Delphi Programming Interview Questions And Answers

Delphi is a product of Borland International. It is a native code compiler that runs under Windows 3.1 and provides visual programming tools somewhat similar to those found in Microsoft Visual Basic 3.0. The underlying language is Object Pascal, which is an extension of the object-oriented Pascal found in Turbo/Borland Pascal starting with version 5.5.

Delphi has been available in beta-test for many months now, and Borland has also given away a large number of "prerelease" copies. As far as the official release is concerned, though, there are two packages: Delphi (sometimes referred to as Delphi Desktop) and Delphi Client/Server. Both are version 1.0.

The only version of Delphi that has been released is for Windows 3.1. There is no reason why it should not run correctly under systems that provide Windows 3.1 emulation, like OS/2 Warp, Windows NT, etc. Borland has announced plans for a 32-bit version to coincide with Windows 95. It is rumored that this might be a free upgrade to users of Delphi 1.0, but I wouldn't count on it. It is also known that Delphi 1.0 does not run correctly on the prerelease version of Windows 95.

Applications built in Delphi are Windows 3.1, 16-bit applications. However, Borland has stated that existing Delphi applications will compile unmodified in 32-bit Delphi.

The minimum installation of Delphi takes about 30Mb, and the full install takes 80Mb. In order to run it well, you'll need a 486 with a minimum of 8Mb of RAM, though I personally wouldn't try to run it in less than about 12Mb. I use a 486DX2/66 at home and a Pentium-90 at work, and to be honest, there's not much difference between them--Delphi's compiles are so fast that the CPU is really not a bottleneck.

If you're lucky, you already have lots of experience with both Object Pascal (or, as it used to be called, Borland Pascal With Objects--essentially, Turbo Pascal v5.5 or later) and with Visual Basic. If you fit this description, then Delphi will be a breeze for you.

Okay, now for everyone else. In order to make full use of the Delphi environment, you have to know Pascal, you have to have some grasp of object orientation, and you have to understand event-driven programming. Once you're over those three hurdles, you've pretty much got it. See section 5 for more information.

On the other hand, most people don't need to make "full" use of the environment. If you just want to pull a simple application together that doesn't do anything too fancy, Delphi shouldn't be any harder to learn than VB--it's just that there's a whole lot more you *can* do in Delphi, which will make you feel more lost than you really are.

Delphi Client/Server includes everything from Delphi Desktop, plus the following:

- SQL-Links 2.5, which includes native client drivers for Oracle, Sybase, Informix, and InterBase, and includes full royalty-free redistribution rights to those drivers, and which costs $995 if bought separately;
- The Local InterBase Deployment Kit, $495 ;
- ReportSmith/SQL, $300;
- "Team development support" -- interoperation with PVCS (obviously, this is no use to you if you don't own PVCS), not available separately;
- The visual query builder, which creates SQL statements for you, also not available separately;
- The VCL source code, which is available separately for $100.

Using regular, non-client/server Delphi, I have developed an application that talks to a Sybase server using ODBC drivers. I didn't have to go to any serious trouble getting it to work, and data access speed is quite acceptable, better than it was under Visual Basic.

Delphi is the descendant of Turbo Pascal, which was first released in 1983 for the CP/M operating system. Turbo Pascal was ported to MS-DOS in early 1984. During the early history of the IBM PC, Turbo Pascal was arguably the most popular language for serious development work--mostly because it was a real compiler, including the program editor and everything, that cost $19.95 and would run on a 64k machine. Borland introduced Turbo Pascal for Windows in 1990. The latest release of Borland Pascal (as it is now called), not including Delphi, was version 7.0 in late 1992.

If you are in the US, You can order Delphi Desktop on CD-ROM for an introductory price of $199 by calling Borland's credit card order desk at 1-800-331-0877. If you are a registered user of any version of Turbo Pascal or Borland Pascal, you qualify for an upgrade price of $149, and you can also buy the Visual Component Library Source Code for $49. The introductory pricing is good through approximately May 15, 1995; after that, the price will be raised to $495. Delphi Client/Server is $1999.

Of course, if you buy through a reseller, you will probably pay less. According to Borland, "Delphi is available through the following US resellers: CompUSA, Best Buy, Elek-Tek, Computer City, Babbages, Software Etc., Fry's, Electronics Boutique, Corporate Software, ASAP Software Express, Egghead Software, Softmart, Software Spectrum, CDW, PC Connection, Programmer's Paradise, Programmer's Warehouse, ProVantage Shop, and Micro Warehouse."

Borland will ship Delphi on 3.5" floppy diskettes, but they charge extra for them. The resellers I have spoken to only carry the CD-ROM version because they don't want to have to stock two different items. I got my copy on CD, but the CD has directories on it called DISK1 through DISK15, so I assume it would be 15 diskettes if you got it that way. If you don't get the CD, you won't get the stuff they use to fill up space on the CD, like an AVI file of an animated spinning Delphi logo.

You get access to the "up and running" line, which will only answer questions related to installing Delphi and getting it to run on your machine. The long hold times that were common when Delphi was first released seem to have improved--the current average seems to be about five minutes or so.

In order to get what borland calls "consultative" tech support (I.E. someone with whom you can discuss reasons why a program is not working), you have to pay $2/minute. You can call (900) 555-1015 and have the charges put on your phone bill, or else (800) 330-3372 and use a credit card. I have called twice, and they couldn't answer my question either time; however, they didn't charge me for the calls.

They don't ship those manuals with Delphi because of their size. The component library reference is over 1000 pages. You can get all this information from the on-line help, or you can ftp the manuals from Borland in Acrobat format, or you can order the actual books for an additional charge.

Delphi is capable of making calls to and receiving callbacks from any standard Windows DLL. In addition, it is said to be possible (though rather tricky) to get Delphi to access classes within C++ DLLs. Borland's WWW server has instructions on how to do this. I wasn't able to get it to work, but I think that's because I have weird DLLs.

Delphi can generate DLLs, which can be called from C, C++, Visual Basic, Powerbuilder, or anything else that understands standard Windows DLLs. There is an example of a database-capable DLL in the DEMOSDBDLL directory.

As is the case in all Windows programming languages that I know of, DLLs are difficult to debug and will crash your system if they aren't quite right. Save your source code frequently.

In general, you have to go to "BDE Config" and define an alias for the database you want to connect to. This allows you to avoid hard-coding a directory path into your application; you just refer to the alias. Then, you create a minimum of three objects on the desktop: A Query or Table object that actually talks to the alias and gets some data; a Data Source object that links between the data and the controls, and at least one data-aware control.

If you've been trying for hours to get this to work, and no matter what you do you just don't see anything happening, try setting the "active" property on the Table or Query to "True." This will open the database. I have seen many people get caught out by this the first time they try it.

Delphi Desktop does not directly support any back-ends except for Local InterBase, and of course the BDE (Borland Database Engine) itself. Delphi Client/Server directly supports Oracle, Sybase, Informix, and InterBase. In both cases, you can use third-party ODBC drivers to connect to any system you want.

Yes, generally. I have not encountered any ODBC drivers that don't work with Delphi, but on the other hand, I haven't tried any really exotic ones.

There is no way to do this within the Delphi VCL. Seems like a pretty serious omission to me. However, it is possible to do it by talking directly to the Borland Database Engine through the interface provided by the DBIPROCS and DBITYPES units. You call DbiDoRestructure, but set all the "number of" variables to zero, which makes it leave the existing values intact. I have code to do this, but it's a bit too long to included here. E-mail graham (at) mhn.org if you want it.

It is sometimes desirable to write code that deals with databases but is not associated with any particular form or user-interface details. To do this, you declare variables of type TDatabase, TTable and/or TQuery, and set their properties in code just as you would in the design environment. Make sure that you include the DB and DBTables units in your uses statement, or you will get "unknown identifier" errors during compilation.

If you look at the declaration, the Sender object is of type TObject, which is the class from which (almost) all other objects are derived. You're probably trying to access a property that isn't defined in TObject, like Text or Caption or something. For this reason, "Sender.Text" will fail, but if (for example) you know that the sender is of type TEdit, then you could use "(Sender As TEdit).Text". If you aren't certain that the Sender object will always be the same type, you can check it with "if (Sender is TEdit) then < blah> ;". See section 5.14.

There is an automated translator that converts Visual Basic projects to Delphi. I have heard from a couple people who have tried using it, and the results seem to be pretty mixed. It's very useful for some people, but not useful at all for others. It is made by:

EarthTrek
7 Mountain Rd.
Burlington, MA 01803
(617) 273-0308

Generally speaking, you call the form's Close method. This runs the OnClose event, which may decide it doesn't want to close, for example if there is unsaved data in the form. Close doesn't free the memory associated with the form, unless of course you put a call to Release in the form's OnClose event.

If you want to close a form without giving it a chance to argue, call the form's Release method. This is similar to Free, but it allows event handlers (e.g. OnDestroy) to finish running before the memory goes away.

Modal forms "end their modal state" when you set the form's ModalResult property to anything greater than zero. If you put a button on a modal form and set the button's ModalResult property to some value, then when the user clicks on that button the form will close with the result you specified. You can find out what the result was by calling ShowModal as a function; i.e. result := Form.ShowModal.

There seems to be a feeling that you need to know more about Windows to use Delphi than you do to use Visual Basic. This isn't really true; you can get by in both environments with a very minimal understanding of Windows internals. However, in both environments, you have to know at least a little about the Windows API in order to really make the most of what you have. The difference is that Delphi gives you a lot more power to do all these interesting things, so you feel more limited if you don't know how.

Well, yes, to a point. Delphi's user interface design tools produce object-oriented code. However, if you're familiar with either Visual Basic or Powerbuilder, you probably have enough understanding of OOP to get by. You can do a great deal in Delphi without having to create your own objects, which is the point at which it becomes important to really understand something about them.

The basic structure goes something like this:

p := new(big_thing);
try
blah(p);
foo(p);
finally
dispose(p);
end;

The first line allocates a big block of memory. Then, in the "try" block, we execute several statements, each of which might produce an error--or, in other words, "raise an event." If an error does occur, the rest of the "try" block will be skipped, "finally" blocks will be executed. If there are no errors, then the "finally" block will be entered when the last statement in the "try" block completes. So, either way, the big block of memory gets freed. These "try/finally" blocks will trap anything up to and including a Windows GPF.