Calling C DLL from C#

I found the reason for my failed attempts by utilising a tool called; Microsoft(R) P/Invoke Interop Assistant as suggested by an answer on this thread.

I utilised this tool to input some of the C function prototypes and get it to generate the required C# prototype on my behalf. The C prototype looked like the following;

long __stdcall TransProjPt(LPSTR psGridFile, long lDirection, double dEasting, double
dNorthing, long lZone, double* pdEastNew, double* pdNorthNew, double* pdEastAcc,
double* pdNorthAcc) 

When entering this into the Interop assistant tool, it showed that rather than using longs (as I had done in my original question), these should be declared as an int. It produced the following output that meant my code above now worked as I'd hoped. Yay.

    /// Return Type: int
    ///psGridFile: LPSTR->CHAR*
    ///lDirection: int
    ///dEasting: double
    ///dNorthing: double
    ///lZone: int
    ///pdEastNew: double*
    ///pdNorthNew: double*
    ///pdEastAcc: double*
    ///pdNorthAcc: double*
    [System.Runtime.InteropServices.DllImportAttribute("<Unknown>", EntryPoint="TransProjPt", CallingConvention=System.Runtime.InteropServices.CallingConvention.StdCall)]
public static extern  int TransProjPt([System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)] System.Text.StringBuilder psGridFile, int lDirection, double dEasting, double dNorthing, int lZone, ref double pdEastNew, ref double pdNorthNew, ref double pdEastAcc, ref double pdNorthAcc) ;

Thanks for everyones help with this.

You may want to define your c# signatures using marshalling attributes for your string parameters.

[DllImport(@"c:\GDAit.dll")]
public static extern long TransGeogPt([MarshalAs(UnmanagedType.LPStr)] string sGridFile, long lDirection, double dLat, double dLong, ref double pdLatNew, ref double pdLongNew, ref double pdLatAcc, ref double pdLongAcc);

[DllImport(@"c:\GDAit.dll")]
public static extern long TransProjPt([MarshalAs(UnmanagedType.LPStr)] string sGridFile, long lDirection, double dLat, double dLong, long lZone, ref double pdLatNew, ref double pdLongNew, ref double pdLatAcc, ref double pdLongAcc);

I'll also piggy-back on Mark Sowul's answer and say try calling with StdCall instead of Cdecl.

Also, as a precaution, I'd probably double-check to make sure that the compiler is set to compile x86 code, in case its compiling for 64-bit.

Try changing string sGridFile to StringBuilder sGridFile

C++ has so many different kinds of strings that marshaling strings between manage and unmanaged code can be tricky.