C# Signature:

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern uint GetLogicalDriveStrings(uint bufferLength, [Out] char[] buffer);

User-Defined Types:

None.

Parameters:

bufferLength, The maximum size of the buffer pointed to by buffer, in TCHARs. This size does not include the terminating null character. If this parameter is zero, buffer is not used.

buffer, A pointer to a buffer that receives a series of null-terminated strings, one for each valid drive in the system, plus with an additional null character. Each string is a device name.

Notes:

DO NOT define using a StringBuilder, because only the first string would be returned.

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern uint GetLogicalDriveStrings(uint bufferLength, [Out] StringBuilder buffer);

Tips & Tricks:

Since arrays are reference types, we can pass an array of chars. Initialize your array prior to calling the function:

char[] buffer = new char[bufferLength];

Sample Code:

using System;
using System.Collections.Specialized;
using System.Runtime.InteropServices;

public class Mainline
{
    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern uint GetLogicalDriveStrings(uint bufferLength, [Out] char[] buffer);

    public static void Main()
    {
       const int size = 512;
       char[] buffer = new char[size];
       uint code = GetLogicalDriveStrings(size, buffer);

       if (code == 0)
       {
      Console.WriteLine("Call failed");
      return;
       }

       StringCollection list = new StringCollection();
       int start = 0;
       for (int i = 0;  i < code;  ++i)
       {
      if (buffer[i] == 0)
      {
         string s = new string(buffer, start, i - start);
         list.Add(s);
         start = i + 1;
      }
       }

       foreach (string s in list)
      Console.WriteLine(s);
    }
}

Alternative Managed API:

public static string[] System.Environment.GetLogicalDrives();

public static System.IO.DriveInfo[] System.IO.DriveInfo.GetDrives();