O'Reilly Book Excerpts: VB.NET Core Classes In a Nutshell
An Introduction to the .NET FCL, Part 1
In part one of a five-part series of book excerpts from VB.NET Core Classes in a Nutshell, learn what it's like to develop without the .NET FCL library and its core classes.
The .NET Framework is Microsoft's new computing platform that simplifies the design, development, and deployment of computer applications. Developed particularly to facilitate the creation of Internet applications and distributed Internet applications, the .NET Framework features the .NET Framework Class Library (FCL), a systematic class framework to be used for the development of system tools and utilities as well as application software. This chapter assesses the significance of the .NET FCL and discusses accessing it from Visual Basic code.
Although programmers using languages like C++ have been using frameworks for system and application development from the very inception of their language (the Microsoft Foundation Class Library, or MFC, for instance, is a framework for developers of Windows applications using C++), comprehensive frameworks or class libraries are comparatively rare in Visual Basic programming. For the most part, programmers of previous versions of Visual Basic depended on two major sources to extend the Visual Basic language: the Win32 API, and ActiveX servers exposed through COM automation.
The Win32 API
The Win32 API is a procedural library that allows the developer to create programs that run under Windows and take advantage of core Windows operating system services. The Win32 API has been enhanced on a regular basis since it was introduced to support Windows NT 3.0, and it now consists of several thousand functions and constants located in a number of dynamic link libraries (DLLs). Because it is a loose collection of functions, there are'nt necessarily any consistent conventions in naming functions or in designating function parameters. The function-based style of programming using the Win32 API has a number of limitations:
- Lack of consistency across the entire Win32 API
Although a collection of Win32 API functions may be interdependent, at the same time each function tends to be a more or less independent entity that is called in isolation from other functions in your program. This tends to make the Win32 API as a whole difficult for all programmers to learn and master.
- Focus on C programmers
The Win32 API originally was developed as a set of functions that would be called primarily from C code. Although the Win32 API can be called from Visual Basic code, writing code that relies heavily on calls to external DLLs has always been something of an adventure in Visual Basic. Much of the challenge centers on the fact that the type systems used by C and Visual Basic are not completely compatible, so that Visual Basic data types have to be converted to data types expected by C language routines.
To get some sense of the difference in style between the function-based, procedural programming that characterizes the Win32 API and the object-oriented programming that characterizes the .NET Framework, Examples 1-1 and 1-2 contain the source code for a console mode routine that launches the application responsible for handling the data file whose name the user enters in a text box. Example 1-1 is written in Visual Basic 6.0 (although it could have run under any version from VB 4 through VB 6) and relies extensively on Win32 API calls, and particularly on calls to the registry API. Example 1-2 is written for Visual Basic .NET and relies on the .NET Framework Class Library, and particularly on its Registry and RegistryKey classes.
Example 1-1. Launching an application using the Win32 API
Option Explicit Private Declare Function RegCloseKey Lib _ "advapi32.dll" ( ByVal hKey As Long) _ As Long Private Declare Function RegOpenKey Lib _ "advapi32.dll" alias "RegOpenKeyA" ( _ ByVal hKey As Long, ByVal lpSubKey _ As String, phkResult As Long) As Long Public Declare Function RegQueryValue Lib _ "advapi32.dll" _ Alias "RegQueryValueA" ( _ ByVal hKey As Long, _ ByVal lpSubKey As String, _ ByVal lpValue As String, _ lpcbValue As Long) As Long Private Declare Function RegQueryValueEx Lib _ "advapi32.dll" _ Alias "RegQueryValueExA" ( _ ByVal hKey As Long, _ ByVal lpValueName As String, _ ByVal lpReserved As Long, _ lpType As Long, _ lpData As Any, lpcbData As Long) As Long Public Declare Function WinExec Lib _ "kernel32" ( _ ByVal lpCmdLine As String, _ ByVal nCmdShow As Long) _ As Long Public Const MAX_PATH = 260 Private Const HKEY_CLASSES_ROOT = &H80000000 Private Const ERROR_SUCCESS = 0& Public Const REG_DWORD = 4 Public Const REG_SZ = 1 Public Const SW_SHOWNORMAL = 1 Private Sub Main() Dim strFile As String, strExten As String Dim strProgID As String, strExe As String Dim lPos As Long Dim hKey As Long, lStrLen As Long strFile = InputBox("Enter Name of File " & _ "to Open: ", "Open File", "") If strFile = "" Then Exit Sub ' Get file extension lPos = InStrRev(1, strFile, ".") If lPos = 0 Then MsgBox "Filename must include an extension." Exit Sub Else strExten = Mid(strFile, lPos) End If ' Get programmatic identifier If RegOpenKey(HKEY_CLASSES_ROOT, _ strExten, _ hKey) <> _ ERROR_SUCCESS Then MsgBox "File extension not found." Exit Sub End If lStrLen = 0 Call RegQueryValue(hKey, "", "", lStrLen) strProgID = Space(lStrLen) Call RegQueryValue(hKey, "", strProgID, lStrLen) RegCloseKey hKey ' Get associated application strProgID = Left(strProgID, lStrLen - 1) & _ "\shell\open\command" If RegOpenKey(HKEY_CLASSES_ROOT, _ strProgID, _ hKey) <> _ ERROR_SUCCESS Then MsgBox "Open command key not found..." Exit Sub End If lStrLen = 0 Call RegQueryValue(hKey, "", "", lStrLen) strExe = Space(lStrLen) Call RegQueryValue(hKey, "", strExe, lStrLen) RegCloseKey hKey ' Launch application and pass its ' filename as a parameter lPos = InStr(1, strExe, " %1") If lPos > 0 Then _ strExe = Left(strExe, lPos) strExe = strExe & " " & strFile Call WinExec(strExe, SW_SHOWNORMAL) End Sub
Example 1-1 is a relatively long program, largely because of the intricacies of working with the Win32 API. We need, of course, to declare all registry-related functions with their parameters, as well as all constants that we intend to use. In addition, each registry access requires that we do the following:
Open the relevant registry key.
Determine the length of the string we want to retrieve.
Set the string buffer to the appropriate length.
Retrieve the registry value.
Adjust the string containing the registry value by removing its terminating null character.
Close the open registry key.
In contrast, the VB.NET program in Example 1-2 is considerably shorter and simpler. In contrast to the numerous Declare and Const statements in Example 1-1, the program only needs to use the Imports statement to indicate which namespaces it will access. Registry access is also significantly simpler. The program relies on two classes: the shared Registry class, which provides access to HKEY_CLASSES_ROOT (HKCR), one of the top-level registry keys; and the RegistryKey class, which represents a registry key. As a result, once the program obtains a reference to HKEY_CLASSES_ROOT, registry access consists of the following steps:
Open the appropriate subkey by calling the top-level key's open method, passing it the path to the subkey to be opened.
Retrieve the newly opened key's default value.
Close the open registry key.
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