Marshalling native .dll in C# with multiple pointers

Question

Having the following code in C++:

• nConId is Connection Identifier
• pParName the parameter name
• pSubName the subParameter Name (if any)
• pValue_out a pointer to a char array of lenght FCL_PAR_VALUE_LENGH
• nValueSize the real size of pValue_out vector (at least FCL_PAR_VALUE_LENGH)

extern "C" MY_API int ReadParameter(const ConnectionId_T nConId, const char* pParName,
    const char *pSubName, char *pValue_out, const int nValueSize );

My try is:

[DllImport("mydll.dll", CharSet = CharSet.Ansi,CallingConvention=CallingConvention.Cdecl)]
public static extern int ReadParameter(ConnectionId_T pConId, IntPtr pParName,
    ref IntPtr pSubName, ref IntPtr[] pValue_out, int nValueSize);

I'm using the following code to call that function:

# nConId is returned from another function and the his value is 0

public const int FCL_PAR_VALUE_LENGH = 128; 

string param_string = "AUXF";
IntPtr pParName = (IntPtr)Marshal.StringToHGlobalAnsi(param_string);

string subparam_string = "T";
IntPtr pSubName = (IntPtr)Marshal.StringToHGlobalAnsi(subparam_string);

IntPtr[] aParValue = new IntPtr[FCL_PAR_VALUE_LENGH]; 

int returnedValue = ReadParameter(nConId, pParName, ref pSubName,
    ref aParValue, FCL_PAR_VALUE_LENGH);

When I run the code I get an AccessViolationException, so I guess there's something wrong in my call.

Do I've my marshall wrong? What do I've to change in the code in order to get the good reponse?

PS: I also know that the call returns also something to aParValue.

Answer 1

You're working too hard with those char*s. It's perfectly legal (and encouraged) to marshal a System.String for input, and a StringBuilder for output.

[DllImport("mydll.dll", CharSet = CharSet.Ansi,CallingConvention=CallingConvention.Cdecl)]
public static extern int ReadParameter(
    ConnectionId_T pConId, 
    string pParName, 
    string pSubName,
    StringBuilder pValue_out,
    int nValueSize);

usage

const int sbLength = 256; //use a domain relevant value
StringBuilder sb = new StringBuilder(sbLength + 1); //for null character, hard to say if you need it without seeing the C++ code, but easier to just add it than find out.
int result = ReadParameter(conId, "paramname", "paramsubname", sb, sbLength);

You haven't given any indication about the underlying type of ConnectionId_T so I'm assuming you've ruled that out as an issue.

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