Home > Articles > Programming

Using COM to Develop UMDF Drivers, Part 1

In part 1 of this two-part series, Ronald D. Reeves, Ph.D., the author of Windows 7 Device Driver, discusses how to work with the Component Object Model (COM) programming model to create lightweight User Mode Driver Framework (UMDF) drivers.
Like this article? We recommend

Like this article? We recommend

In previous articles we looked at the architecture of the Windows 7 User Mode Driver Framework (UMDF), the programming approach for developing UMDF drivers, and specifically in the previous article how to program drivers for the UMDF. In this article, we'll examine the Component Object Model (COM), and its use in creating user mode drivers. COM is a specification for a way of building applications based on using components. In the traditional sense, applications were divided into files, modules, or classes; then they were linked to form a monolithic application. By contrast, a component is like a small application that comes packaged as a binary piece of code that is compiled, linked, and ready to use. This piece of code then links with other components at runtime to form an application. One of the great features of this approach is that you can change or enhance the application by replacing one of the components.

This article introduces COM and its use in creating UMDF drivers. For our work in UMDF driver development, we must use COM objects that are part of the Windows Driver Foundation (WDF) framework. We need to create a number of UMDF driver callback objects, based on COM-based callback objects. In the UMDF driver development, we won't use the COM runtime, which contains a good deal of complexity, but we will use the essential core of the COM programming model. This keeps the UMDF driver fairly lightweight and thus relatively easy to implement.

Getting Started

In general, UMDF drivers are programmed using C++, and COM objects are developed and also written in C++. It's good to have an understanding of class structure, such as the struct and class keywords, public and private members, static methods, constructors, destructors, and pure abstract classes. Also, you should understand object creation, which includes base and derived classes, multiple inheritance, and pure virtual methods. Paul Deitel and Harvey M. Deitel's book C++ How to Program, Seventh Edition is a good reference to review to help you get an understanding of these basic concepts. In UMDF drivers, the operator overloading or templates are not necessary. The UMDF drivers can use-but are not required to use-the C++ standard template libraries.

COM Fundamentals

Let's start by looking at some of the fundamental aspects of COM:

  • IUnknown is the core COM interface, from which all other COM interfaces derive. Every COM object exposes this interface, and it's essential to the object's operation.
  • One significant difference between objects in COM and other object-oriented programming (OOP) models is that COM has no fundamental object pointers. COM exposes interfaces that are groups of related methods. Objects typically expose at least two and sometimes many interfaces. Thus, when you obtain a COM object, you're given a pointer to one of the object's interfaces-not to the object itself.
  • Globally unique identifiers (GUIDs) are used by COM to identify unique COM interfaces. COM uses GUIDs for two primary purposes:

    • Interface ID (IID). An IID is a GUID that uniquely identifies a particular COM interface. The interface always has the same IID, regardless of which object exposes it.
    • Class ID (CLSID). A CLSID is a GUID that identifies a particular COM object. CLSIDs are required for COM objects that are created by a class factory, but optional for objects that are created in other ways. With UMDF, only the driver callback object has a class factory or a CLSID.

      Some COM objects have GUID identifiers, in which case they're referred to as CLSIDs. GUIDs are referred to as IIDs when you request an interface pointer by using these IIDs.

      To simplify using GUIDs, an associated header file usually defines friendly names that conventionally have a prefix of either IID_ or CLSID_ followed by the description name. For example, the friendly name for the GUID associated with IDriverEntry is IID_IDriveEntry. For convenience, the UMDF documentation usually refers to interfaces by the name used in their implementation, such as IDriverEntry, rather than the IID.

  • Any of the methods on an interface can be used with an interface pointer. If you want access to a method on another interface, you must obtain another interface pointer by using the IUnknown::QueryInterface method.
  • There is no public data member's exposure in COM objects. Public data is exposed through methods called accessors. In UMDF, you use a Get/Retrieve or Set/Assign prefix for its read and write accessors, respectively. Figure 1 shows the logical relationship between an object and its contents.
Figure 1

Figure 1 COM object, interfaces, and methods.

All access to COM objects is through a virtual function table—commonly called a VTable—that defines the physical memory structure of the interface. The VTable is an array of pointers to the implementation of each of the methods that the interface exposes. When a client gets a pointer to an interface, it's actually a pointer to the VTable pointer, which in turn points to the method pointer. For example, Figure 2 shows the memory structure of the VTable for IWDFloRequest.

Figure 2

Figure 2 VTable and interface pointers.

The VTable is exactly the memory structure that many C++ compilers create for a pure abstract base class. This is one of the main reasons that COM objects are normally implemented in C++, with interfaces declared as pure abstract base classes. You can then use C++ inheritance to implement the interfaces in your objects, and the VTable is created for you by the compiler.

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