Returning a string from a C++ dll

[howardnl]

Hello,

I have a C++ dll that needs to return a string to a C# program. What's the best way to do this? I've tried several things, but I can't seem to get it to work.

I really appreciate any help.

Thanks,
Nick

[MartinL]

In C++

/* Dll export specifier*/ LONG SomeFuncName(BSTR* bstrRetString)
{
    // some stuff
    // Fill *bstrRetString with appropriate string
    _bstr_t bs("this is some string which I want to return to the c# prog");
    *bstrRetString = bs.copy();

    return 0L;
}

You can define this function when it is imported into the C# as following:

class DllWrapper
{
    [DllImport("YourDll.dll")]
    public static extern long SomeFuncName(out string str);
}

than you can use it:

void SomeOtherFunc()
{
    string str;
    long lRet = DllWrapper.SomeFuncName(out str);
    MessageBox.Show("Returned string: " + str);
}

Martin

[howardnl]

Martin,

Thank you very much for your help. I've got everything running, but for some reason I only seem to be getting the first letter of the string that I am returning. For testing I used your test string below and I always get "t" returned. What does OL need to be that I return? Anything specific?

Thanks again for your help.

Regards,
Nick

[pareshgh]

try this one out,
CString s( "abcd" );
LPTSTR p = s.GetBuffer( 10 ); // LPTSTR will be ur ref string
strcpy( p, "Hello" ); // directly access CString buffer
s.ReleaseBuffer( );

<edit>
this will be in your C++ code. if CString is not allowed (forexample in case of embeded environment) then just use std::string of STL

</edit>
Paresh

[MartinL]

No paresh... That is not the thing...

I just trying it... I wrote the answer down and didn't test it...

I wrote win32 dll which exports just one function. Function returns int and takes BSTR* as parameter...

Then, I wrote C# client (console app) as I post in my answer. It really gets just the first character of the string I want to pass to the caller of the dll's function...

Then, I wrote C prog for testing that dll. It works ok...

I think, the problem is in using unicode string. However I still didn't find solution. I used the same mechanism like that in my answer in several COM objects I wrote. When I used those COM objects from the C#, they worked well... I really don't know why I can't get unicode string from the DLL into this C# app...

I attached the project to this mail...
ConsApp is C# app for testing dll
TryDll is Win32 DLL
TryCpp is C prog for testing dll

solution file is in TryDll dir...

Martin

[howardnl]

Hello Paresh,

Thank you very much for your help. I am using CStrings. It works if I set the CString to a constant, but I am calling a function that returns a CString in my dll.

Here is some more detail. I've got an open CDatabase object and I want to return the GetConnect() value back to the C# program. Here is what I've got:

CString getConnectString;
getConnectString= myDB.GetConnect();

LPTSTR p = getConnectString.GetBuffer(getConnectString.GetLength());
strcpy( p, connectStr );
connectStr.ReleaseBuffer( );

My definitions still look basically like what Martin suggested, but now I'm using LPTSTR. Would that be correct?

Thanks again for your help.

Regards,
Nick

[pareshgh]

ya, you can use LPTSTR
in you C# program pass string as a ref
and take LPTSTR (actually a pointer to char) and then by allocating the size and releasing the buffer is fine.
check out Martin's sample also.

tnx
Paresh

[howardnl]

Paresh,

When I run Martin's sample I only get the first character of the string that is returned (when using the C# client). I think he said the same thing. When I run the C++ program that calls the dll it returns the whole string.

Thanks,
Nick

[GMV]

Hi guys!
Try this!
It should be work!

// Dll C++
extern "C" DllExport int GetStringFromDLL(WCHAR** ppString)
{
LPTSTR p = getConnectString.GetBuffer(getConnectString.GetLength()); // or something else!
*ppString = (LPTSTR)p;
return 1;
}


// C#
[DllImport("Your.dll")]
public static extern int GetStringFromDLL(ref IntPtr p);


void SomeFunction()
{
IntPtr ptr = IntPtr.Zero;
int res = GetStringFromDLL(ref ptr);
string MyString = Marshal.PtrToStringUni(ptr);
}

Good luck!

Max

[pareshgh]

and yes marshalling is requred here. i forgot to mention. thanks GMV

Paresh

[howardnl]

You all are geniuses! Thank you very much for your help.

[MartinL]

Yea... I was working too long yesterday... And even made more mistakes...

[pareshgh]

Martin,
i don't think so you made mistakes


Paresh

[Sarah Leach]

Hi there,

I have a 'similar' problem - except I want to return a string from a C# DLL to a C/C++ application as an output parameter. I've just about given up. Help!

Sarah

[Sarah Leach]

After a lot of trial and error (and much hair pulling) I have finally managed to get a string out of a C# DLL into a C/C++ application. So unless anyone has a better suggestion: use unsafe pointers in the C# code. I used the C# sbyte* type which corresponds to the C/C++ char* type. Note that the C# code has to be compiled with /unsafe.