vba implements multiple interfaces

Once defined, the interface So, for example, if the user wants to sort by first name, the code will execute the line. Note that the PersonalData interface is implemented with members that are named with the interface name PersonalData_ as a prefix. Next, we need a class to compare string taking upper and lower case into account. Because this can have completely different logic in the IComparer_Compare function.The only a Long value of -1, 0, or +1. vbBinaryCompare rather that vbTextCompare to the StrComp function so that VBA began life as a simple scripting language that allowed you to create simple macros to automate In this way you could, for example, exchange information between modules, e.g. Once you have defined your interface class module, you must not change the interface. should be viewed as a contract with the outside world, because other objects depend on the form of the interface. Use LB to indicate the index of first position to sort and UB to indicate the last position In this example, it very simple, but in the the other object. subs of a class, but contains no executable code. But it offers both objects (two players and two speaker systems here) the same standard possibilities; to be connected with each other and to forward the sound signal to the output. So less coding, and you can easily add a filtering option (as discussed by Rob Van Gelder and myself a few years back on Dick Kusleikas blog Daily Dose of Excel). You can see that if you try to access "Me.IStdFunctions_BeforeQuitprogram" - IntelliSense will not show that in the list as "Me" also it is only able to see the "Public" procedures, the same as any external object trying to access it. to run across code that uses them, and even more rare to find code that uses them correctly and efficiently. on the same way) into a collection and then use this: All completely different objects in the collection use the same functionality, they all can be used as interface objects and they all can decide how the functionality is implemented in their specific instances. Why not have one CarClass? You can call that here now if you enter "objIStdFunctions.BeforeQuitProgram False". These can be implemented by delegating to the PersonalData interface implementations. First, the interface is not dependent on your form. sort the data. If your return type is "DAO.Recordset" you will get a Set property and a "Set" statement in properties, but as there is no way to distinguish between a Set or Let in functions by VBE the CCInterface code would need to decide if it would need a let or set here so I added also a "TODO" comment here instead and always use Let in all cases. called "CompareLastName" and insert the following code: In this class, we use the Spit function to break the input parameters X and Y into arrays of strings, All players and all output devices will use the same interface so the manufacturers of an output device don't need to think about what player it will be interfaced with nor do the manufacturers of the players need to think about which output device will be connected. to sort the data. In the Collection example I think you are referring to (People container with Person instances) it simply assumes that each instance has the same attributes. as a different class. Swimming under the water, in depth look at classes and objects, and how to to use them. We name it "IStdFunctions" here (usually interface class modules are named with a big "I" at the beginning of the name). of any of those classes and assign it the to varible Sorter. But if you are We can now count the number of strings that were sorted: While it is technically allowed to implement multiple interfaces in a single class, doing so can become complicated and I would recommend that you not do it until you you want to sort the entire array, so set both of these values to -1. which are important when designing and developing large and complicated applications. Between 9:00 AM and 7:00 PM, The world's choice for creating NET-based Commercial Quality Add-Ins for Office implements both interfaces. outside world, but does not contain or define how any of these procedures are carried out. The interface Flyable is a class module with the following code: A class module, Airplane, uses the Implements keyword to tell the compiler to raise an error unless it has two methods: a Flyable_Fly() sub and a Flyable_GetAltitude() function that returns a Long. If X is to be considered greater than Y, then Compare should return techniques. A quick hint: if you want to sort the array in reverse (descending) order, just change the order of the parameters you will see the following: This is the only sub which is in the interface object so no surprise. of Strings is to be supported. than the other. Because each of our comparison class modules Implement IComparer, only a subset of the array. The Implements statement can't appear in a standard module. Copy and paste the header code (the private variable declaration "prv_objIStdFunctions" in the example and the "ObjIStdFunctions" properties) from another module and deactivate the "Implements". If you don't place code in one of the procedures in a class you are implementing, you can raise the appropriate error (Const E_NOTIMPL = &H80004001) so a user of the implementation understands that a member is not implemented. For example, Like classes in general, interfaces allow you to isolate and modularize your code, which promotes more solid and stable code, as well as promoting code reusability, all of That means, the deactivated "Implements", the new property and variable "ObjIStdFunctions" and all interface procedures are now Public. In this example, we have only one sorting class, but there is nothing to prevent you from having many sorting classes, and as long as they all implement

It can then be assigned objects of different class types that implement the interface. Creating a Custom Class With VBA Implements you can make sure each container has the sample attributes AND the same methods so there is no need to create an Interface simply for objects with the same attributes but without any methods (functions/procedures). An interface module contains only the procedure If you were to use a single large complicated sort procedure, adding sorting complex objects, you would need to define what property of the object defines greater than whose data type is ICountable: But since we changed the CSorter class to implement both the IComparer and ICountable interfaces, we can use the existing CSorter class variable: Note that since the Counter variable was declared as ICountable, only the methods of the ICountable interface will be available through the Counter variable, even though the CSorter class propery (e.g, VIN Number, Manufacturer, Price, etc) is to be compared in order to indicate Create a class module named CompareIgnoreCase and enter the following greater than X, then Compare should return +1. In the same way as the MP3 player uses the phone jack, the CD player or mobile phone or amplifier could insert the phone jack. We need to create four classes, one

ISortable, you can assign any of them to the Sorter variable. So instead of rewriting the code I created a wrapper class which should be the replacement of the real interface class. "Stable" is unfortunately the big problem using interfaces. for an automobile dealership, and you need to create a report listing the cars in inventory. for each type of compare. It already exists, we have created that above. a variety of algorithms that can be used for sorting. You could build this support for reversing the order into the IComparer interface module and provide support for it in the Compare class modules. However, it are definitely an intermediate to advanced level technique, and many developers don't want to take the time to learn how to use them. Class definition PorscheCayenneClass that implements CarInterface. As it is a not often used feature of VBA it seems that it is not really tested by the Microsoft developers thoroughly. We now need a third module that wants to use the interface. Create as class named "ISortable" and enter in the code below: This is an interface class that defines a single function named Sort which takes in the array to sort, and If Y is to be considered You can try to add a space into the "Implements" line so it creates a syntax error, leave the line, go back, remove the syntax error and compile it again. Then the form is opened and the "frm" variable is assigned to the opened form. module: As you can see, this is exactly the same as the previous class, but we use pass If you Since all sort algorithms need to compare two items and determine which of the two is greater, Interfaces allow you to treat multiple different Classes that implement then as if they were defined as the same Class. Why even call the classes Porsche/Skoda. along with their input parameters (if any) and the return type for functions, but no code This decouples the code of both forms completely so it is never a problem to rename a control on the main or the popup form as both never accesses a control name of the other form directly. It shows the labels but no results. The form may look for unsaved data and set the "Cancel" variable to "True" if the user must do anything here before stopping the program, the report may stop printing the report and clear the printer queue for example. Simply the best place to learn Excel and Analytics, Home VBA Implements Tutorial Defining a Class Interface in VBA. if the user chooses to sort by last name, we use the code Set Comparer = New CompareLastNameThenFirstName to sort by last name. hundreds of lines of conventional VBA than to learn how to do the same thing with a few dozen lines of code using interfaces. if you use "Implements" very often in a bigger project the project gets into a state where it is not possible to keep it compiled. When a Visual Basic class implements an interface, the Visual Basic class provides its own versions of all the Public procedures specified in the type library of the Interface. case, 2) we can treat them as simple strings and compare them taking upper and lower case into Instead of. But as there are many possibilities to initialize and use that, it only rewrites the declaration and adds a "TODO:" comment to it so you can easily find that and add the adjustment (adding ".ObjIStdFunctions" when assigning the reference). can implement the new interface, but existing code will not be broken when using the original interface. In this example, we are going to use only one type of sort, a modified version the standard QSort algorithm. Interfaces may seem complicated at first, but once an interface. This is not good. The result of ISortable_Sort is an array of strings sorted in the specified manner. interface and a regular class, other than that the procedures defined within an interface

You can try to deactivate all "Implements" statements in the entire project using "find and replace" and then compile it, reactivate all again and compile it again, maybe close and open the project in-between. Fortunately these are not only pure class modules but also forms and reports which have a code module - these are also class modules (that's the reason why you can open more than one equal form at the same time). does include two very important OOP features: Classes and Interfaces. First we declared two object variables, one as the interface class and one as the specific form class. In this example, we have only one class for doing the actual sort, which implements ISortable. I copied all of P-Class except define Porsche unique values and when I run the result is it shows the details for the Porsche but not the Skoda. So the interface object offers to the player a way to be connected to speakers or headphones, and in the same way it offers to the headphones a way to use an MP3 or CD player (and a lot of others). Interfaces are certainly under used in the VBA world. ISortable interface. For now you need to insert the code modules "modCCInterface" and "clsCCInterface" into your project. That was not as easy as the above steps. Change the "Private"s to "Public"s for all interface procedures. The prefix "IStdFunctions_" should make the procedure unique in your form's code. Normally, Now it is time to get to the meat of the application. An interface would represent all those common characteristics that the Car Class would share. Under the covers, there is no difference between a class used as an In VBA, using interfaces lets the compiler check that a module implements all of its methods. your code more stremalined, faster, easier to understand, and easier to If you've converted everything you can remove the "modCCInterface", "clsCCInterface" and "modRecreate" modules and remove the above inserted reference. This is the procedure in the code above named ISortable_Sort. Your email address will not be published. There are The Classes page provides a rather Assigning an object to variables declared by using different interfaces provides a polymorphic behavior. So this is why I programmed this little tool to help me doing that. An Interface is a way to define a set of behaviors that a class will perform. already supplies a function that does this, StrComp, so we just call that and return the Create a class module named "ICountable" and insert the following code: Then modify the existing CSorter class to implement both the IComparer and ICountable interfaces. Now, we decide that given an array of Strings consisting of people's names, there are four different If the variable is defined specifically as a Duck, it would have to be called as Flyable_Fly. The payoff when creating applications far outweighs the time it takes to learn how to use them. By defining an Interface you could create a Collection or Array of Cars ignoring the fact they are of different classes and set or obtain their common variables or execute their common methods. IComparer has only one function, Compare, we create a function called IComparer_Compare and insert the following code. database, or perhaps by VIN number, or manufacturer, or price. Moreover it contains a "Cancel" parameter which should give the using object the opportunity to avoid that the program will be closed (maybe because the user needs to decide what to do with unsaved data). It may seem like a lot of work to create one or more interfaces and the classes that implement those interfaces, and in a trivially simple task like the example described above, it If you want to use more than one interface in the For loop you could add the form and report references to the collection and use "For Each obj in MyCollection" where "obj" is declared as "Object" and then use "obj.ObjIStdFunctions" or "obj.ObjIOtherInterface". within the procedure. It cannot produce sound and it cannot play. All sort algorithms have one thing in common: they repeatedluy compare two elements and determine which is the The wrapper should now handle this so we need to change the code a little bit: The difference is that the interface sub is now declared as "Public" and a new object variable for the wrapper class module is created and a property which automatically initialises it if it was not created before or lost due an unhandled error (of course unhandled errors only exist during design time). You'll see that it takes a lot longer to compile as now all interface codes are compiled again. It should only describe the interface generally without being itself an active object. Create If you tried to do the sort with these options in one big monlithic sort function, the code would very quickly become very For example, suppose that in addition to sorting, we want to count the number of strings being sorted. If X and Y are to be considered equal, Compare should return 0. It can be used in any VBA application. It is possible for a class to implement more than one interface. Site Last Updated: 30-Jun-2022, Email: chip@cpearson.com you read this page before continuing with this Interfaces page. You could have several sort classes that all implement ISortable and choose the appropriate sort class at So the popup form could be used for two different main forms which should select a supplier for example and return the chosen supplier without directly accessing the specific main form - because you forward a reference to the main form's interface object only using an interface of the popup form and the popup form accesses this interface object reference only and can be sure that it will be sent to the right main form. Put the following code in this class If you modify an interface, any class or project To give you an example imagine you want to implement a family of VBA Classes, each representing a different Car (like in my VBA Class Tutorial). That said, implementing mutiple interfaces in a single class can provide powerful features Classes provide the code used when each function is called by a controller of the class. If you've adjusted the references, start "Debug" - "Compile" to make sure the code is compiled and all references are correct. a form reference) and code to call the objects in the form (or whatever class module using the interface). consideration, 3) we can treat them as names and sort by last name, or, finally, 4) we can treat code: The line of code Implements IComparer tell the compiler that all You now have an alternative way of calling a list of procedures in a form. , String Literals - Escaping, non-printable characters and line-continuations, Searching within strings for the presence of substrings, Assigning strings with repeated characters, Working With Files and Directories Without Using FileSystemObject, Reading 2GB+ files in binary in VBA and File Hashes, Multiple Interfaces in One Class - Flyable and Swimable, Automation or Using other applications Libraries, Macro security and signing of VBA-projects/-modules. SS(1) which contains the last name. In For me, your article didnt cast any more light. To use this in code, you will have to declare a new variable Maybe you want to implement another interface later which itself also has a sub "BeforeQuitProgram" - this would not be possible if you doesn't have a unique name. implement this interace. Therefore, we declare a variable as Dim Sorter As ISortable. If the project is not compiled like described above Access crashes in many cases when you try to open the module where the interface is implemented. Created By Chip Pearson at Pearson Software Consulting, This Page: www.cpearson.com/excel/Implements.aspx used as an interface, and 'class module' when refering to a regular class module. 2.2 What does that mean for programmers?

It would be up to you to determine what Depending on your application and its data, you may want to be able to specify which sort Returning any other value may result in Moreover, suppose that later the dealership Nevertheless, the LB and UB parameters To give an example from the real world: If you want to listen to your favorite music you need a device which outputs sound. exists within any of these procedures. Really great what you can do with an interface and how it can make your life easier and the code more stable. You can compile it, save it, no problem. Learn more about Excel and VBA You can now compile your project because now the contract is fulfilled, all procedures of your interface module are implemented into the form module now. VBA Implements is about building custom containers with same interfaces but with different implementations. Nothing happens, because there's no code implementation anywhere. allow you to do this should the need arise. We are going to make the assumption All of the new code logic is isolated in the new compare class, which doesn't interact with the outside world except through Visual Basic does not implement derived classes or interfaces. Of course you can also copy the interface procedures from another module instead of using "Implements" to do that. However, if you consider a large, real-world application, the benefits of interfaces and implementations become clear. This example was written in Excel and gets its initial values from worksheet cells, but the entire concept and practice of implementing interfaces So what are the advantages of using an interface now? In addition to providing a mapping between the interface prototypes and your procedures, the Implements statement causes the class to accept COM QueryInterface calls for the specified interface ID. We live in a world of interfaces like the one in the title picture. an interface. But we still haven't gotten to the actual sorting. In the MsgBox seleciton logic, we determine based on the user's responses which comparison class to use. -1, 0, or +1. Phone: (816) 325-9822 USA Central Time (-6:00 UTC) to sort. That was easy. As we know cars basically do the same thing they accelerate, break, turn lights on or off etc. that implements that inteface will not compile and all the classes will need to be modifed to reflect the changes in the interface. For example, suppose you were working on a project for an automobile dealer and

How that value is determined is completely up to you and your application's requirements. Earlier, we created the four classes, each of which implements IComparer, to determine how a form without an implemented interface and then open the form with the interface. You don't have to do anything special to the class module to indicate that it is to be used as In short in OOP an Interface defines a set of properties (variables) and methods (Subs or Functions) that should be defined in the Class that implements the interface. That's the same here; you want to tell different objects to be informed about quitting the program. -1. Learn how your comment data is processed. declarations, no code. To rewrite the declarations you can use this one: In future, if you want to insert a new module with the interface and you already have the wrapper module you can use "Implements" to insert all the procedures, before compiling that to check if everything is OK. Have a question about something in this article? ways we can sort them: 1) we can treat them as simple strings and compare them ignoring upper and lower So, just what is an interface? In a large application, Now you can go to the immediate window and if you enter "objIStdFunctions." Next, we need the form implementing the interface. Now how you can insert the new "phone jack" into your module? Sign up for a free trial to get started. The module can find the object that declares " As IStdFunctions" and rewrite that. object to determine how to sort the strings. The original input array, ArrayToSort is not modified. An interface defines what a class exposes to the last names will be in SS1(1) and SS2(1). Since Comparer is declared as IComparer and the class CompareFirstNameThenLastName implements IComparer, the assignment succeeds. This could be a CD player or an MP3 player. In the example above I created a CarInterface that represents 2 variables common for all cars name and top speed. This brings up an important point to be made. However, all Classes and Interfaces are a must for Object Oriented Programming (OOP). I suppose this is because they require a higher level of ability and understanding than routine VBA code. But it is definitely worth the time and effort to learn You wrote "Access 2010 (and maybe later versions)". If you look at the The code supporting the supplier data is in a class module called Supplier: The following code supports the Selector form: The following code supports the Data Entry form: Note how, in the data entry form, the m_pd variable is declared by using the PersonalData interface, and it can be assigned objects of either the Customer or Supplier class because both classes implement the PersonalData interface. It would not be an easy task. In this example the form "frmTest" was rewritten to use the wrapper instead. (Visual Basic for Applications). This should be used to inform the objects that the, Public Sub BeforeQuitProgram(ByRef Cancel As Boolean), Private Sub IStdFunctions_BeforeQuitProgram(Cancel As Boolean), For Each objIStdFunctions In MyCollection, objIStdFunctions.BeforeQuitProgram bolCancel, ' --------- Interface Wrapper for interface IStdFunctions------------, Public Property Get ObjWithInterface() As Object, Set ObjWithInterface = prv_objWithInterface, Public Property Set ObjWithInterface(obj As Object), If Not prv_objWithInterface Is Nothing Then, prv_objWithInterface.IStdFunctions_BeforeQuitProgram Cancel, Private prv_objIStdFunctions_Wrapper As IStdFunctions_Wrapper, Public Property Get ObjIStdFunctions() As IStdFunctions_Wrapper, If prv_objIStdFunctions_Wrapper Is Nothing Then, Set prv_objIStdFunctions_Wrapper = New IStdFunctions_Wrapper, Set prv_objIStdFunctions_Wrapper.ObjWithInterface = Me, Set ObjITest1 = prv_objIStdFunctions_Wrapper, Public Sub IStdFunctions_BeforeQuitProgram(Cancel As Boolean), Dim ObjIStdFunctions As IStdFunctions_Wrapper, Set ObjIStdFunctions = frm.ObjIStdFunctions, CCIF.CreateInterfaceWrapper "IStdFunctions", CCIF.RewriteModuleWithInterface "IStdFunctions", "Form_frmTest", CCIF.ReplaceInterfaceUsageWithWrapper "IStdFunctions", https://www.experts-exchange.com/articles/30739/Interfaces-in-VBA-How-to-use-them-and-how-to-work-around-them.html, Checks the implementation of an interface in a class module, Converts a class module by changing the normally as "Private" declared objects (subs, function, properties) into "Public" declarations, Creates a new interface wrapper class module and inserts the required code into this wrapper class and also into the class module which uses the interface, Disables the "Implements" statement with a comment character ('), Needs a reference to "Microsoft Visual Basic for Applications Extensibility" (only during design time), The name of the sub has the prefix "IStdFunctions_". in VBA and how to use them. The following example shows how to use the Implements statement to make a set of declarations available to multiple classes. In the ISortable_Sort function, This is by no means required, but it keeps things orgnainzed. There are many algorithms for sorting, each with its own strenghts and weaknesses. StrComp takes upper and lower case into account when comparing strings the one IComparer_Compare function, so you won't run the risk of introducing errors into the main program. Excel / VBA / C# enthusiast and hobbist. that the names are in the format of FirstName LastName. This is We will declare a set of interface variables and methods. However, when you declare the variable in code, you need to specify which interface you are going to use run time. I wanted to be sure that I can provide the functionality of a normal interface so that I don't need to reprogram my whole project which is a lot code. This is only needed at design time, you can remove the reference and the two "CCInterface" modules for the final release of your project. A class provides an implementation of all the methods and properties of one or more interfaces. encountered a situation in which I wanted to sort only a subset of the array.

The specific object (like the form implementing the interface) doesn't need to know who has called the interface code and doesn't need to access it directly in any way (it can't as it has no reference to it). you could create any number of sorting classes, each of which uses a different sorting algorithm and each of which Implments ISortable. Thanks to the VBA Interface we can now treat all Car classes similar which makes working with them much easier. This is the code for the new standard module: Run the code now, it will break at the "Stop" command highlighting it. It seems a bit wasteful to have to duplicate code like that. It is also named a "contract" between the one and the other class module which wants to use it - therefore it doesn't contain any code. Sometimes it helps for a while. You now need to do that for any object using the interface, don't forget to compile and save before you go on to the next object. unpredicable behavior. Recursion This modified text is an extract of the original, Multiple Interfaces in One Class - Flyable and Swimable, Assigning strings with repeated characters, Automation or Using other applications Libraries, Macro security and signing of VBA-projects/-modules, Reading 2GB+ files in binary in VBA and File Hashes, Searching within strings for the presence of substrings, String Literals - Escaping, non-printable characters and line-continuations, Working With Files and Directories Without Using FileSystemObject. In the This works because Comparer is declared as An interface in a programming language like VBA is a collection of procedures (subs, functions or properties). IComparer, use. It is also recommended to insert the "modRecreate" module which I wrote to create a new ADP from an existing one by exporting and importing all objects and settings.

Then you'll find all procedures of the interface in the right drop-down list: Selecting the procedure will insert it into your form code now, but because we only have one it was already inserted by selecting the interface name.

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vba implements multiple interfaces