How to pass .net (C#) string to unmanaged C++ dll?

Tesic Goran

Hi, We have an issue - how to pass .net (C#) string to unmanaged C++ dll? We tried something, but this error came up: "An unhandled exception of type 'System.AccessViolationException' occurred in mscorlib.dll Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt." Does anybody know how to do this? We need the code sample. Thank you in advance. Goran

Luc Pattyn

Should be no problem. A managed string can be passed to a native char* (read-only) in a straightforward way; for a writeable char*, you need a StringBuilder. See this little article[^]. :)

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DaveyM69

As Luc Says, it shoul be no problem. To help any further we would need to see the unmanaged function signature.

Dave
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Tesic Goran

Thank you. I've just tried it, but the same error came up. Our C# code looks as follows:

\[ComVisible(true)\]
\[InterfaceType(ComInterfaceType.InterfaceIsDual)\]
\[Guid("F4100FB3-75C5-4C4F-8034-9B836598DD1E")\]
public interface IDecoder
{
    \[return: MarshalAs(UnmanagedType.BStr)\]
    StringBuilder Decode(\[MarshalAs(UnmanagedType.BStr)\] ref string encodedString);

public class UfoDecoder : IDecoder
{
    public StringBuilder Decode(ref string encodedString)
    {
        byte\[\] dec64 = Base64Decode(encodedString);
        byte\[\] unzipped = CompressionUtility.Decompress(dec64);

        return BinToXml(unzipped);
    }

And our C++ code looks as follows:

extern "C" __declspec(dllexport) void decode(char *encString, char *answer )
{

// Initialize COM.
    HRESULT hr = CoInitialize(NULL);
IDecoderPtr pIDec(\_\_uuidof(UfoDecoder));

    BSTR k1 = bstr\_t(encString);
BSTR k2 = bstr\_t(answer);

pIDec -> Decode(&k1, &k2);

The error comes up on calling last line in C++ block.

modified on Monday, May 30, 2011 5:05 AM

Tesic Goran

Yes. I've just posted it up there.

Luc Pattyn · modified on Monday, May 30, 2011 5:05 AM

get rid of the ref keyword, objects always get passed by reference, your native code will get a pointer without using ref. FWIW: With ref, it would be a char**. [ADDED] On a second read, I don't understand your native code at all. It seems to me the decode() function isn't relevant. All that matters is Decode(), of which you didn't show the actual signature. However your prototype suggests it returns a StringBuilder (bad idea, that is a managed type, no native function returns managed types automatically), and yet your decode() just ignores the return value!?!? If you need more help, please show the declaration of the native Decode() function. If it doesn't require BSTR (say it just takes two char*, then things should be simple). [/ADDED] :)

Luc Pattyn [My Articles] Nil Volentibus Arduum

The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
Please use <PRE> tags for code snippets, they improve readability.
CP Vanity has been updated to V2.3

modified on Monday, May 30, 2011 5:38 AM

Tesic Goran · modified on Monday, May 30, 2011 5:38 AM

Do you mean on this:

\[ComVisible(true)\]
\[InterfaceType(ComInterfaceType.InterfaceIsDual)\]
\[Guid("F4100FB3-75C5-4C4F-8034-9B836598DD1E")\]
public interface IDecoder
{
    \[return: MarshalAs(UnmanagedType.BStr)\]
    StringBuilder Decode(\[MarshalAs(UnmanagedType.BStr)\] string encodedString);

public class UfoDecoder : IDecoder
{
    public StringBuilder Decode(string encodedString)
    {
        byte\[\] dec64 = Base64Decode(encodedString);
        byte\[\] unzipped = CompressionUtility.Decompress(dec64);

        return BinToXml(unzipped);
    }

If so, I have the same error.

Luc Pattyn

sorry, we were both typing at the same time. I expanded my earlier reply. :)

Luc Pattyn [My Articles] Nil Volentibus Arduum

The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
Please use <PRE> tags for code snippets, they improve readability.
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Tesic Goran

Ok. The whole code is here:

extern "C" __declspec(dllexport) void decode(char *encString, char *answer )
{
// Initialize COM.
HRESULT hr = CoInitialize(NULL);
IDecoderPtr pIDec(__uuidof(UfoDecoder));

BSTR k1 = bstr\_t(encString);
BSTR k2 = bstr\_t(answer);

pIDec->Decode(&k1,&k2);

SysFreeString(k1);
SysFreeString(k2);

}

int _tmain(int argc, _TCHAR* argv[])
{
char *xml = (char*)malloc(5000000*sizeof(char));
char* str = "Some encoded string";
int length = strlen(str);

decode(str, xml);

printf("Decoded = %s",xml);

int vol = strlen(xml);
return 0;

}

\[ComVisible(true)\]
\[InterfaceType(ComInterfaceType.InterfaceIsDual)\]
\[Guid("F4100FB3-75C5-4C4F-8034-9B836598DD1E")\]
public interface IDecoder
{
    \[return: MarshalAs(UnmanagedType.BStr)\]
    string Decode(\[MarshalAs(UnmanagedType.BStr)\] ref string encodedString);


\[ComVisible(true)\]
\[Guid("85F2DFFB-5655-40DD-A779-94388A2773DC")\]
\[ProgId("UfoDecodeWrapper.UfoDecoder")\]
\[ClassInterface(ClassInterfaceType.None)\]
\[ComDefaultInterface(typeof(IDecoder))\]
public class UfoDecoder : IDecoder
{
    public string Decode(ref string encodedString)
    {
        byte\[\] dec64 = Base64Decode(encodedString);
        byte\[\] unzipped = CompressionUtility.Decompress(dec64);

        return BinToXml(unzipped);
    }

[ADDED] If str variable in _tmain function in this line: char* str = "Some encoded string"; is not so long we're getting Xml. But, if str is very long, the error comes up. [/ADDED]

modified on Monday, May 30, 2011 6:22 AM

Luc Pattyn · modified on Monday, May 30, 2011 6:22 AM

Sorry, I give up. It is all just too confusing for me. I now think you have native code calling managed code, not the other way around. :doh:

Luc Pattyn [My Articles] Nil Volentibus Arduum

The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
Please use <PRE> tags for code snippets, they improve readability.
CP Vanity has been updated to V2.3

modified on Friday, June 10, 2011 8:47 PM

Tesic Goran · modified on Friday, June 10, 2011 8:47 PM

I've added some explanation up there: [ADDED] If str variable in _tmain function in this line: char* str = "Some encoded string"; is not so long we're getting Xml. But, if str is very long, the error comes up. [/ADDED]

Tesic Goran · modified on Friday, June 10, 2011 8:47 PM

"decode" function is relevant because it's placed in this function:

int _tmain(int argc, _TCHAR* argv[])
{
char *xml = (char*)malloc(5000000*sizeof(char));
char* str = "Some encoded string";
int length = strlen(str);

decode(str, xml);

printf("Decoded = %s",xml);

int vol = strlen(xml);
return 0;

}

MicroVirus · modified on Monday, May 30, 2011 6:22 AM

Please avoid using things like:

char* str = "A string literal here";

If you define it this way, it should be defined as a const char*. If it needs to be editted, define it as a char array with a large enough size. Also, why are you calling pIDec->Decode(&k1,&k2);. Why not just pIDec->Decode(k1,k2);? And then... why are you defining Decode as a function with one parameter and yet calling it with two parameters?