses has been set to 0 when an instance is created. This is true for both static members and instance members. Your goal is to make sure that you initialize all the values the way you want and execute that initialization code only once. Use initializers to initialize simple resources. Use constructors to initialize members that require more sophisticated logic. Also factor calls to other constructors to minimize duplication.

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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