C# Signature:

[DllImport("setupapi.dll", CharSet = CharSet.Auto)]
static extern IntPtr SetupDiGetClassDevs(
                                              ref Guid ClassGuid,
                                              [MarshalAs(UnmanagedType.LPTStr)] string Enumerator,
                                              IntPtr hwndParent,
                                              uint Flags
                                             );

Notes:

This signature has the drawback that it does not allow passing a null pointer as the first parameter, which is one of the common usages cited at SetupDiGetClassDevs: "To return devices for all device setup classes, set the DIGCF_ALLCLASSES flag, and set the ClassGuid parameter to NULL."

Alternate Signatures:

[DllImport("setupapi.dll", CharSet = CharSet.Auto)]
static extern IntPtr SetupDiGetClassDevs(           // 1st form using a ClassGUID only, with null Enumerator
   ref Guid ClassGuid,
   IntPtr Enumerator,
   IntPtr hwndParent,
   int Flags
);

[DllImport("setupapi.dll", CharSet = CharSet.Auto)]     // 2nd form uses an Enumerator only, with null ClassGUID
static extern IntPtr SetupDiGetClassDevs(
   IntPtr ClassGuid,
   [MarshalAs(UnmanagedType.LPTStr)] string Enumerator,
   IntPtr hwndParent,
   int Flags
);

ClassGuid should be IntPtr.Zero

Flags for the second form should be: DIGCF_DEVICEINTERFACE | DIGCF_ALLCLASSES

The signatures above allow substitution of a null by specifying IntPtr.Zero as one of the first two parameters. They must use IntPtrs for the Enumerator and ClassGuid parameters, instead of int or Int32, or they will not work on 64-bit platforms.

The Flags parameter may alternatively be declared as taking an enum value:

        [Flags]
        public enum DiGetClassFlags : uint {
            DIGCF_DEFAULT       = 0x00000001,  // only valid with DIGCF_DEVICEINTERFACE
            DIGCF_PRESENT       = 0x00000002,
            DIGCF_ALLCLASSES    = 0x00000004,
            DIGCF_PROFILE       = 0x00000008,
            DIGCF_DEVICEINTERFACE   = 0x00000010,
        }

VB Signatures:

    <DllImport("setupapi.dll", _
    EntryPoint:="SetupDiGetClassDevsW", _
    SetLastError:=True, _
    CharSet:=CharSet.Unicode, _
    ExactSpelling:=True, _
    PreserveSig:=True, _
    CallingConvention:=CallingConvention.Winapi)> _
    Private Shared Function SetupDiGetClassDevs( _
    ByRef ClassGuid As GUID, _
    ByVal Enumerator As Integer, _
    ByVal hwndParent As Integer, _
    ByVal Flags As Integer) As Integer
    End Function

    <DllImport("setupapi.dll", _
    EntryPoint:="SetupDiGetClassDevsW", _
    SetLastError:=True, _
    CharSet:=CharSet.Unicode, _
    ExactSpelling:=True, _
    PreserveSig:=True, _
    CallingConvention:=CallingConvention.Winapi)> _
    Private Shared Function SetupDiGetClassDevs( _
    ByRef ClassGuid As GUID, _
    ByVal samDesired As Integer, _
    ByVal Flags As Integer, _
    ByRef hwndParent As String, _
    ByVal Reserved As Integer) As Integer
    End Function

VB 9 Syntax

Public Declare Auto Function SetupDiGetClassDevs Lib "setupapi.dll" Alias "SetupDiGetClassDevsW" ( _
    ByRef ClassGuid As Guid, _
    ByVal Enumerator As IntPtr, _
    ByVal hwndParent As IntPtr, _
    ByVal Flags As UInteger) As IntPtr

User-Defined Types:

const int DIGCF_DEFAULT = 0x1;
const int DIGCF_PRESENT = 0x2;
const int DIGCF_ALLCLASSES = 0x4;
const int DIGCF_PROFILE = 0x8;
const int DIGCF_DEVICEINTERFACE = 0x10;
Guid GUID_DEVINTERFACE_DISK = new Guid(0x53f56307, 0xb6bf, 0x11d0, 0x94, 0xf2, 0x00, 0xa0, 0xc9, 0x1e, 0xfb, 0x8b);

Alternative Managed API:

Do you know one? Please contribute it!

Notes:

On 32bit platforms, all SetupApi structures are 1-Byte packed. On 64bit platforms the SetupApi structures are 8-byte packed.

Tips & Tricks:

When finished using the device information set, the application should free the resources used by calling SetupDiDestroyDeviceInfoList

Sample Code:

Guid DiskGUID = new Guid(GUID_DEVINTERFACE_DISK);

// We start at the "root" of the device tree and look for all
// devices that match the interface GUID of a disk
IntPtr h = SetupDiGetClassDevs(ref DiskGUID, IntPtr.Zero, IntPtr.Zero, DIGCF_PRESENT | DIGCF_DEVICEINTERFACE);
if (h != (IntPtr)INVALID_HANDLE_VALUE)
{
   bool Success = true;
   int i = 0;
   while (Success)
   {
     // create a Device Interface Data structure
     SP_DEVICE_INTERFACE_DATA dia = new SP_DEVICE_INTERFACE_DATA();
     dia.cbSize = Marshal.SizeOf(dia);

     // start the enumeration
     Success = SetupDiEnumDeviceInterfaces(h, IntPtr.Zero, ref DiskGUID, i, ref dia);
     if (Success)
     {
       // build a DevInfo Data structure
       SP_DEVINFO_DATA da = new SP_DEVINFO_DATA();
       da.cbSize = Marshal.SizeOf(da);

       // build a Device Interface Detail Data structure
       SP_DEVICE_INTERFACE_DETAIL_DATA didd = new SP_DEVICE_INTERFACE_DETAIL_DATA();
       didd.cbSize = 4 + Marshal.SystemDefaultCharSize; // trust me :)

       // now we can get some more detailed information
       int nRequiredSize = 0;
       int nBytes = BUFFER_SIZE;
       if (SetupDiGetDeviceInterfaceDetail(h, ref dia, ref didd, nBytes, ref nRequiredSize, ref da))
       {
     // current InstanceID is at the "USBSTOR" level, so we
     // need up "move up" one level to get to the "USB" level
     IntPtr ptrPrevious;
     CM_Get_Parent(out ptrPrevious, da.DevInst, 0);

     // Now we get the InstanceID of the USB level device
     IntPtr ptrInstanceBuf = Marshal.AllocHGlobal(nBytes);
     CM_Get_Device_ID(ptrPrevious, ptrInstanceBuf, nBytes, 0);
     string InstanceID = Marshal.PtrToStringAuto(ptrInstanceBuf);

     Marshal.FreeHGlobal(ptrInstanceBuf);
       }
     }
     i++;
   }
}
SetupDiDestroyDeviceInfoList(h);