Passing strings to C++ DLL

[Andy Baker]

I am attempting to write a .NET wrapper for a C++ DLL file, but am having
problems with passing strings as parameters. How should I be writing my C#
function call when the C header file is definined as taking a char * as an
argument? For example the C++ header says
SDCERR GetCurrentConfi g(DWORD *num, char *name);
I am using Uint for the *num parameter, which returns the correct value
but for *name, I always get back a string of 6 squares. I have tried using
string, StringBuilder and char arrays, and passing by ref and not, but
always get the same result.
Connected to this I am also having problems with passing structures for
C++ functions - the structures contain strings as well. Whenever I call a
function that requires a structure as a parameter, I always get a
NotSupportedExc eption, does this just mean that I have defined my structure
incorrectly? Do I have to initailise the structure beforehand (have tried
this but still didn't work)?
I am using Visual Studio 2005, and if it makes a difference it is for a
Compact Framework device. Thanks in advance for any help.

Andy Baker

[puggid@googlemail.com]

Re: Passing strings to C++ DLL

Have you tried passing a StringBuilder?

[Andy Baker]

Re: Passing strings to C++ DLL

Yes, I tried StringBuilder. From what I read on the net, that's what I
thought I should be doing. The code I am using is as follows:-
uint Number = 0;
StringBuilder Name = new StringBuilder(C ONFIG_NAME_SZ);
SDCERR Result = GetCurrentConfi g(ref Number, Name);
Result contains a valid value, as does Number, but Name just contains
rubbish. If I pass Name with the ref keyword I get a NotSupportedExc eption.
To convert the StringBuilder to a string, I am just using ToString - is this
correct?

Andy Baker

<puggid@googlem ail.comwrote in message
news:5b2bd884-532d-456a-9852-c4bf3bae2387@f6 3g2000hsf.googl egroups.com...

Have you tried passing a StringBuilder?

[Pavel Minaev]

Re: Passing strings to C++ DLL

On Jul 2, 3:17 pm, "Andy Baker" <aba...@NOSPAMv anputer.comwrot e:

I am attempting to write a .NET wrapper for a C++ DLL file, but am having
problems with passing strings as parameters. How should I be writing my C#
function call when the C header file is definined as taking a char * as an
argument? For example the C++ header says
    SDCERR GetCurrentConfi g(DWORD *num, char *name);
    I am using Uint for the *num parameter, which returns the correct value
but for *name, I always get back a string of 6 squares. I have tried using
string, StringBuilder and char arrays, and passing by ref and not, but
always get the same result.
    Connected to this I am also having problems with passing structures for
C++ functions - the structures contain strings as well. Whenever I call a
function that requires a structure as a parameter, I always get a
NotSupportedExc eption, does this just mean that I have defined my structure
incorrectly? Do I have to initailise the structure beforehand (have tried
this but still didn't work)?
    I am using Visual Studio 2005, and if it makes a difference it is for a
Compact Framework device. Thanks in advance for any help.

Can you please show your P/Invoke declaration for this function?
Perhaps you've forgotten CharSet?
Try this also:

[DllImport(CharS et = CharSet.Ansi)]
SDCERR GetCurrentConfi g(ref uint num, StringBuilder name);

[=?Utf-8?B?S0g=?=]

RE: Passing strings to C++ DLL

Andy, a couple options:

1) Look at the MarshalAsAttrib ute which tells .NET how to translate its
unicode string between it and unmanaged code; the attribute is applied to the
argument, here's the example from MSDN:

void MyMethod( [MarshalAs(LPStr )] string s);

2) The other option is to write the wrapper in managed C++; if you go this
route your method calls will look something like this:

void MyMethod(String ^ s)
{
IntPtr ptr = Marshal::String ToHGlobalAnsi(s tr);;

try
{
SDCERR err = GetCurrentConfi g( (char*)ptr.ToPo inter() );
}
finally
{
if (ptr.ToPointer( )) Marshal::FreeHG lobal(ptr);
}
}

HTH - KH



"Andy Baker" wrote:

I am attempting to write a .NET wrapper for a C++ DLL file, but am having
problems with passing strings as parameters. How should I be writing my C#
function call when the C header file is definined as taking a char * as an
argument? For example the C++ header says
SDCERR GetCurrentConfi g(DWORD *num, char *name);
I am using Uint for the *num parameter, which returns the correct value
but for *name, I always get back a string of 6 squares. I have tried using
string, StringBuilder and char arrays, and passing by ref and not, but
always get the same result.
Connected to this I am also having problems with passing structures for
C++ functions - the structures contain strings as well. Whenever I call a
function that requires a structure as a parameter, I always get a
NotSupportedExc eption, does this just mean that I have defined my structure
incorrectly? Do I have to initailise the structure beforehand (have tried
this but still didn't work)?
I am using Visual Studio 2005, and if it makes a difference it is for a
Compact Framework device. Thanks in advance for any help.
>
Andy Baker
>
>
>
>

[Andy Baker]

Re: Passing strings to C++ DLL

My P/Invoke declaration is as follows:
[DllImport("VPSu mmit.dll", CharSet = CharSet.Unicode "]
public static extern SDCERR GetCurrentConfi g(ref unit num, StringBuilder
name);
I have also tried CharSet.Auto - CharSet.Ansi is not a option for the
Compact Framework (I think)

"Pavel Minaev" <int19h@gmail.c omwrote in message
news:1f2323cd-7b69-47a4-96ac-1f64b32c8003@w7 g2000hsa.google groups.com...
On Jul 2, 3:17 pm, "Andy Baker" <aba...@NOSPAMv anputer.comwrot e:

I am attempting to write a .NET wrapper for a C++ DLL file, but am having
problems with passing strings as parameters. How should I be writing my C#
function call when the C header file is definined as taking a char * as an
argument? For example the C++ header says
SDCERR GetCurrentConfi g(DWORD *num, char *name);
I am using Uint for the *num parameter, which returns the correct value
but for *name, I always get back a string of 6 squares. I have tried using
string, StringBuilder and char arrays, and passing by ref and not, but
always get the same result.
Connected to this I am also having problems with passing structures for
C++ functions - the structures contain strings as well. Whenever I call a
function that requires a structure as a parameter, I always get a
NotSupportedExc eption, does this just mean that I have defined my
structure
incorrectly? Do I have to initailise the structure beforehand (have tried
this but still didn't work)?
I am using Visual Studio 2005, and if it makes a difference it is for a
Compact Framework device. Thanks in advance for any help.

Can you please show your P/Invoke declaration for this function?
Perhaps you've forgotten CharSet?
Try this also:

[DllImport(CharS et = CharSet.Ansi)]
SDCERR GetCurrentConfi g(ref uint num, StringBuilder name);

[Andy Baker]

Re: Passing strings to C++ DLL

Thanks for the advice. I have been looking at the MarshalAs attribute in
relation to passing structures, but hadn't tried it in the DllImport
declaration. However, I am obviously doing something wrong as every time I
try it I am getting a NotSupportedExc eption.
As I understand even less about C++ that I do about C# (which isn't
alot!) I don't think the managed C++ route is the one to go down. I think I
need someone more experienced to do this job for me! Thanks again.

Andy Baker

"KH" <KH@discussions .microsoft.comw rote in message
news:9C5B61AE-FAD6-496B-94B0-8D104F6C24D0@mi crosoft.com...

Andy, a couple options:
>
1) Look at the MarshalAsAttrib ute which tells .NET how to translate its
unicode string between it and unmanaged code; the attribute is applied to
the
argument, here's the example from MSDN:
>
void MyMethod( [MarshalAs(LPStr )] string s);
>
2) The other option is to write the wrapper in managed C++; if you go this
route your method calls will look something like this:
>
void MyMethod(String ^ s)
{
IntPtr ptr = Marshal::String ToHGlobalAnsi(s tr);;
>
try
{
SDCERR err = GetCurrentConfi g( (char*)ptr.ToPo inter() );
}
finally
{
if (ptr.ToPointer( )) Marshal::FreeHG lobal(ptr);
}
}
>
HTH - KH
>
>
>
"Andy Baker" wrote:
>

>I am attempting to write a .NET wrapper for a C++ DLL file, but am having
>problems with passing strings as parameters. How should I be writing my
>C#
>function call when the C header file is definined as taking a char * as
>an
>argument? For example the C++ header says
> SDCERR GetCurrentConfi g(DWORD *num, char *name);
> I am using Uint for the *num parameter, which returns the correct
>value
>but for *name, I always get back a string of 6 squares. I have tried
>using
>string, StringBuilder and char arrays, and passing by ref and not, but
>always get the same result.
> Connected to this I am also having problems with passing structures
>for
>C++ functions - the structures contain strings as well. Whenever I call a
>function that requires a structure as a parameter, I always get a
>NotSupportedEx ception, does this just mean that I have defined my
>structure
>incorrectly? Do I have to initailise the structure beforehand (have tried
>this but still didn't work)?
> I am using Visual Studio 2005, and if it makes a difference it is for
>a
>Compact Framework device. Thanks in advance for any help.
>>
>Andy Baker
>>
>>
>>
>>