Calling an unmanaged c++ function having arguments as pointers from VB.Net

Question

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Hi,

I've the following function written in C++ in Win32 Library Project

C++

__declspec(dllexport) void __stdcall SetupChat(char *chatName, bool Advertiser, int &maxchars)

I'm trying to call this function from VB.Net application (Framework 3.5) using the following code:

VB

[dllimport("chatlib32.dll"] _
Private Shared Sub SetupChat(<marshalas(unmanagedtype.lparray)> chatName As Char(), ByVal asAdvertiser As Boolean, ByRef size As Integer)
End Sub

But in the function SetupChat in the C++ it only prints the first character. So I tried the following code instead:

VB

Declare Auto Sub SetupChat Lib "ChatLib32.dll" (name As String, ByVal asAdvertiser As Boolean, ByRef size As Integer)

And I also got the same result of the previous function.

So can anyone help me?

Thanks a lot.

Posted 24-May-13 3:27am

Mr.HA-AS

Updated 24-May-13 3:39am

Richard MacCutchan

v2

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Comments

Richard MacCutchan 24-May-13 9:42am

The default character type in VB.NET is Unicode, but char in C++ is ASCII. You need to convert your characters to ASCII in the VB.NET code, or modify the dll code to expect a Unicode string.

Sergey Alexandrovich Kryukov 24-May-13 11:04am

This is the default marshaling. Other cases are resolved by applying [MarshalAs] attribute to a parameter...
Please see OP's comment below.
—SA

Richard MacCutchan 24-May-13 11:19am

From the comments it sounds as though the C++ code gets a Unicode string passed in.

Sergey Alexandrovich Kryukov 24-May-13 14:30pm

And then the parameter should come with appropriate [MarshalAs]...
—SA

Mr.HA-AS 24-May-13 10:01am

In my c++ project I add UNICODE word in Preprocessor Definitions. Also in my function in VB.NET I added thisenumeration to the SetupChat function CharSet.Unicode as following:

<DllImport("ChatLib32.dll", CallingConvention:=CallingConvention.Cdecl, CharSet:=CharSet.Unicode)> _
Private Shared Sub SetupChat(<marshalas(unmanagedtype.lparray)> chatName As Char(), ByVal asAdvertiser As Boolean, ByRef size As Integer)
End Sub

I assume what I did let both VB.Net and VC++ use UNICODE Character Set .But I still get the same result.

Richard MacCutchan 24-May-13 11:18am

Yes, but your function definition in the DLL uses char, which is always ASCII. If you want it to use Unicode then you must use the wchar_t type.

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Mr.HA-AS · Answer 1 · 2013-05-24T09:45:00

Solution 1

I got an answer from StackOverFlow as following and it worked with, thanks for everyone supports:

ASM

<DllImport("ChatLib32.dll", CharSet:=CharSet.Ansi)> _
Private Shared Sub SetupChat(ByVal chatName As String, ByVal asAdvertiser As Boolean, ByRef size As Integer)
End Sub