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Calling C Type DLL from VB.net to return Char() or String (passed as intptr)

Posted on 2004-04-06
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Last Modified: 2011-10-03
How do I retrieve a string / char() value from a dll which only accepts IntPtr(at least all my other attempts failed with eroors) as Variable.
I have a perfectly fine working c/c++ code to call the c-type dll which goes as follows (Hope I include all the relevant parts):

typedef KPDCStatus (*KPDCGetAttributeInfoFunc)(
                  KPDCOpaqueRef inRef,
                  KPDCAttributeID inAttrID,
                  KPDCDataType *outAttrType,
                  KPDCUInt32 *outAttrSize);

typedef KPDCStatus (*KPDCGetAttributeValueFunc)(                                        
                  KPDCOpaqueRef inRef,                                    
                  KPDCAttributeID inAttrID,
                  KPDCUInt32 *inOutAttrSize,
                  void* outAttrValue);

//The functions are loaded into the theProcs-Structure

char theMemoryPath[255];

theStatus = theProcs.GetAttributeInfo(theCamera, KPDCSerialNumberStringID,&theDataType, &theAttributeSize );

// Now get actual serial number and store it in the memory path
memset(theMemoryPath, 0, sizeof(theMemoryPath) );
theStatus = theProcs.GetAttributeValue( theCamera, KPDCSerialNumberStringID,&theAttributeSize, theMemoryPath);

The function returns : P14N-15076 (That's what I want)


Trying to transfer this call to VB.net my current version looks like this (aftertaking many forms in the meantime, trying some MarshalAs and whatever I read about the topic):

    <DllImport("DCSPro4SLR.dll")> Shared Function KPDCGetAttributeInfo(ByVal inRef As IntPtr, _
               ByVal inAttrID As UInt32, ByRef outAttrType As UInt32, ByRef outAttrSize As UInt32) As Integer
    End Function

    <DllImport("DCSPro4SLR.dll")> Shared Function KPDCGetAttributeValue(ByVal inRef As IntPtr, _
               ByVal inAttrID As UInt32, ByRef inOutAttrSize As UInt32, ByRef outAttrValue As IntPtr) As Integer
    End Function
'The KPDCGetAttributeValue Function is the one troubling me

Private sub Test
            theStatus = KPDCGetAttributeInfo(theCamera, Convert.ToUInt32(KPDCCamAttr.KPDCSerialNumberStringID), _
                    theDataType, theAttributeSize)

            Dim tmpMemPath As IntPtr = Nothing
            theMemoryPath = ""

            theStatus = KPDCGetAttributeValue(theCamera, Convert.ToUInt32(KPDCCamAttr.KPDCSerialNumberStringID), _
                      theAttributeSize, tmpMemPath)
end sub

Everything seems to work fine. The returned status is OK, similar to the c call, theAttributeSize changes from 11 to 10, but I can't find a way to get the string value based on the tmpMemPath = 1312043344 value returned by the function.
E.g. trying:
      theMemoryPath = Marshal.PtrToStringAuto(tmpMemPath)                  
 yields "" while all the other versions of Marhal.PtrToString...(tmpMemPath [,]) result in an reference not set to an object exception.
Just ask if additional information is needed.
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Question by:Der_Steppenwolf
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3 Comments
 
LVL 48

Accepted Solution

by:
AlexFM earned 200 total points
Comment Utility
In C code you allocate memory for output outAttrValue parameter:

char theMemoryPath[255];
memset(theMemoryPath, 0, sizeof(theMemoryPath) );
theStatus = theProcs.GetAttributeValue( theCamera, KPDCSerialNumberStringID,&theAttributeSize, theMemoryPath);

In VB you just pass NULL-pointer:
Dim tmpMemPath As IntPtr = Nothing
theStatus = KPDCGetAttributeValue(theCamera, Convert.ToUInt32(KPDCCamAttr.KPDCSerialNumberStringID), _
                      theAttributeSize, tmpMemPath)

Allocate memory by the same way as in C:

Dim tmpMemPath As IntPtr
tmpMemPath = Marshal.AllocHGlobal(255)
theStatus = KPDCGetAttributeValue(theCamera, Convert.ToUInt32(KPDCCamAttr.KPDCSerialNumberStringID), _
                      theAttributeSize, tmpMemPath)

Now try to extract valur written to tmpMemPath by KPDCGetAttributeValue.

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Author Comment

by:Der_Steppenwolf
Comment Utility
Thanxs for the quick help, one additional twist was, that the outAttrValue has to be passed as ByVal rather than ByRef. Based on your answer I finally grabbed the concept of passing pointers to dll :-)
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by:AlexFM
Comment Utility
Right. I didn't see this.

ByVal outAttrValue As IntPtr        void* outAttrValue        
ByRef outAttrValue As IntPtr        void** outAttrValue
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