Home > Articles > Programming > C#

This chapter is from the book

4.4 Constructed Types

A generic type declaration, by itself, denotes an unbound generic type that is used as a “blueprint” to form many different types, by way of applying type arguments. The type arguments are written within angle brackets (< and >) immediately following the name of the generic type. A type that includes at least one type argument is called a constructed type. A constructed type can be used in most places in the language in which a type name can appear. An unbound generic type can be used only within a typeof-expression (§7.6.11).

Constructed types can also be used in expressions as simple names (§7.6.2) or when accessing a member (§7.6.4).

When a namespace-or-type-name is evaluated, only generic types with the correct number of type parameters are considered. Thus it is possible to use the same identifier to identify different types, as long as the types have different numbers of type parameters. This is useful when mixing generic and nongeneric classes in the same program:

namespace Widgets
    class Queue {...}
    class Queue<TElement> {...}

namespace MyApplication
    using Widgets;

    class X
        Queue q1;               // Nongeneric Widgets.Queue
        Queue<int> q2;          // Generic Widgets.Queue

A type-name might identify a constructed type even though it doesn’t specify type parameters directly. This can occur where a type is nested within a generic class declaration, and the instance type of the containing declaration is implicitly used for name lookup (§

class Outer<T>
    public class Inner {...}

    public Inner i;                     // Type of i is Outer<T>.Inner

In unsafe code, a constructed type cannot be used as an unmanaged-type (§18.2).

4.4.1 Type Arguments

Each argument in a type argument list is simply a type.

    < type-arguments >

    type-arguments , type-argument


In unsafe code (§18), a type-argument may not be a pointer type. Each type argument must satisfy any constraints on the corresponding type parameter (§10.1.5).

4.4.2 Open and Closed Types

All types can be classified as either open types or closed types. An open type is a type that involves type parameters. More specifically:

  • A type parameter defines an open type.
  • An array type is an open type if and only if its element type is an open type.
  • A constructed type is an open type if and only if one or more of its type arguments is an open type. A constructed nested type is an open type if and only if one or more of its type arguments or the type arguments of its containing type(s) is an open type.

A closed type is a type that is not an open type.

At runtime, all of the code within a generic type declaration is executed in the context of a closed constructed type that was created by applying type arguments to the generic declaration. Each type parameter within the generic type is bound to a particular runtime type. The runtime processing of all statements and expressions always occurs with closed types, and open types occur only during compile-time processing.

Each closed constructed type has its own set of static variables, which are not shared with any other closed constructed types. Since an open type does not exist at runtime, there are no static variables associated with an open type. Two closed constructed types are the same type if they are constructed from the same unbound generic type, and their corresponding type arguments are the same type.

4.4.3 Bound and Unbound Types

The term unbound type refers to a nongeneric type or an unbound generic type. The term bound type refers to a nongeneric type or a constructed type.

An unbound type refers to the entity declared by a type declaration. An unbound generic type is not itself a type, and it cannot be used as the type of a variable, argument, or return value, or as a base type. The only construct in which an unbound generic type can be referenced is the typeof expression (§7.6.11).

4.4.4 Satisfying Constraints

Whenever a constructed type or generic method is referenced, the supplied type arguments are checked against the type parameter constraints declared on the generic type or method (§10.1.5). For each where clause, the type argument A that corresponds to the named type parameter is checked against each constraint as follows:

  • If the constraint is a class type, an interface type, or a type parameter, let C represent that constraint with the supplied type arguments substituted for any type parameters that appear in the constraint. To satisfy the constraint, it must be the case that type A is convertible to type C by one of the following:
    • An identity conversion (§6.1.1).
    • An implicit reference conversion (§6.1.6).
    • A boxing conversion (§6.1.7), provided that type A is a non-nullable value type.
    • An implicit reference, boxing, or type parameter conversion from a type parameter A to C.
  • If the constraint is the reference type constraint (class), the type A must satisfy one of the following:
    • A is an interface type, class type, delegate type, or array type. Both System.ValueType and System.Enum are reference types that satisfy this constraint.
    • A is a type parameter that is known to be a reference type (§10.1.5).
  • If the constraint is the value type constraint (struct), the type A must satisfy one of the following:
    • A is a struct type or enum type, but not a nullable type. Both System.ValueType and System.Enum are reference types that do not satisfy this constraint.
    • A is a type parameter having the value type constraint (§10.1.5).
  • If the constraint is the constructor constraint new(), the type A must not be abstract and must have a public parameterless constructor. This is satisfied if one of the following is true:
    • A is a value type, since all value types have a public default constructor (§4.1.2).
    • A is a type parameter having the constructor constraint (§10.1.5).
    • A is a type parameter having the value type constraint (§10.1.5).
    • A is a class that is not abstract and contains an explicitly declared public constructor with no parameters.
    • A is not abstract and has a default constructor (§10.11.4).

A compile-time error occurs if one or more of a type parameter’s constraints are not satisfied by the given type arguments.

Since type parameters are not inherited, constraints are never inherited either. In the example below, D needs to specify the constraint on its type parameter T so that T satisfies the constraint imposed by the base class B<T>. In contrast, class E need not specify a constraint, because List<T> implements IEnumerable for any T.

class B<T> where T: IEnumerable {...}

class D<T>: B<T> where T: IEnumerable {...}

class E<T>: B<List<T>> {...}

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