Example 1-2. Launching an application using the .NET FCL

Option Strict On
  
Imports System
Imports Microsoft.Win32
Imports Microsoft.VisualBasic
  
Public Module modMain
   Public Sub Main()
      Dim strExten, strProgID, strExe As String
      Dim oProgID, oOpenCmd As RegistryKey
  
      Dim strFile As String = InputBox("Enter Name" & _ 
                                       " of File" & _ 
                                       " to Open: ", _
                                       "Open File", "")
      If strFile = "" Then Exit Sub
  
      ' Get file extension
      Dim iPos As Integer = InStrRev(strFile, ".")
      Try
         strExten = Mid(strFile, iPos)
      Catch
         MsgBox("Filename must include an extension.")
         Exit Sub
      End Try
  
      ' Get Programmatic Identifier
      Dim oHKCR As RegistryKey = Registry.ClassesRoot
      Try 
         oProgID = oHKCR.OpenSubkey(strExten)
         strProgID = CStr(oProgID.GetValue(Nothing))
         oProgID.Close()
      Catch
         MsgBox("File extension not found.")
         Exit Sub
      End Try
  
      ' Get associated application
      Try
         oOpenCmd = oHKCR.OpenSubkey(strProgID & _
                    "\shell\open\command")
         strExe = CStr(oOpenCmd.GetValue(Nothing))
         oOpenCmd.Close()
      Catch
         MsgBox("Open command key not found...")
         Exit Sub
      End Try
  
      ' Launch application and pass its 
      ' filename as a parameter
      iPos = InStr(1, strExe, " %1")
      If iPos > 0 Then _
         strExe = Left(strExe, iPos)
      strExe = strExe & " " & strFile
  
      Call Shell(strExe, AppWinStyle.NormalFocus)
   End Sub
End Module

COM Automation

In place of the function-based programming using the Win32 API, COM automation represented a clear step forward. COM was a more or less object-oriented technology that held out the promise of language independence; as long as a language understood the Component Object Model, it should be able to access and take advantage of COM components.

Example 1-3 shows a VB 6 program written using COM automation that, like the programs in Examples 1-1 and 1-2, launches the application responsible for handling the data file whose name the user enters in a text box. Like the VB.NET program in Example 1-2, it is a short and fairly simple program that relies on the WScript object available from the Windows Script Host object model.

Example 1-3. Launching an application using COM automation

Option Explicit
  
Private Sub Main()
   On Error Resume Next
  
   Dim lPos As Long
   Dim strFile As String, strExten As String
   Dim strProgID As String, strExe As String
  
   strFile = InputBox("Enter Name of File to Open: ", _
                      "Open File", "")
   If strFile = "" Then Exit Sub
  
   ' Get file extension
   lPos = InStrRev(strFile, ".")
   If lPos = 0 Then
      MsgBox "Filename must include an extension."
      Exit Sub
   Else
      strExten = Mid(strFile, lPos)
   End If
  
   ' Initialize WSH Shell object
   Dim oShell As WshShell
   Set oShell = New WshShell
  
   ' Get programmatic identifier
   strProgID = oShell.RegRead("HKCR\" & strExten & "\")
   If Err.Number <> 0 Then
      MsgBox "File extension not found."
      Exit Sub
   End If
  
   ' Get associated application
   strProgID = "HKCR\" & strProgID & "\shell\open\command\"
   strExe = oShell.RegRead(strProgID)
   If Err.Number <> 0 Then
      MsgBox "Open command key not found..."
      Exit Sub
   End If
  
   ' Launch application and pass it 
   ' filename as a parameter
   lPos = InStr(1, strExe, " %1")
   If lPos > 0 Then _
      strExe = Left(strExe, lPos)
   strExe = strExe & " " & strFile
  
   oShell.Run strExe, 5, True
End Sub

Despite its substantial popularity, COM suffered from a number of limitations:

• COM itself offered a model for binary code reuse; it did not offer a model for source code reuse. An implication of this is that, although COM offered interfaced-based inheritance (a feature that predominantly advanced programmers were interested in), it did not support code-based inheritance.

In This Series An Introduction to the .NET FCL, Part 5 In this final installment from VB.NET Core Classes in a Nutshell, learn how to approach the .NET FCL. An Introduction to the .NET FCL, Part 4 In part four in this five part series on the .NET FCL from VB.NET Core Classes in a Nutshell, learn the types of a .NET namespace. An Introduction to the .NET FCL, Part 3 In part three in this series of book excerpts on the .NET Framework Class Library from VB.NET Core Classes in a Nutshell, learn how to work with the .NET FCL. An Introduction to the .NET FCL, Part 2 In part two of this book excerpt series on the .NET Framework Class Library from O'Reilly's VB.NET Core Classes in a Nutshell, get an introduction to the .NET Framework Class Library (FCL).

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• Although COM offered the promise of a language-independent architecture, reality often fell far short of the promise. The root of the problem was the fact that seamless interoperability with COM presupposed that each language was able to create and manipulate common automation-compatible data types. This, however, was not the case. As a result, although COM made some real advances in the area of language independence, it also had some real weaknesses.

• COM was extremely complex, and for the most part only C++ programmers were able to work with COM directly. For VB programmers, the Visual Basic environment masked much of the complexity of COM. The inevitable result was that Visual Basic failed to give the developer full control over COM when it was needed, and many Visual Basic programmers often lacked sufficient familiarity with COM to take advantage even of those features that they were able to control.

In addition, COM did not offer an integrated class library comparable to the .NET FCL. Instead, the developers of each application or operating system service were free to implement whatever object model made sense to extend their application. As a result, there are major gaps in the functionality made available through COM automation, and there is not a good deal of consistency across object models.

The .NET platform and the .NET Framework Class Library were developed in an effort to address these weaknesses of COM.

In the next installment, learn about the .NET Framework Class Library.