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Open a file using its extension

In order to open a file without knowing which application handles it, or to be more precise whithout knowing the exact path of the application executable, you need to search the regisrty for file extension associations (the file extension must be registered).

Each file extension maps to one file type, file types map to applications. So you need to search twice the registry to find the executable that handles a specific file.

Step 1 - Search for file type mapped to file extension
Open the registry key:

   HKEY_CLASSES_ROOT\.[ext]

Note the dot '.' before the file extension. The default value contains the file type (e.g. for '.txt' files, the type is 'txtfile').

Step 2 - Search for registered application
Open the registry key:

   HKEY_CLASSES_ROOT\[file-type]\shell\open\command

The default value contains the executable path handling this file type (e.g. for 'txtfile' files, the executable path is '%SystemRoot%\system32\NOTEPAD.EXE %1').

Public Shared Function OpenFile(ByVal FileToOpen As String, _

            ByVal WindowState As Microsoft.VisualBasic.AppWinStyle, _

            ByRef ExceptionMessage As String) As Integer

    Dim ret As Integer = 0

    Dim FileExtension As String = ""

    Try

        If Not FileToOpen Is Nothing AndAlso FileToOpen.Length > 0 Then

            FileExtension = FileToOpen.Substring(FileToOpen.IndexOf("."))

            If Not FileExtension Is Nothing AndAlso FileExtension.Length > 0 Then

                Dim RegPerm As System.Security.Permissions.RegistryPermission = _

                    New System.Security.Permissions.RegistryPermission(System.Security.Permissions.RegistryPermissionAccess.Read, "\\HKEY_CLASSES_ROOT")

                Dim ClassesRoot As Microsoft.Win32.RegistryKey = Microsoft.Win32.Registry.ClassesRoot

                Dim TypeKey As Microsoft.Win32.RegistryKey = ClassesRoot.OpenSubKey(FileExtension)

                Dim AppFilePath As String = ""

 

                If Not TypeKey Is Nothing Then

                    Dim FileType As String = TypeKey.GetValue("", "").ToString

                    If FileType.Length > 0 Then

                        Dim TypeKey2 As Microsoft.Win32.RegistryKey = ClassesRoot.OpenSubKey(FileType & "\shell\open\command")

 

                        If Not TypeKey Is Nothing Then

                            AppFilePath = TypeKey2.GetValue("", "").ToString

 

                            If AppFilePath.Length > 0 AndAlso AppFilePath.ToLower.IndexOf(".exe") > -1 Then

                                If AppFilePath.StartsWith("""") Then

                                    AppFilePath = AppFilePath.Substring(1)

                                End If

                                AppFilePath = AppFilePath.Substring(0, AppFilePath.ToLower.IndexOf(".exe") + ".exe".Length)

 

                                If System.IO.File.Exists(AppFilePath) Then

                                    ret = Shell(String.Format("""{0}"" {1}", AppFilePath, FileToOpen), WindowState)

                                Else

                                    Throw New Exception(String.Format("File does not exist [{0}].", AppFilePath))

                                End If

                            Else

                                Throw New Exception(String.Format("No executable is defined for open command {0}.", AppFilePath))

                            End If

                        Else

                            Throw New Exception(String.Format("No open command is registered for file type {0}.", FileType))

                        End If

                    Else

                        Throw New Exception(String.Format("No file type is registered for extension {0}.", FileExtension))

                    End If

                Else

                    Throw New Exception(String.Format("No file type is registered for extension {0}.", FileExtension))

                End If

            Else

                Throw New Exception(String.Format("No file extension in args(0) [{0}].", FileToOpen))

            End If

        Else

            Throw New Exception("No file parameter is entered.")

        End If

    Catch ex As Exception

        ret = -1

        ExceptionMessage = ex.ToString

    End Try

    Return ret

End Function