Error :Unable to load DLL (Setupapi.lib)

In my C# code: public class Win32 { ...... [DllImport("Setupapi.lib", CharSet=CharSet.Auto)] public static extern IntPtr SetupDiGetClassDevs( [In]System.Guid ClassGuid, [In]string Enumerator, [In]IntPtr hwndParent, [In]int Flags ); ...... } public class usbdevice { ...... hDevInfo = Win32.SetupDiGetClassDevs(Win32.GUID_DEVINTERFACE_USB_DEVICE, //usb class GUID null, // Enumerator ...... } when invoke the SetupDiGetClassDevs(),a error occurs,it says: "An unhandled exception of type 'System.DllNotFoundException' occured in usbdevice.exe" "Additionas information:Unable to loas DLL (Setupapi.lib)". what's the problem? Thanks! new IntPtr(null), Win32.DIGCF_PRESENT | Win32.DIGCF_DEVICEINTERFACE); //flags