Marshaling char * in .NetcompactFramework(Windows CE) - windows-ce

Question

I have the following signature from c++
IDTECHREADER_EXPORTS void ReadTwoBlocks(char *pathConfig,
char *datablock1, char *datablock2, int timeout, bool &ret )
I was able to Marshal this correctly on the Full .NetFrameWork and it works as below
[DllImport("IDTechReader.dll", EntryPoint = "ReadTwoBlocks" )]
private static extern void _ReadTwoBlocks(
[In][MarshalAs(UnmanagedType.LPStr)] string pathConfig,
[Out][MarshalAs(UnmanagedType.LPStr)] StringBuilder datablock1,
[Out] [MarshalAs(UnmanagedType.LPStr)] StringBuilder datablock2,
int TimeOut,
ref bool result);
However using the same Marshaling as above doesn't work on the NetcompactFramework(Windows CE) it gives an error "NotSupported Exception"
How do we correctly Marshal the above C++ method signature so that it will work correctly on the .NET CompactFramework(windows CE)
any ideas are apperciated...thanks.

Answer 1

The marshaler is probably choking on the MarshalAs(UnmanagedType.LPStr). You're either going to have to change the signature to a fixed size byte[] and do the string conversion in managed code using Encoding.ASCII.GetString(), or you could use an IntPtr type and allocate the memory using Marshal.AllocHGlobal/FreeHGlobal and deal with the string conversion in your code.
I think this might work..
private const int MAX_STRING = 256;
[DllImport("IDTechReader.dll", EntryPoint = "ReadTwoBlocks")]
private static extern void _ReadTwoBlocks(
byte[] pathConfig,
[Out] byte[] datablock1,
[Out] byte[] datablock2,
int TimeOut,
ref bool result);
public void ReadTwoBlocks(string pathConfig,
StringBuilder datablock1,
StringBuilder datablock2,
int TimeOut,
ref bool result)
{
var pathConfigBuff = new byte[MAX_STRING];
var datablock1Buff = new byte[MAX_STRING];
var datablock2Buff = new byte[MAX_STRING];
// Convert unicode string to null terminated single byte charater string
Array.Copy(Encoding.ASCII.GetBytes(pathConfig), pathConfigBuff, pathConfig.Length);
// Call your native method
_ReadTwoBlocks(pathConfigBuff, datablock1Buff, datablock2Buff, TimeOut, ref result);
// If success, copy the datablocks to the StringBuffers
if (result)
{
datablock1.Append(Encoding.ASCII.GetString(datablock1Buff, 0, MAX_STRING).Replace('\0', ' ').Trim());
datablock2.Append(Encoding.ASCII.GetString(datablock2Buff, 0, MAX_STRING).Replace('\0', ' ').Trim());
}
}

Answer 2

It would be something like this:
[DllImport("IDTechReader.dll")]
private static extern void ReadTwoBlocks(IntPtr pathConfig,
IntPtr datablock1, IntPtr datablock2, int timeout, ref bool ret);
and when you use it like this:
string pathConfig = "..\program.ini";
IntPtr ptrPathConfig = IntPtr.Zero;
ptrPathConfig = Marshal.StringToHGlobalAnsi(pathConfig);
IntPtr ptrDatablock1 = IntPtr.Zero;
IntPtr ptrDatablock2 = IntPtr.Zero;
int timeout = 300;
bool ret = false;
ReadTwoBlocks(ptrPathConfig, ptrDatablock1, ptrDatablock2, timeout, ref ret);
string db1 = Marshal.PtrToStringAnsi(ptrDatablock1);
string db2 = Marshal.PtrToStringAnsi(ptrDatablock2);