Char* in C++ 6 to C# .Net [modified] - C# Discussion Boards

22-Mar-11 4:06

Hi Guys,

I have a DLL created in Visual Studio 6 / C++ with the following function

MyDLL_API int __stdcall MyMethod( char *strA, char *strB, char *strC )

i am trying to reference it like so in c# 2005

[DllImport("mydll.dll", CharSet = CharSet.Auto)]
        public static extern int MyMethod(ref string strA, ref string strB, ref string strC);

Here is the debugging code i added to MyMethod but the params are garbage when presented on screen:

MyDLL_API int __stdcall MyMethod( char *strA, char *strB, char *strC )	
        int   ret_val            = 0;
	char  cA[161]   = {0};
	char  cB[161] = {0};
	char  cC[161]   = {0};
	LPSTR buffer             = NULL;
	char  cMsg[161] = {0};

	buffer = (LPSTR)strA;
	strncpy( cA, buffer, 161);

	buffer = (LPSTR)strB;
	strncpy( cB, buffer, 161);

	buffer = (LPSTR)strC;
	strncpy( cC, buffer, 161);

	sprintf( cMsg, "Params A=%s,B=%s,C=%s", strA, strB, strC );
	MessageBox( NULL, cMsg, "Dispense", MB_OK );

the function gets called but all i see is garbage in the parameters passed in, other than a string what else can i use to pass to the Char*?

Thanks In Advance

modified on Tuesday, March 22, 2011 10:29 AM

Richard MacCutchan22-Mar-11 4:34

22-Mar-11 4:34

The default marshalling type for C# strings is Unicode (2 bytes per character) as described here[^]. You will need to modify your C++ code to handle Unicode characters, or add a front end converter from WCHAR to CHAR.

I must get a clever new signature for 2011.

KeithF22-Mar-11 4:40

22-Mar-11 4:40

Thanks Richard,

I cant modify the dll as its from a third party and they just sent me on a samepl of the code they use how would I go about adding a front end converter from WCHAR to CHAR?

Thanks In Advance

Richard MacCutchan22-Mar-11 5:10

22-Mar-11 5:10

Just write your own C/C++ function that interfaces to the C# code and takes the three strings and converts them from Unicode to char using the WideCharToMultiByte()[^] function. You can then pass the converted strings to the DLL routines.

I must get a clever new signature for 2011.

KeithF22-Mar-11 4:37

22-Mar-11 4:37

Just an update on this, I found that if i do the following in C#

[DllImport("mydll.dll", CharSet = CharSet.Auto)]
        public static extern int MyMethod(
            [param: MarshalAs(UnmanagedType.VBByRefStr)]
            ref string strA,
            [param: MarshalAs(UnmanagedType.VBByRefStr)]
            ref string strB,
            [param: MarshalAs(UnmanagedType.VBByRefStr)]
            ref string strC);

That my messagebox in MyMethod in the dll shows the first character of the strings eg passing 1, 234, 5 the results are A=1,B=2,C=5

Richard MacCutchan22-Mar-11 5:13

22-Mar-11 5:13

That sounds about right as the Unicode characters for these values are the same as ANSI. However, because the Unicode are 2-byte characters, and the second byte in these cases will be zero, you effectively see a 1 character string in each parameter.

I must get a clever new signature for 2011.

KeithF22-Mar-11 5:30

22-Mar-11 5:30

Hi Richard,

Thanks for all the help.

After playing around with a few other things i found that this works

[DllImport("MyDll.dll", CharSet = CharSet.Unicode)]
        public static extern int MyMethod(
            [param: MarshalAs(UnmanagedType.LPStr)]
            StringBuilder strA,
            [param: MarshalAs(UnmanagedType.LPStr)]
            StringBuilder strB,
           [param: MarshalAs(UnmanagedType.LPStr)]
            StringBuilder strC);

changeg need were to remove the ref keyword, changes string to stringbuilder and the unmanaged type to LPStr

Richard MacCutchan22-Mar-11 8:16

22-Mar-11 8:16

Great stuff. I see that you also got some help from Ian, and Luc who knows far more about this than I do.

I must get a clever new signature for 2011.

Re: Char* in C++ 6 to C# .Net [modified]

Ian Shlasko22-Mar-11 5:32

Ian Shlasko

22-Mar-11 5:32

I'm no expert on C++ interfacing, so I'd defer to Richard on this, but I think marshaling as UnmanagedType.LPStr might take care of this.

According to the MSDN docs, that'd send them through as single-byte, null-terminated strings. Would eliminate the need to convert from Unicode.

EDIT: Never mind, looks like you already figured this out Smile | :)

Proud to have finally moved to the A-Ark. Which one are you in?
Author of the Guardians Saga (Sci-Fi/Fantasy novels)

Luc Pattyn22-Mar-11 6:16

Luc Pattyn

22-Mar-11 6:16

Some of the replies you got contain too much attributes IMO. Passing strings between C# and native code is pretty easy, you don't even need all those Marshal thingies, a simple CharSet = CharSet.Unicode
[ADDED] or CharSet = CharSet.ASCII [ADDED]
in your DllImport should do it. You must get rid of the ref keywords obviously, as a string is always passed as a pointer anyway. And if one or more of those strings are meant to be output parameters, the easiest way is by using a StringBuilder. I think I covered all these essentials in this article[^].

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum

Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.

modified on Tuesday, March 22, 2011 12:32 PM

KeithF22-Mar-11 6:24

22-Mar-11 6:24

Hi Luc,

I tried what you said as well but I only get see the first character in the C++ method, here is the code:

[DllImport("MyDll.dll", CharSet = CharSet.Unicode)]
        public static extern int MyMethod(
            string strA,
            string strB,
            string strC);

I took out the

[param: MarshalAs(UnmanagedType.LPStr)]

but it seems that this is needed

Luc Pattyn22-Mar-11 6:31

Luc Pattyn

22-Mar-11 6:31

sorry, my mistake, if your native code wants ASCII, just say CharSet.ASCII. There is automatic marshaling that will allocate and later free buffers, translate the strings and everything for you.

Warning: you are not allowed to remember those string pointers (actually any pointer unless the object remains pinned) and use them after the function return!

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum

Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.

KeithF22-Mar-11 6:40

clickonce Deployment [modified]

22-Mar-11 6:40

Hi Luc,

Thanks that works perfectly, you learn something new every day Wink | ;)

arkiboys22-Mar-11 3:40

arkiboys

22-Mar-11 3:40

I have developed a windows application in visual studio 2008.
This application connects to sql server 2005 Express by using a login in sql server.
As soon as the appplication runs (When the form is loaded), there is connection to the sql server, data is retrieved and displayed on the form.

In the clickonce, I just created the setup. After installtion from the CD onto the client machine, the form appears but obviously because there is no sql server 2005 express installed on the client, then the application fails to load data.
I am trying not to install the sql server 2005 express onto the machine and would like the clickonce to take care of the .mdf file.

Question
Using the clickonce procedure to create a setup onto a CD, is there an option to add the .mdf to the setup too? so that I do not have to install the sql server separately?

Note that I do NOT have .mdf in the solution of VS and the application uses a config file to connect to sql server as a connectionstring.

Thanks

modified on Tuesday, March 22, 2011 9:47 AM

Dave Kreskowiak22-Mar-11 4:47

Dave Kreskowiak

22-Mar-11 4:47

You can add the .MDF all you want, but without Sql Server 2005 Express installed, the file is completely useless.

You simply have no choice. As for installing SQL Server Express with ClickOnce, read this[^].

A guide to posting questions on CodeProject[^]

Dave Kreskowiak

arkiboys22-Mar-11 4:51

arkiboys

22-Mar-11 4:51

I see.
In that case I can just do the following:
1- Install sql server 2005 express.
2- Run the setup from the CD as it is (Without the .mdf). There is a connectionstring in the code which points to the local sql server.

What do you think?

Thanks

Rajesh R Subramanian23-Mar-11 4:47

Rajesh R Subramanian

23-Mar-11 4:47

Hi Dave,

Did you receive my email? Otherwise, can you email me at rajesh AT indiamvps DOT net?

Thanks.

"Real men drive manual transmission" - Rajesh.