How To Use Visual Basic to List Active Logical Drives

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This article was previously published under Q291573

SUMMARY

You can determine which drive letters are in use on a local computer by using the GetLogicalDriveStrings Win32 API. This list of drive letters will be the drive letters of all active drives on the local computer, including floppy drives, hard disks, mapped drives, and other drives that are mapped to a drive letter.

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MORE INFORMATION

The GetLogicalDriveStrings API function returns a string containing a list of all active drive letters on the local computer. The format of the string is a null-separated list of drive letters with a terminating null at the end of the string. For example, a computer with a single floppy drive (A:) and a single hard disk drive (C:) would have a string of the following format returned by GetLogicalDriveStrings:

A:\<null>C:\<null><null>

This string can be parsed to list the active drives.

The following Visual Basic code sample demonstrates how to retrieve and parse a list of drives. Note that this can also be used to create a control or feature similar to the DriveListBox.

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Step-by-Step Example

Start a new Visual Basic Standard EXE project. Form1 is created by default.
Add a new command button (Command1) and a listbox (List1) to Form1.

Add the following code to Form1's code window:

Option Explicit

Private Declare Function GetLogicalDriveStrings Lib "kernel32" _
    Alias "GetLogicalDriveStringsA" _
    (ByVal nBufferLength As Long, ByVal lpBuffer As String) As Long

Private Function GetDriveStrings() As String
    ' Wrapper for calling the GetLogicalDriveStrings API
    
    Dim result As Long          ' Result of our api calls
    Dim strDrives As String     ' String to pass to api call
    Dim lenStrDrives As Long    ' Length of the above string
    
    ' Call GetLogicalDriveStrings with a buffer size of zero to
    ' find out how large our stringbuffer needs to be
    result = GetLogicalDriveStrings(0, strDrives)
    
    strDrives = String(result, 0)
    lenStrDrives = result
    
    ' Call again with our new buffer
    result = GetLogicalDriveStrings(lenStrDrives, strDrives)
    
    If result = 0 Then
        ' There was some error calling the API
        ' Pass back an empty string
        ' NOTE - TODO: Implement proper error handling here
        GetDriveStrings = ""
    Else
        GetDriveStrings = strDrives
    End If
End Function

Private Sub Command1_Click()
    Dim strDrives As String
    
    ' Find out what drives we have on this machine
    strDrives = GetDriveStrings()
    
    If strDrives = "" Then
        ' No drives were found
        MsgBox "No Drives were found!", vbCritical
    Else
        ' Walk through the string and list each drive
        DisplayDriveTypes strDrives
    End If
End Sub

Private Sub DisplayDriveTypes(drives As String)
    ' Walk through the logical drive string and display the drive
    ' letters. The logical drive string is a null seperated
    ' double null terminated string.
    
    Dim pos As Long
    Dim drive As String
    
    List1.Clear
    pos = 1
    
    Do While Not Mid$(drives, pos, 1) = Chr(0)
        drive = Mid$(drives, pos, 3)
        pos = pos + 4
        List1.AddItem UCase(drive)
    Loop
End Sub