Home > Articles > Programming > Windows Programming

Delegates vs. Interfaces in .NET

  • Print
  • + Share This
When you write .NET code that takes callback parameters, your first instinct may be to use delegates, but it may not occur to you that you can use an interface instead. Learn why and when an interface may be a better answer.
Like this article? We recommend

You've almost certainly written at least one callback function. Callback functions are functions you don't call directly. They're called, using a pointer, from within some library code that you do call directly, passing the callback function's address as a parameter. The odds aren't quite so high that you've written code that takes and uses callback parameters, but the odds aren't all that low, either, as this is also a fairly common task.

Sometimes, the callback is only called once, given access to a specific part of a monstrous or opaque data structure. More commonly, your callback is called many times, in the process of selecting items from a list, or telling library code when to stop looping.

Chances are, you probably prefer to write callback code that takes strongly typed parameters. That lets you be reasonably sure a non-null parameter is a pointer to a method with the right prototype, and not a pointer to a method with a different prototype, or even a pointer to data.

When it comes to writing .NET code that takes callback parameters, your first instinct may be to use delegates, which look a lot like strongly typed method pointers in C++ or Delphi. It may not occur to you that you can implement a callback parameter as an interface instead. Interfaces and delegates share the key property of allowing you to call a method with the right prototype without knowing which object implements the method, which instance is bound to the call, and even the name of the method you're calling.

Interface calls are faster than delegate calls. An interface reference is a reference to an instance of an object which implements the interface. An interface call is not that different from an ordinary virtual call to an method. A delegate reference, on the other hand, is a reference to a list of method pointers. While invoking a delegate looks like you're making an indirect call through a method pointer, it's actually a subroutine call that walks the list of method pointers. The overhead involved in making the call and walking the list means that delegate invocation can be two or three times slower than calling a method through an interface reference.

Interfaces are also a bit more general than are delegates. A single interface reference gives you access to all the methods of the interface. You can also check if the interface is implemented by This object type, or if the object also implements That other interface. If it does, you can cast the interface reference to an instance reference, or to a reference to another interface. Conversely, you can not go from a delegate to the instances it will call, or to any other methods those instances may support.

However, don't conclude from this that you should always implement callbacks via interfaces, not delegates. One key difference between delegates and interfaces is that you can create a delegate to any method with the right prototype.

However, you can only convert an instance reference to an interface reference if the object explicitly implements the interface. That is, while an object can “implement” an interface entirely with inherited methods, it is not enough for an object to support all the interface's methods. This means that you can't use an interface reference to callback to methods of value types or of sealed classes, unless they already support the interface.

For example, given

interface IToString
{
  string ToString();
}
class Implementor: IToString
{
}

you can say

IToString Reference1 = new Implementor()

because the Implementor class implements IToString with the ToString method it inherits from System.Object. But you can't say

IToString Reference2 = new object()

because the System.Object class does not explicitly implement IToString.

In other words, callback code that takes delegate parameters can be a bit easier to use than callback code that takes interface parameters. You can create a delegate to any (reference or value type) object which has a method of the right signature. If you want to get an interface reference to an object which does have a method of the right signature but which doesn't explicitly support the right interface, you do have to create a subclass (like the Implementor class) which implements the interface with inherited methods.

This is not much of a limitation. Callback methods are often methods of the class that calls the code that does the callback, or perhaps they are methods of a small class nested within the class that calls the code that does the callback. It's easy for the author of the class that calls the code that does the callback to explicitly support a callback interface—and using a callback interface instead of a callback predicate means your code will be faster, especially if you call the callback many times.

In general, the only time you should use delegates for callbacks is when there is some chance that a user might want to pass a method of an existing sealed type.

  • + Share This
  • 🔖 Save To Your Account

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