Question

I am trying to use the GPS_DEVICE structure from within .NET CF 3.5 and I keep getting an error (87) returned from the call to GPSGetDeviceState. As far as I know, 87 means invalid parameter, but I do not know which parameter is invalid!

Can anybody please advise to what I have done wrong with my code as I have now spent the best part of two nights getting nowhere.

The simple test case is…

NativeMethods.GpsDevice gpsDevice = new NativeMethods.GpsDevice();
int result = NativeMethods.GPSGetDeviceState(ref gpsDevice);

My interop is defined as follows…

private const string GpsApi = @"gpsapi.dll";

private const int GPS_VERSION_1 = 1;
private const int GPS_MAX_SATELLITES = 12;
private const int GPS_MAX_PREFIX_NAME = 16;
private const int GPS_MAX_FRIENDLY_NAME = 64;

[DllImport(GpsApi)]
public static extern int GPSGetDeviceState(ref GpsDevice pGPSDevice);

[StructLayout(LayoutKind.Sequential)]
public class GpsDevice {

public UInt32 dwVersion;

 public UInt32 dwSize;

 public IoctlServiceStatus dwServiceState;

 public IoctlServiceStatus dwDeviceState;

 public FileTime ftLastDataReceived;

 [MarshalAs(UnmanagedType.ByValTStr, SizeConst = NativeMethods.GPS_MAX_PREFIX_NAME)]
 public string szGPSDriverPrefix;

 [MarshalAs(UnmanagedType.ByValTStr, SizeConst = NativeMethods.GPS_MAX_PREFIX_NAME)]
 public string szGPSMultiplexPrefix;

 [MarshalAs(UnmanagedType.ByValTStr, SizeConst = NativeMethods.GPS_MAX_FRIENDLY_NAME)]
 public string szGPSFriendlyName;

 public GpsDevice() {

  this.dwVersion = NativeMethods.GPS_VERSION_1;
  this.dwSize = (UInt32)Marshal.SizeOf(this);

 }

}

[StructLayout(LayoutKind.Sequential)]
public struct FileTime {
 UInt32 dwLowDateTime;
 UInt32 dwHighDateTime;
}

public enum IoctlServiceStatus : uint {

 Off = 0,

 On = 1,

 StartingUp = 2,

 ShuttingDown = 3,

 Unloading = 4,

 Uninitialised = 5,

 Unknown = 0xffffffff

}

I hope I have included all information that may be required; if not, please prompt me for more.

Thanks in advance.

Answer 1

I changed the definition of the GpsDevice from a class to a structure and it works!

[StructLayout(LayoutKind.Sequential)]
public struct GpsDevice {
...

}

Had to remove the constructor though (which was actually the reason that I coded it as a class in the first place – so that I could automatically initialise the dwSize and dwVersion members).

NativeMethods.GpsDevice gpsDevice = new NativeMethods.GpsDevice {
    dwVersion = NativeMethods.GPS_VERSION_1,
    dwSize = (uint)Marshal.SizeOf(typeof(NativeMethods.GpsDevice))
};

result = NativeMethods.GPSGetDeviceState(ref gpsDevice);

Real shame that you cannot have constructors on structs!