Introduction to the problem:
I've got to control a certain device through API provided with a DLL file, LIB file, and c header files whose functions are declared as dllimport.
When I use the API in a C++ project everything worked just fine - I included the headers, lib , dll, and called the functions as declared in the header files.
The problem begins when trying to call those functions from a C#.NET project, using [DllImport] attribute: The functions were declared with the exact name and parameters, and running the code did not throw any exception. And yet the device did not respond at all, like the functions had never been actually called.
How it is declared in the C header:
int __declspec(dllimport) Init_Card_RTx(unsigned short device_num, unsigned short enabled_channels, unsigned short xmt_channels);
How it is declared in C#:
[DllImport(Path, CallingConvention = CallingConvention.Cdecl, EntryPoint = "Init_Card_RTx")]
public static extern int Init_Card_RTx(UInt16 device_num, UInt16 enabled_channels, UInt16 xmt_channels);
The questions:
Is that because the functions in the headers are declared dllimport?
In that case, do I have to wrap the DLL with C++ functions declared as dllexport?
What are the steps required for a C DLL to be accessible from C#?
In the C# project, do I have to include the LIB file as well? not just the DLL file itself?
Is that because the functions in the headers are declared dllimport?
Possibly. The functions need to be exported. Whether they are exported or not depends on how they DLL was compiled by whoever gave it to you. I'd guess that they should be though (or the C++ code would try import them and fail).
The first thing I do when troubleshooting this kind of thing is to load the DLL in Dependency Walker which will show you all the exported functions. If they're not showing up, they're not exported, and C# can't call them. If they are showing up, then you're ok and you don't need to change any of your C code or create any wrapper functions.
In that case, do I have to wrap the DLL with C++ functions declared as dllexport?
No. C# can only call C functions using DllImport. C++ does name mangling when exporting functions, which makes things a mess and generally not workable.
What you need to do is make your functions exported somehow. My guess is that they are already exported, but if not, you could make some wrapper functions like this.
int __declspec(dllexport) Init_Card_RTx(unsigned short device_num, unsigned short enabled_channels, unsigned short xmt_channels) {
// just call the other function here
}
These should make the function exported and you should see it show up in dependency walker.
What are the steps required for a C DLL to be accessible from C#?
The CLR has to be able to find the dll (usually you just put it in the same directory as the C# exe), and the functions have to be exported as C functions. That's pretty much it.
In the C# project, do I have to include the LIB file as well? not just the DLL file itself?
You don't need to include anything in the C# project at all, just the public static extern wrapper functions with their DllImport attributes on them. So long as the CLR can find the dll at runtime, that's all you need. If it can't find it at runtime, you should get an exception when you call the first imported method.
PS: Get Dependency walker. I can't recommend it more highly for this kind of thing
This kind of thing is always a pain in the butt. You have to make sure that the byte structure of the data being passed matches what your DLL is expecting, and exactly. The native structures in C# don't know that they are being passed externally, so you have to structure the code more rigorously.
See here for more info:
http://msdn.microsoft.com/en-us/library/awbckfbz(v=vs.110).aspx
The reason the functions are declared as dllimport is (correct me if I'm wrong) the the DLL is provided by your card vendor, that's ok because dllexport is only needed when building the DLL.
[DllImport(Path, CallingConvention = CallingConvention.Cdecl, EntryPoint = "Init_Card_RTx")]
The first argument Path is there only for ilustration purposes, am I rigth?
Can you open the DLL with depends.exe or PE Explorer(http://www.heaventools.com/download-pe-explorer.htm) and see the DLL's export table?
I'm asking this because the function name maybe decorated, that could happen if the DLL was compiled as C++ instead of plain C.
Try declacring it like this (you only do this once, in a CPP file, NOT a header)
extern "C" __declspec(dllexport) int __stdcall Init_Card_RTx(unsigned short device_num, unsigned short enabled_channels, unsigned short xmt_channels)
{
//
}
and in C#:
[DllImport("MyDll.dll")]
int Init_Card_RTx(ushort device_num, ushort enabled_channels, ushort xmt_channels);
you need to export it and declare with extern "C". __stdcall is convenient as it's the default on the .net side. I have a macro in my C++ project to make sure I type it the same everytime.
And yes, get dependencywalker....
Related
A third-party app I have can call extension DLLs if they have C-compliant interfaces as described below.
I would like the third-party app to call my C# DLL, since I would rather write in C# than C. (Maybe my other choice would be to write a C DLL wrapper to call my C# DLL, which is an extra step that I might not get right).
I've searched the net and SO but haven't found (or recognized) a good match to my question.
The best suggestion seemed to be this special marshalling declarations in the C# code here: Calling a C# DLL from C; linking problems
Is it possible to write a C# DLL that has a C-compliant interface like the one below? If so, could someone tell me what I have to do or point me to some documentation?
The third-party app documentation says, "The DLL function must be a PASCAL function that takes a single LPCSTR (long pointer to a constant string) argument. C or C++ DLL functions called must have prototypes equivalent to:
extern "C" __declspec(dllexport) void __stdcall fn(LPCTSTR szParam );
Take a look at this article: https://www.codeproject.com/Articles/12512/Using-the-CDECL-calling-convention-in-C-changing
It discusses a different problem (opposite to yours) but also explains the linkage of C# functions - by default C# uses __stdcall (and it cannot be overridden). This means your third party app should be able to call the C# function as it stands. Have you tried it? Did you get any errors?
I have to rebuild software that was written in C++/MFC & Unicode. There are two assemblies, one Server and one Client. The Client has to be rebuilt in C#+WPF.
Originally there was a .h file included by both assemblies, to share some global values. I want to use these values in the new C# assembly, too.
My first approach is to use SWIG. So I created a test-Project (that Outputs a DLL) that contains the .h file and let do SWIG its Magic.
This results in a C++ Wrapper for the C++ Project and some .cs files.
Then I created a C# WPF application, included the DLL and the generated cs files from swig.
So far so good.
The .h file contains basically int variables, classes that contain an enum each and some LPCTSTR strings (that are wchar_t * since I use Unicode).
const LPCTSTR ProductName = _T("XYZ");
const int MAX_MESS_ITEMS = 200;
class SPS
{
public:
enum SPS
{
SPS1 = 1,
SPS2 = 2,
SPS3 = 3,
};
};
Using the int and enums is no Problem, but swig isn't able to convert the LPCTSTR to C# string (without doing some customizing). Swig generates a SWIGTYPE_p_LPCTSTR type which does not allow me to get the original Content of the string as defined in the .h file.
As a next Approach I walked through the SWIG Documentation (I use version 3.0.12) and found out about typedef and typemaps.
I tried to include the windows.i and that gave me the first letter of the string assigned to a LPCTSTR variable.
Next I tried to figure out how to use a typemap for this, but I can't get this to work. For me it is pretty difficult to understand how to create a typemap (I have some Problems understanding what is done there and how the Syntax works). Also the lib (that I downloaded with the swigwin.zip) that contains some typemaps for std::string etc. does not help me.
Also Research on stackoverflow and MSDN did not take me closer to a solution.
Are there any good instructions or examples on how to crate a typemap for converting strings (especialy LPCTSTR) ?
What other easy-to-use tools can make the Job? Should I stop using SWIG and use _declspec(dllexport) stuff (with what I don't have any experience)?
I'm not in the Need to convert functions or highly-desgined classes. Just the strings, int, and enums.
Since the two assemblies are communicating over a namedpipe, I concerned to send some data over the pipe when starting the applications, but there are some Information from the global variables needed before the pipe starts doing its work.
Any help concerning this is appreciated :)
I'm trying to find a tutorial on creating a C++ class in a DLL that can be called from C#. I've found a few tutorials but they either do not compile in VS 2013 Community or are not on point. The C++ object has only a few exported functions (i.e., that are called from the consuming C# app like ptr->StartAction()). It has about 70 procedures that are not exposed, in assembly language which is why I need to use C++. Currently I expose some functions in the DLL via dllimport etc. But C# requires using fixed(...) (to prevent garbage collection) which slows everything down quite a bit (and they are called 1,000,000s of times). Hence the need to move all that into the DLL. So I need a class that I can instantiate in the C# calling program and then call the public methods in the dll. I'm using .NET 4.0. The DLL will only be used by this one C# app.
You can achieve it through the help of COM component and IDL with an output of Interoperability dll where you can use in your c# project. For more details follow this link.
You can use the DllImport attribute of C#. Here is a sample signature:
[DllImport("path to dll", CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.BStr)]
public static extern string methodName(string parameterName);
C# will call the method with the same name in the DLL.
Its impossible to say what you really need, because we don't have your method declarations.
I want to create a DLL using C#, which can be then called from VBA in the following manner -
Private Declare Function invokePath Lib "\\shared_computer\Projects\somedll\myDLL.dll" (ByVal strUName As String, ByVal strSFile As String) As String
The idea is, if the path of the DLL is changed I need to only update the path in the Private Declare function line....
I have searched a lot for it, and also not sure whether this arrangement is possible - where we can call a DLL function from a network path without referencing or registering.
Actually you can.
Check out unmanagedexports
It is a complete walk-through and you'll be able to create entry points like this:
[DllExport("GenerateSomehting", CallingConvention = CallingConvention.Cdecl)]
public static int GenerateSomehting(int somevalue, ref IntPtr pointer)
and call it just like any other unmanaged dll.
[DllImport("MyGenerator.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int GenerateSomehting(int somevalue, ref IntPtr pointer);
You can not create unmanaged DLLs with C#, as C# is a .NET only language. Use C++.
The only way to use a .NET DLL from unmanaged code (for example VBA) is to use COM interop.
Check P/Invoke here, The article contains a useful table of how to translate managed to unmanaged types, short code examples (in C#), tips and a list of resources.
Essential P/Invoke
C# Dll cannot export functions that can be called by any other non-CLR code.
This is not possible.
This question might seem a repeat of previous ones. I have read through a series of posts, but not completely clear for my situation.
I have a c++ library which is created using momentics IDE. I have to be able to use this library into a c# project.
Someone had been working on this project before being handed over to me. Currently, there are 2 layers for making this possible. First, a c++ project includes the complete library with a c++ wrapper. This project creates a dll as the output. This c++ dll is then fed to a c# project, which has dllimport calls to the c++ dll. This c# project again creates a dll. Finally, in order to use the library in c# application, I have to include a reference to both of these dlls.
Is this the correct way to get it working? I was thinking probably there should be a way to simplify the process.
Can someone please help me with this question?
Given that you're using a C++ library, I'm assuming it takes advantage of C++ semantics like classes, rather than just exposing procedures. If this is the case, then the way this is typically done is via a manually-created managed C++ interop library.
Basically, you create a managed C++ library in Visual Studio, reference your existing C++ library, and create a managed wrapper around your existing C++ classes. You then reference this (managed) C++ assembly in your C# project and include the original (unmanaged) C++ library in your C# assembly just as a file that gets placed in the build directory.
This is required because there is no way to reference things like C++ classes via P/Invoke (DllImport) calls.
If your base library is just a series of functions, then you can reference that directly in the C# project via P/Invoke functions.
Either way, all of the libraries mentioned above (for the first, the unmanaged C++ library, the managed C++ assembly, and the C# project, or, for the second, the unmanaced C++ library and the C# project) must be included in any project that references them. You cannot statically link the unmanaged library into the managed assembly.
Seems like you have one wrapper too many but perhaps someone is implementing some sort of facade or adding properties or something. The basic connection between managed and unmanaged will be either DllImport of "flat" function calls - not member functions - or C++/CLI code calling member functions. If the wrapper is C++/CLI it's easiest to write (just include the header for the C++ library) and easiest to call (C# code just adds a .NET reference to it and carries on as normal) so it would be my first choice if there's C++ expertise on the project.
It sounds like whoever you were taking over from is doing it the hard way. If there are less than 20 methods, I would suggest starting over.
You can use:
DllImport
class Example
{
// Use DllImport to import the Win32 MessageBox function.
[DllImport("user32.dll", CharSet = CharSet.Unicode)]
public static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);
static void Main()
{
// Call the MessageBox function using platform invoke.
MessageBox(new IntPtr(0), "Hello World!", "Hello Dialog", 0);
}
}