If your Delphi application needs to operate on various types of files, one of the tasks you might have for your application is to allow the user of the application to print a file, whatever the file type is.
Most document-oriented applications, like MS Word, MS Excel or Adobe can easily print documents created in that program. For example, Microsoft Word saves the text you write in documents with DOC extension. Since Word determines what is the "raw" contents of a .DOC file it knows how to print .DOC files. The same applies for any "known" file type holding some printable information.
What if you need to print various types of documents/files from your application? Can you know how to send the file to the printer in order for it to be printed correctly?
Print From Delphi
We can ask Windows what application can print, for example, a PDF file. Or, even better, we can tell Windows, here's one PDF file, send it to the application associated / in charge of printing PDF files.
To do this, open up Windows Explorer, navigate to a directory containing some printable files. For most of the file types on your system, when you right-click a file in Windows Explorer, you will locate the "Print" command. Executing the Print shell command will result in the file being sent to the default printer. Well, that's exactly what we want: for a file type, call a method that will send the file to the associated application for printing. The function we are after is the ShellExecute API function.
ShellExecute: Print / PrintTo
However, ShellExecute can do much more. ShellExecute can be used to launch an application, open Windows Explorer, initiate a search beginning in the specified directory, and-what's of greatest interest to us-print the specified file.
Using the above call, a document "document.doc" located on the root of the C drive will be sent to the Windows default printer. ShellExecute always uses the default printer for the "print" action. What if you need to print to a different printer, what if you want to allow the user to change the printer?
The PrintTo Shell Command
Before you copy and paste: the Printer global variable (TPrinter type) available in all Delphi programs can be used to manage any printing performed by an application. The printer is defined in the "printers" unit, ShellExecute is defined in the "shellapi" unit.
- Drop a TComboBox on a form. Name it "cboPrinter". Set Style to csDropDownLidt
- Put the next two lines in the form's OnCreate even handler:
//have available printers in the combo boxcboPrinter.Items.Assign(printer.Printers);//pre-select the default / active printercboPrinter.ItemIndex := printer.PrinterIndex;
use to print any document type to a specified printer
Note: some document types do not have an application associated with printing. Some do not have the "printto" action specified.