Returning a string from a C++ dll

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

In C++

Code:

---

/* 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:

Code:

---

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

---

than you can use it:

Code:

---

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

---

Martin

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

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

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

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

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

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

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

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

Paresh

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

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

Martin,
i don't think so you made mistakes
;)

Paresh

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

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.

could you show us your code atleast so that others can get idea.

thanx
Paresh

Yep, it's pretty straightforward when you know how.

Here's the C# method:

public unsafe void GetString( sbyte* String )
{
string TestString = "Hello World";

// Convert string to unsafe pointer
char[] CharArray = new char[TestString.Length];
CharArray = TestString.ToCharArray();

foreach ( char Char in CharArray )
{
*String++ = (sbyte)Char;
}

// Add terminating null
*String = (sbyte)'\0';
}

and the C/C++ code to call it:

char String[30];
GetString( String );

Hope that helps (obviously need to check for string overflow)

Sarah ;)

thanks...;)

Hi guys,
Interesting thread. I have same problem, but I have exported function which I cannot change. The declaration is:
short MyFunc(TCHAR* cRefNumber);
and the export is defined in the *.def file.

And your advice does not work with this scenario. I got the error below on the line MyString = Marshal.PtrToStringUni(ptr);
“An unhandled exception of type ‘System.NullReferenceException’ occurred in mscorlib.dll”.
:confused:

Could you help me, please?
Btw: I am new in .NET and C#.

Thanks,
David

to me it looks like you haven't initialized the string.

paresh

I initialized the string.
I think the main difference is between declartions of exported function. GMV’s solution contains two pointers, but I have only one:
GetStringFromDLL(WCHAR** ppString);
MyDLLFunc(TCHAR* pszRefNumber);
But I cannot change it.

thanks anyway,
David

Hello Experts,

I also have some problems related to string ...
Sorry for posting this question again ...

I want to use a function written in C++ VS6.0 dll.
The function takes "CString&" as an argument.
I want to call this function from a Managed C++ dll.

// .....
CString strTest;
CString strTest1;

SomeFnInVC6( strTest );
strTest1 = strTest; // ----->>> Exception here !!!
// ....


Is it possible to do like this ? Please give me a solution.
Thanks.