VCL Controls: The Open File Dialog Box





A file is a series of bits of data that are arranged in a particular way to produce a usable document. For easy storage, location, and management, the bits are stored on a medium such as a hard disc, a floppy disc, a compact disc, or any valid and supported type of storage. When these bits belong to a single but common entity, the group is referred to as a file. For even greater management, files can be stored in a parent object called a directory or a folder. Since a file is a unit of storage and it stores information, it has a size which is the number of bits it contains. To manage it, a file also has a location also called a path that specifies where and/or how the file can be retrieved. Also, for better management, a file has attributes that indicate what can be done on a file or that provide specific information that the programmer or the operating system can use when dealing with the file.

File processing consists of creating, storing, and/or retrieving the contents of a file from a recognizable medium. For example, it is used to save word-processed files to a hard drive, to store a presentation on floppy disk, or to open a file from a CD-ROM. To perform file processing on VCL applications, you have four main choices, two derived from C and C++ languages, one or a few classes provided by the Visual Component Library, or use the Win32 API.

Characteristics of a File

In order to manage files stored in a computer, each file must be able to provide basic pieces of information about itself. This basic information is specified when the file is created but can change during the life time of a file.

To create a file, a user must first decide where it would be located: this is a requirement. A file can be located on the root drive. Alternatively, a file can be positioned inside of an existing folder. Based on security settings, a user may not be able to create a file just anywhere in the (file system of the) computer. Once the user has decided where the file would reside, there are various means of creating files that the users are trained to use. When creating a file, the user must give it a name following the rules of the operating system combined with those of the file system.

Author Note At the time of this writing, the rules for file names were on the MSDN web site at Windows Development\Windows Base Services\Files and I/O\SDK Documentation\Storage\Storage Overview\File Management\Creating, Deleting, and Maintaining Files\Naming a File (because it is a web site and not a book, its pages can change anytime).

The most fundamental piece of information a file must have is a name. Once the user has created a file, whether the file is empty or not, the operating system assigns basic pieces of information to it. Once a file is created, it can be opened, updated, modified, renamed, etc.

Introduction to Common File Dialog Boxes

Because files on a computer can be stored in various places, Microsoft Windows provides various means of creating, locating, and managing files through objects called Windows Common Dialog Boxes. Indeed, these dialog boxes are part of the operating system and are equipped with all the necessary operations pertinent to their functionality. To support this, Borland C++ Builder ships these ready-made dialog boxes so that, instead of, or before creating a new commonly used dialog box, first find out if C++ Builder already provides an object that can do the job. The objects of C++ Builder are highly efficient and were tested enough to be reliable. This means that whenever possible, you should use them.

To use a standard Windows dialog box, from the Dialogs tab of the Component Palette, click the desired dialog’s button and click anywhere on the form. The position of the control on the form has no importance because it is only a representative. It will not appear when the form is running. Once the desired dialog’s icon is on the form, place a button that will be used to call the dialog. A dialog is called using the DialogName->Execute() method. You can find out what button the user clicked when closing the dialog, and act accordingly.

Practical Learning Practical Learning: Introducing Common Dialogs

  1. Start Borland C++ Builder or create a new project with its default form
  2. Save it in a new folder named FileProcess1
  3. Save the unit as Exercise and save the project as FileProc
  4. Change the Caption to File Processing

The Open File Dialog Box



One of the most usual involvements with computer files consists of opening them for review or for any other reason the user judges appropriate. Microsoft Windows provides a convenient dialog box to handle the opening of files. The job is performed by using the Open File dialog box:


Open File Dialog Box Creation

To provide the means of opening files, you can use the TOpenDialog class. The easiest way to use it is to click the OpenDialog button from the Dialogs tab of the Component Palette and click on the form. The OpenDialog icon can be positioned anywhere on the form because it would not be seen at run time. After placing it on the form, you can use the Object Inspector to configure it.

If you prefer to dynamically create an Open dialog box, declare a pointer to TOpenDialog and use the new operator to call its constructor and specify its owner. The technique is the same we applied for the TSaveDialog class. Here is an example:

TOpenDialog *OpenMe = new TOpenDialog(Form1);

Characteristics of an Open Dialog Box

One of the most important properties of an Open dialog box is the file it presents to the user. This is represented by the FileName property. If you want a default file to be specified when the dialog box comes up, you can specify this in the FileName property of the Object Inspector. If you need to use this property, you should make sure the file can be found. If the file is located in the same folder as the application, you can provide just its name. If the file is located somewhere in the hard drive, you should provide its complete path. Most of the time, you will not be concerned with this property if you are creating an application that will allow the user to open any file of her choice. Once a file is located, it can be accessed using the TOpenDialog::FileName property.

To make your application more effective, you should know what types of files your application can open. This is taken care by specifying a list of extensions for the application. To control the types of files that your application can open, specify their extensions using the Filter Property. The Filter string is created exactly like that of a SaveDialog control as we saw earlier.

Like the SaveDialog control, the default extension is the one the dialog box would first filter during file opening. If you want the Open File dialog to easily recognize a default type of file when the dialog box opens, you can specify the extension's type using the DefaultExt property.

For convenience, or for security reasons, Open File dialog boxes of applications are sometimes asked to first look for files in a specific location when the Open File dialog box comes up. This default folder is specified using the InitialDir property.

The essence of using the Open File dialog box is to be able to open a file. This job is handled by the Execute() method which is easily called using a pointer to the OpenDialog object you are using.


Practical Learning Practical Learning: Using the Open Dialog Box

  1. On the Dialogs tab of the Component, click the OpenDialog button and click the form
  2. While the OpenDialog1 icon is still selected on the form, in the Object Inspector, click DefaultExt and type rtf
  3. Click Filter and click its ellipsis button
  4. Complete the Filter Editor dialog box as follows:
  5. Click OK
  6. Click Title, type Open an Existing Document and press Enter
  7. Click an unoccupied area on the form to select it and, in the Object Inspector, click the Events property page
  8. Double-click the OnMouseDown field and implement the event as follows:
    void __fastcall TForm1::FormMouseDown(TObject *Sender, TMouseButton Button,
    				      TShiftState Shift, int X, int Y)
    	if( Button == mbRight )
    		if( OpenDialog1->Execute() == True )
    			ShowMessage("The Open button was clicked or the Enter key was pressed"
    				    "\nThe " + OpenDialog1->FileName +
    				    " file would have been opened.");
    			ShowMessage("The Cancel button was clicked or Esc was pressed");
  9. Test the application:
  10. After using it, close it and return to Bcb
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