Get default application in Windows XP or Vista with VB.NET

When you double click a file, Windows Explorer opens it with associated default application. Windows Explorer determines current default application with file's extension and with information that is stored in the Windows registry. Obtaining the default application with VB.NET is not a difficult task. Default application can be found in both Windows XP and Windows Vista with the same VB.NET code.

Windows stores known file extensions in HKEY_CLASSES_ROOT registry hive. So, the first thing is to check if file extension exists in the registry. If the file extension exists, it has a so called ProgID associated with it. For example, file extension ".txt" has a ProgID "txtfile". Next step is to locate this ProgID registry key and check if it has a sub key "\shell\open\command". This sub key contains finally the path and the name of the default application. In the case of "txtfile" registry key, the default application could be "%SystemRoot%\system32\NOTEPAD.EXE %1".

The VB.NET function below returns the name of the default application, if it exists. Additionally it returns a ready-to-execute string which can be used with VB.NET's Shell command. First parameter in the function is a parameter string for default application. Second parameter is the actual file extension we are searching for.

''' <summary>
''' Return registered application by file's extension
''' </summary>
''' <param name="ParamFileName">Parameter for ShellAppName</param>
''' <param name="FileExtension">File extension</param>
''' <param name="AppName">Returns application name if any</param>
''' <param name="ShellAppName">Returns a string with application name and file name as its parameter</param>
''' <returns>True if the default application for this file type was found</returns>
''' <remarks>This function is Windows XP and Vista compatible</remarks>
Public Function GetRegisteredApplication(ByVal ParamFileName As String, ByVal FileExtension As String, _
 ByRef AppName As String, ByRef ShellAppName As String) As Boolean
 '
 ' Return registered application by file's extension
 '
 Dim StrExt As String
 Dim StrProgID As String
 Dim StrExe As String
 Dim oHKCR As RegistryKey ' HKEY_CLASSES_ROOT
 Dim oProgID As RegistryKey
 Dim oOpenCmd As RegistryKey
 Dim TempPos As Integer

 Try
   ' Add starting dot to extension
   StrExt = "." & FileExtension
   ' Get Programmatic Identifier for this extension
   Try
     oHKCR = Registry.ClassesRoot
     oProgID = oHKCR.OpenSubKey(StrExt)
     StrProgID = oProgID.GetValue(Nothing).ToString
     oProgID.Close()
   Catch
     ' No ProgID, return false
     Return False
   End Try
   ' Get associated application
   Try
     oOpenCmd = oHKCR.OpenSubKey(StrProgID & "\shell\open\command")
     StrExe = oOpenCmd.GetValue(Nothing).ToString
     oOpenCmd.Close()
   Catch
     ' Missing default application
     Return False
   End Try
   TempPos = StrExe.IndexOf(" %1")
   If TempPos > 0 Then
     ' Replace %1 placeholder with ParamFileName
     StrExe = StrExe.Substring(0, TempPos)
     AppName = StrExe
     StrExe = StrExe & " " & Convert.ToChar(34) & ParamFileName & Convert.ToChar(34)
     ShellAppName = StrExe
   Else
     ' No %1 placeholder found, append ParamFileName
     AppName = StrExe
     ShellAppName = StrExe & " " & Convert.ToChar(34) & ParamFileName & Convert.ToChar(34)
   End If
   Return True
 Catch ex As Exception
   Return False
 End Try

End Function

The function returns value True if the default application was found. If the function returns False, there was some error or some issue with registry permissions. In the latter case see System.Security.AccessControl namespace. That namespace provides methods to handle permission issues and how to set registry permissions.

To test this function, here's a code snippet for testing purpose. First create a new text file to C-drives root and name it "test.txt". When you run this snippet, it opens "test.txt" file with Notepad and displays message box:

This is, if you have Notepad as your default text editor.

Dim ApplicationName As String
Dim ShellApplicationName As String
Dim FileExtension As String
Dim ParamFileName As String

ApplicationName = ""
ShellApplicationName = ""
FileExtension = "txt"
ParamFileName = "C:\test.txt"
If GetRegisteredApplication(ParamFileName, FileExtension, ApplicationName, ShellApplicationName) Then
 Shell(ShellApplicationName)
 MessageBox.Show("Default application for the files of type '" & FileExtension & "'" & _
   " is '" & ApplicationName & "'", _
   "Default Application", _
   MessageBoxButtons.OK, _
   MessageBoxIcon.Exclamation)
Else
 MessageBox.Show("No default application found for the files of type '" & FileExtension & "'", _
   "Default Application", _
  MessageBoxButtons.OK, _
  MessageBoxIcon.Exclamation)
End If

Final words of warning. Always be careful when handling registry with your application. I have tested and used this code, and it does only read from the registry. But I will not give any kind of warranty if your registry gets messed up. So backup your registry first or otherwise make sure that you can restore your system if something goes wrong. You use this code totally at your own risk!