return value from c dll to c#

return value from c dll to c# - missing chars

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Simon - 06 Jan 2006 08:53 GMT

Hi

I have written a DLL in eVC4 and exported a function that returns a value. I
have told the DLL to message box the results before returning, and this shows
what I expect, in the example code below its "123456789". When it returns to
c# I again message box the result which shows "56789". Any help would be
greatly appreciated.

c++ code
extern "C"
{
    __declspec(dllexport)LPTSTR testreturn()
    {

        LPTSTR test;
        test = L"123456789";
        MessageBox(NULL, test, L"TEST", MB_OK);
        return test;

    }

}

c# code

[DllImport("test.dll", EntryPoint = "testreturn")]
public static extern string testreturn();

string h = testreturn();
MessageBox.Show(h , "TEST c#");

Reply to this Message

<ctacke/> - 06 Jan 2006 13:07 GMT

It's not a good idea to return a string pointer from a native function, as
the memory pointed to is owned by the callee, not the caller.

Try this:

extern "C"
{
__declspec(dllexport)void testreturn(TCHAR *data)
{
_tcscpy(data, _T("123456789"));
}

}

c# code

[DllImport("test.dll", EntryPoint = "testreturn")]
public static extern void testreturn(StringBuilder sb);

StringBuilder sb = new StringBuilder(1024);
testreturn(sb);
MessageBox.Show(sb.ToString());

-Chris

> Hi
>
[quoted text clipped - 29 lines]
> string h = testreturn();
> MessageBox.Show(h , "TEST c#");

Reply to this Message

Simon - 06 Jan 2006 14:00 GMT

Cheers Chris works a treat

> It's not a good idea to return a string pointer from a native function, as
> the memory pointed to is owned by the callee, not the caller.
[quoted text clipped - 54 lines]
> > string h = testreturn();
> > MessageBox.Show(h , "TEST c#");

Reply to this Message

MSenne - 06 Jan 2006 14:31 GMT

Just as an aside, you should be sure to use StringBuilder when dealing
with character string pointers (ie. if you want to get a string back
from your native Dll call). Using a 'string' object doesn't do the
job. And be sure to initialize your StringBuilder with sufficient
length or it won't go as planned.

Reply to this Message

Lochan Pandya - 24 Mar 2006 13:32 GMT

Hi Cris/simon,
Well I too tried to do the same thing but in a different way.
I want to transfer a string from c# to eVC++.

c# code :

[DllImport("CryptoUtil.dll")]
private static extern int fnDecryptPayload(StringBuilder sb);

StringBuilder sb1 = new StringBuilder(strTokenKeyFile.Length);
sb1.Append(strTokenKeyFile)

int CharsReturned = fnDecryptPayload(sb1);

c++ code:

DECRYPTPAYLOAD_API long fnDecryptPayload(TCHAR * tokenFile)

But if I print the "tokenfile" string in c++ i get exactly the half of
string that I send from c#.

Please help.

Thanks,
Lochan.

> Cheers Chris works a treat
>
[quoted text clipped - 56 lines]
> > > string h = testreturn();
> > > MessageBox.Show(h , "TEST c#");

Reply to this Message

Chris Tacke [MVP] - 24 Mar 2006 19:30 GMT

Show us the code you're using to print the data in C++.

-Chris

> Hi Cris/simon,
> Well I too tried to do the same thing but in a different way.
[quoted text clipped - 87 lines]
>> > > string h = testreturn();
>> > > MessageBox.Show(h , "TEST c#");

Reply to this Message

Lochan Pandya - 27 Mar 2006 08:08 GMT

First of all, thanx for brisk reply chris.

My C++ code:

DECRYPTPAYLOAD_API long fnDecryptPayload(TCHAR * tokenFile)
{
HANDLE hlochan = 0;
DWORD dwRetSize = 0;
hlochan = CreateFile(L"c:\\lochan.txt",GENERIC_WRITE,0,0,CREATE_ALWAYS,0,0);
WriteFile(hlochan,tokenFile,wcslen(tokenFile),&dwRetSize,NULL);
if(hlochan != 0)
{
CloseHandle(hlochan);
}
}

In c++:
I am trying to write the string variable "tokenFile" in the file lochan.txt

In C#:
In the variable "tokenFile" i'll pass the token file's absolute path eg.
"c:\\windows\\token.clear"

Thanks,
Lochan

> Show us the code you're using to print the data in C++.
>
[quoted text clipped - 91 lines]
> >> > > string h = testreturn();
> >> > > MessageBox.Show(h , "TEST c#");

Reply to this Message

<ctacke/> - 27 Mar 2006 14:03 GMT

1. CE has no concept of drives, so "C:\{anything}" is meaningless.
2. Don't mix your macros, if you use TCHAR, use _tcslen, not wcslen. If you
compile that under ANSI it will break.
3. WriteFile takes a number of bytes to write, not a number of characters.
Since under unicode TCHAR is 2 bytes per character, your wcslen will return
1/2 the number of bytes in the string, and won't account for a terminating
null.

Should be something like this:

WriteFile(hlochan, tokenFile, (_tcslen(tokenFile) + 1) * sizeof(TCHAR),
&dwRetSize, NULL);

-Chris

> First of all, thanx for brisk reply chris.
>
[quoted text clipped - 122 lines]
>> >> > > string h = testreturn();
>> >> > > MessageBox.Show(h , "TEST c#");

Reply to this Message

Lochan Pandya - 28 Mar 2006 12:17 GMT

Chris,
I use:
# include <tchar.h>

changed the WriteFile to:

WriteFile(hlochan,tokenFile,(_tcslen(tokenFile) + 1)*
sizeof(TCHAR),&dwRetSize,NULL);

I get the error while compiling :
error C2664: 'strlen' : cannot convert l from 'TCHAR *' to 'const char *'

If I use WCHAR instead of TCHAR, this is what I do:

WriteFile(hlochan,tokenFile,(wcslen(tokenFile))* sizeof(WCHAR),
&dwRetSize,NULL);

I am getting the string but i think the string end is not interpreted
properly.
If I give the same string c:\\token.clear, it is transported with three
spaces appended to it.
If I send string c:\\lochan\\token.clear i get the strange string :
"㩃屜潬档湡屜潴敫⹮汣慥rWS/Microsoft.NET/Framework/v1.1.4322/System.dll"

Actually first I am testing this application on my desktop and then will try
it on my PDA.

Thanks,
Lochan

> 1. CE has no concept of drives, so "C:\{anything}" is meaningless.
> 2. Don't mix your macros, if you use TCHAR, use _tcslen, not wcslen. If you
[quoted text clipped - 137 lines]
> >> >> > > string h = testreturn();
> >> >> > > MessageBox.Show(h , "TEST c#");

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