Home > Articles > Programming > Windows Programming

This chapter is from the book

Rule 2-3: Define Custom Interfaces Separately, Preferably Using IDL

Interface-based programming is a powerful mechanism for building systems that evolve more easily over time. The importance of this programming style is evident in the design of Java, which raised interfaces to the same level as classes. Of course, COM is another compelling example of the significance of interfaces.

A custom interface is nothing more than a set of method signatures defined as a stand-alone entity. As discussed in the previous rule, however, this simple concept enables a wide range of advantages—in design, reuse, and evolution. The proposal here is to take this one step further and to define the interfaces separately.

In VB interfaces are typically defined as PublicNotCreatable class modules and are then implemented in MultiUse class modules within the same project (see the previous rule if you have never worked with custom interfaces). Although this is a perfectly adequate approach to implementation, there is a significant drawback: When you hand out your interfaces as class modules, you are essentially giving out the source code. This opens up the possibility that others may change your interfaces, thus altering your design and breaking compatibility with your clients. Whether the changes are accidental or intentional, this is a dangerous loophole.

As shown in Figure 2.7, the simplest precaution is to define your interfaces in a separate ActiveX DLL project, compile and hand out the resulting DLL file. Class implementers then create a separate VB project, set a project reference to this DLL, and implement the custom interfaces as before. Likewise, the client (typically denoted by a standard EXE project) must also set a reference to the interface's DLL. This scenario is depicted in Figure 2.8. Note that the client project actually references both the interface's DLL and the class's DLL. The former is needed to declare variables referencing a custom interface, whereas the latter is necessary to instantiate classes using New. For example,

Dim rEmp As IEmployee    '** need interface information
Set rEmp = New CConsultant '** new class information

Figure 2.7 Defining custom interfaces separately in VB

Figure 2.8 Accessing the custom interface's DLL in other VB projects

Keep in mind that a COM-based DLL must be registered on your machine before it can be referenced from a VB project. This can be done using the Windows utility RegSvr32, or through the Browse button available off the Project >> References menu item in VB.

Although separating your interfaces into a separate VB project is an improvement, it is somewhat confusing to use an executable DLL to represent entities that contain no implementation! Furthermore, because ActiveX DLL projects must contain at least one public class, to make this work you must define a dummy MultiUse class along with your PublicNotCreatable interface classes (see Figure 2.7). 1 However, the most significant disadvantage to this approach is that VB allows you to extend your custom interfaces, without warning, even though the result breaks compatibility with your clients. For example, accidentally adding a method to an interface and recompiling the interface's DLL will break both the class implementer and the client application. This is true regardless of VB's compatibility mode setting when working with your interfaces (see rule 2-5).

The alternative, and better approach, is to do what C++ and Java programmers have been doing for years: defining their interfaces separately using IDL. IDL is a small C-like language solely for describing interfaces, and is often called "the true language of COM" because it is what enables clients and COM components to understand each other. Thus, the idea is to abandon VB, define your interfaces as a text file using IDL, compile this file using Microsoft's IDL compiler (MIDL), and deploy the resulting binary form, known as a type library (TLB).2 This is outlined in Figure 2.9. Once you have a TLB definition of your interfaces, your clients simply set a project reference to the TLB file instead of the interface's DLL file (Figure 2.10). Note that TLBs are registered using the RegTLib utility, or via the Browse button under VB's Project >> References.3

Figure 2.9 Defining custom interfaces separately using IDL

Figure 2.10 Accessing the custom interface's TLB in other VB projects

The advantage to using IDL and MIDL is that you, and only you, can change an interface or break compatibility. You have complete control over your design, and exactly when and how it evolves. The drawback is that you have to learn yet another language. The good news is that we'll show you a way to generate automatically 98 percent of the IDL you'll need. But first, let's take a peek at what IDL looks like. As an example, consider the following VB interface IEmployee:

Public Name As String

Public Sub ReadFromDB(rsCurRecord As ADODB.Recordset)
End Sub

Public Function IssuePaycheck() As Currency
End Function

To make things more clear, let's first rewrite this as a custom interface (no data members), with parameter passing explicitly defined:

Private Property Get Name() As String
End Property

Private Property Let Name(ByVal sRHS As String)
End Property

Public Sub ReadFromDB(ByRef rsCurRec As ADODB.Recordset)
End Sub

Public Function IssuePaycheck() As Currency
End Function

Now, here is the equivalent COM-based interface in IDL:

[ 
	uuid(E1689529-01FD-42EA-9C7D-96A137290BD8),
	version(1.0),
	helpstring("Interfaces type library (v1.0)")
]
library Interfaces
{
	importlib("stdole2.tlb");

	[ 
		object,
		uuid(E9F57454-9725-4C98-99D3-5F9324A73173),
		oleautomation
	]
	interface IEmployee : IUnknown {
		[propget] HRESULT Name([out, retval] BSTR* ps);
		[propput] HRESULT Name([in] BSTR s);

		HRESULT ReadFromDB([in, out] _Recordset** pprs);
		HRESULT IssuePaycheck([out, retval] CURRENCY* pc);
	};
};

This IDL description defines a TLB named Interfaces, which is tagged with three attributes (values within square brackets). When registered, the helpstring attribute makes the TLB visible to VB programmers as "Interfaces type library (v1.0)," whereas internally it is represented by the globally unique identifier (GUID) E1689529-01FD-42EA-9C7D-96A137290BD8 because of the uuid attribute. The library contains one interface, IEmployee, uniquely identified by the GUID E9F57454-9725=4C98-99D3-5F9324A73173. The remaining attributes define the version of IDL we are using (object) and enable automatic proxy/stub generation (oleautomation). IEmployee consists of four method signatures, each of which is defined as a function returning a 32-bit COM error code (HRESULT). Note that VB functions (such as IssuePaycheck) are redefined to return their values invisibly via an additional out parameter. Finally, for each method, the VB parameter type is translated to the equivalent IDL data type, and the parameter-passing mechanism (ByVal versus ByRef) is transformed to its semantic equivalent (in versus in/out). The most common mappings from VB to IDL data types are shown in Table 2.1.

Table 2.1 VB-to-IDL data type mappings

VB

IDL

Byte

unsigned char

Integer

short

Long

long

Single

float

Double

double

Array

SAFEARRAY(<type>) *

Boolean

VARIANT_BOOL

Currency

CURRENCY

Date

DATE

Object

IDispatch *

String

BSTR

Variant

VARIANT


In general, a TLB may contain any number of interface definitions. When writing IDL, the first step is to assign the TLB and each interface a GUID. GUIDs can be generated using the GuidGen utility: Select Registry Format, press New GUID, then Copy, and paste the resulting GUID into your IDL file. Next, assign the TLB and interfaces the same set of attributes shown earlier. Finally, define each interface. Once you have the IDL file, simply run MIDL to compile it (see Figure 2.9). For example, here's the compilation of Interfaces.idl:

midl Interfaces.idl 

This produces the TLB Interfaces.tlb. The interfaces are now ready for use by your clients (see Figure 2.10).4 Note that your clients do not have to be written in VB. For example, class implementers can use C++ if they prefer. In fact, another advantage of using IDL is that MIDL can automatically generate the additional support files needed by other languages.

Although writing IDL is not hard, it is yet another language that you must learn. Furthermore, you must be careful to use only those IDL constructs and types that are compatible with VB. This is because of the fact that IDL is able to describe interfaces for many different object-oriented programming languages (C++, Java, and so on), but only a subset of these interfaces are usable in VB. Thus, writing IDL from scratch is not a very appealing process for VB programmers.

Luckily, Figure 2.11 presents an easy way to generate VB-compatible IDL automatically. Given a VB ActiveX DLL (or EXE), the OLEView utility can be used as a decompiler to reverse engineer the IDL from the DLL's embedded TLB (put there by VB). Obviously, if we start with a DLL built by VB, the resulting IDL should be VB compatible! The first step is to define your interfaces using VB, as discussed earlier (see Figure 2.7). Then, run OLEView (one of the Visual Studio tools available via the Start menu) and open your DLL file via File >> View TypeLib. You'll be presented with the reverse-engineered IDL. Save this as an IDL file. Edit the file, defining a new GUID for the TLB as well as for each interface, Enum, and UDT. Now compile the IDL with MIDL, unregister the VB DLL, and register the TLB. In a nutshell, that's it.

Figure 2.11 Reverse-engineering IDL using OLEView

Unfortunately, OLEView is not perfect: You will want to modify the resulting IDL file before compiling it with MIDL. If your interfaces use the VB data type Single, the IDL will incorrectly use Single as well. Search the file and change all occurrences of Single to float. Also, if your interfaces define a UDT called X, the equivalent IDL definition will be incorrect. Manually change struct tagX {...} to struct X {...}. Finally, you'll want to delete some unnecessary text from the IDL file. In particular, it will contain one or more coclass (or COM class) definitions, including the dummy class you may have defined for VB to compile your interfaces classes. For example, suppose Interfaces.DLL contains the IEmployee interface class we discussed earlier, as well as a CDummy class. Decompiling the DLL with OLEView yields

// Generated .IDL file (by the OLE/COM Object Viewer)
// 
// typelib filename: Interfaces.DLL
[
	uuid(E1689529-01FD-42EA-9C7D-96A137290BD8),
	version(1.0),
	helpstring("Interfaces type library (v1.0)")
]
library Interfaces
{
	// TLib : // TLib : Microsoft ADO : {...}
	importlib("msado15.DLL");
	// TLib : // TLib : OLE Automation : {...}
	importlib("stdole2.tlb");

	// Forward declare all types defined in this typelib
	interface _IEmployee;
	interface _CDummy;

	[
		odl,
		uuid(E9F57454-9725-4C98-99D3-5F9324A73173),
		version(1.0),
		hidden,
		dual,
		nonextensible,
		oleautomation
	]
	interface _IEmployee : IDispatch {
		[id(0x40030000), propget]
		HRESULT Name([out, retval] BSTR* Name);
		[id(0x40030000), propput]
		HRESULT Name([in] BSTR Name);
		[id(0x60030000)]
		HRESULT ReadFromDB([in, out] _Recordset** );
		[id(0x60030001)]
		HRESULT IssuePaycheck([out, retval] CURRENCY* );
	};

	[ ... ]
	coclass IEmployee {
		[default] interface _IEmployee;
	};

	[ ... ]
	interface _CDummy : IDispatch {
		[id(0x60030000)]
		HRESULT foo();
	};

	[ ... ]
	coclass CDummy {
		[default] interface _CDummy;
	};
};

At the very least, your IDL file must contain the code shown in boldface and italic. The rest can be safely deleted. Note that VB defines the name of an interface by starting with the _ character (e.g., _IEmployee). Delete this character as well. Next, you should change each interface odl attribute to object, and IDispatch reference to IUnknown. Finally, consider adding helpstring attributes not only to your interfaces, Enums, and UDTs, but to their individual elements as well. This information is visible when others browse your TLB, yielding a convenient form of documentation.

You are now ready to begin using IDL for defining your interfaces, and to reap the advantages that C++ and Java programmers have been enjoying for years: separating design from implementation, and retaining complete control over when and how your interfaces change. No one else can alter your design, and only you can break compatibility with clients. The many advantages of custom interfaces rely on your ability to maintain compatibility, and IDL is the best way to go about doing this.

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