I am learning C# from my C++/CLR background by rewriting a sample C++/CLR project in C#.
The project is a simple GUI (using Visual Studio/ Windows Forms) that performs calls to a DLL written in C (in fact, in NI LabWindows/CVI but this is just ANSI C with custom libraries). The DLL is not written by me and I cannot perform any changes to it because it is also used elsewhere.
The DLL contains functions to make an RFID device perform certain functions (like reading/writing RFID tag etc). In each of these functions, there is always a call to another function that performs writing to a log file. If the log file is not present, it is created with a certain header and then data is appended.
The problem is: the C++/CLR project works fine.
But, in the C# one, the functions work (the RFID tag is correctly written/read etc.) but there is no activity regarding the log file!
The declarations for DLL exports look like this (just one example, there are more of them, of course):
int __declspec(dllexport) __stdcall Magnetfeld_einschalten(char path_Logfile_RFID[300]);
int save_Logdatei(char path_Logdatei[], char Funktion[], char Mitteilung[]);
The save_Logdatei function is called during execution of Magnetfeld_einschalten like this:
save_Logdatei(path_Logfile_RFID, "Magnetfeld_einschalten", "OK");
In the C++/CLR project, I declared the function like this:
#ifdef __cplusplus
extern "C" {
#endif
int __declspec(dllexport) __stdcall Magnetfeld_einschalten(char path_Logfile_RFID[300]);
#ifdef __cplusplus
}
#endif
then a simple call to the function is working.
In the C# project, the declaration goes like:
[DllImport("MyDLL.dll", CallingConvention = CallingConvention.StdCall, EntryPoint = "Magnetfeld_einschalten", CharSet = CharSet.Ansi, ExactSpelling = false)]
private static extern int Magnetfeld_einschalten(string path_Logfile_RFID);
and, as I said, although the primary function is working (in this case, turning on the magnetic field of the RFID device), the logging is never done (so, the internal DLL call to save_Logdatei is not executing correctly).
The relevant code in the Form constructor is the following:
pathapp = Application.StartupPath;
pathlog = string.Format("{0}\\{1:yyyyMMdd}_RFID_Logdatei.dat", pathapp, DateTime.Now);
//The naming scheme for the log file.
//Normally, it's autogenerated when a `save_Logdatei' call is made.
Magnetfeld_einschalten(pathlog);
What am I missing? I have already tried using unsafe for the DLL method declaration - since there is a File pointer in save_Logdatei - but it didn't make any difference.
===================================EDIT==================================
Per David Heffernan's suggestion, i have tried to recreate the problem in an easy to test way. For this, i have created a very simple DLL ("test.dll") and I have stripped it completely from the custom CVI libaries, so it should be reproducible even without CVI. I have uploaded it here. In any case, the code of the DLL is:
#include <stdio.h>
int __declspec(dllexport) __stdcall Magnetfeld_einschalten(char path_Logfile_RFID[300]);
int save_Logdatei(char path_Logdatei[], char Funktion[], char Mitteilung[]);
int __declspec(dllexport) __stdcall Magnetfeld_einschalten(char path_Logfile_RFID[300])
{
save_Logdatei(path_Logfile_RFID, "Opening Magnet Field", "Success");
return 0;
}
int save_Logdatei(char path_Logdatei[], char Funktion[], char Mitteilung[])
{
FILE *fp; /* File-Pointer */
char line[700]; /* Zeilenbuffer */
char path[700];
sprintf(path,"%s\\20160212_RFID_Logdatei.dat",path_Logdatei);
fp = fopen (path, "a");
sprintf(line, "Just testing");
sprintf(line,"%s %s",line, Funktion);
sprintf(line,"%s %s",line, Mitteilung);
fprintf(fp,"%s\n",line);
fclose(fp);
return 0;
}
The C# code is also stripped down and the only thing i have added to the standard Forms project, is Button 1 (and the generated button click as can be seen). The code is this:
using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;
namespace TestDLLCallCSharp
{
public partial class Form1 : Form
{
public int ret;
public string pathapp;
public string pathlog;
[DllImport("test", CallingConvention = CallingConvention.StdCall, EntryPoint = "Magnetfeld_einschalten", CharSet = CharSet.Ansi, ExactSpelling = false)]
private static extern int Magnetfeld_einschalten(string path_Logfile_RFID);
public Form1()
{
pathapp = #"C:\ProgramData\test";
pathlog = string.Format("{0}\\20160212_RFID_Logdatei.dat", pathapp);
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
}
private void button1_Click(object sender, EventArgs e)
{
ret = Magnetfeld_einschalten(pathlog);
}
}
}
As can be seen, I have avoided using an automatic naming scheme for the log file (normally i use the date) and in both the dll and the C# code, the log file is "20160212_RFID_Logdatei.dat". I have also avoided using the app path as the directory where to put the log file and instead I have opted for a folder named test i created in ProgramData
Again, no file is created at all
This looks like a simple typo in your calling code. Instead of:
ret = Magnetfeld_einschalten(pathlog);
you mean to write:
ret = Magnetfeld_einschalten(pathapp);
In the C# code, these two strings have the following values:
pathapp == "C:\ProgramData\\test"
pathlog == "C:\ProgramData\\test\\20160212_RFID_Logdatei.dat"
When you pass pathlog to the unmanaged code it then does the following:
sprintf(path,"%s\\20160212_RFID_Logdatei.dat",path_Logdatei);
which sets path to be
path == "C:\\ProgramData\\test\\20160212_RFID_Logdatei.dat\\20160212_RFID_Logdatei.dat"
In other words you are appending the file name to the path twice instead of once.
An extensive overview for P/Invoke in C# is given in Platform Invoke Tutorial - MSDN Library.
The problematic bit is you need to pass a fixed char array rather than the standard char*. This is covered in Default Marshalling for Strings.
The gist is, you need to construct a char[300] from your C# string and pass that rather than the string.
For this case, two ways are specified:
pass a StringBuilder instead of a string initialized to the specified length and with your data (I omitted non-essential parameters):
[DllImport("MyDLL.dll", ExactSpelling = true)]
private static extern int Magnetfeld_einschalten(
[MarshalAs(UnmanagedType.LPStr)] StringBuilder path_Logfile_RFID);
<...>
StringBuilder sb = new StringBuilder(pathlog,300);
int result = Magnetfeld_einschalten(sb);
In this case, the buffer is modifiable.
define a struct with the required format and manually convert your string to it:
[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
struct Char300 {
[MarshalAs(UnmanagedType.ByValTStr,SizeConst=300)]String s;
}
[DllImport("MyDLL.dll")]
private static extern int Magnetfeld_einschalten(Char300 path_Logfile_RFID);
<...>
int result = Magnetfeld_einschalten(new Char300{s=pathlog});
You can define an explicit or implicit cast routine to make this more straightforward.
According to UnmanagedType docs, UnmanagedType.ByValTStr is only valid in structures so it appears to be impossible to get the best of both worlds.
The String is in Unicode format, convert it to byte[]
Encoding ec = Encoding.GetEncoding(System.Threading.Thread.CurrentThread.CurrentCulture.TextInfo.ANSICodePage);
byte[] bpathlog = ec.GetBytes(pathlog);
and change parameter type to byte[]
[DllImport("MyDLL.dll", CallingConvention = CallingConvention.StdCall, EntryPoint = "Magnetfeld_einschalten", CharSet = CharSet.Ansi, ExactSpelling = false)]
private static extern int Magnetfeld_einschalten(byte[] path_Logfile_RFID);
For me it is working
JSh
i have a simple function in c++ (not a method of a class)
__declspec(dllexport) extern "C" void __stdcall TestFunc();
i try to call it from c#:
[DllImport("ImportTest.dll")]
public static extern void TestFunc();
...
TestFunc();
It throws an "entry point could't be found" exception.
Whats wrong?
Thank you for helping me :)
Try (guessing, that DLL is written in VS)
extern "C" __declspec(dllexport) void __stdcall TestFunc();
That's:
__declspec(dllexport) to notify compiler, that this function is to be exported from the DLL;
extern "C" mainly to prevent function name decorations;
__stdcall, because this is default calling convention if you specify none in [DllImport] directive.
In the future, you can check if your function is exported from DLL using Dll export viewer.
In C++ function , at header(if your function is declared in header) add
extern "C" _declspec(dllexport) void TestFunc();
at the function definition use
_declspec(dllexport) void TestFunc()
{
}
At C# side,you need to declare a function like
[DllImport(#"ImportTest.dll",
EntryPoint = "TestFunc",
ExactSpelling = false,
CallingConvention = CallingConvention.Cdecl)]
static extern void NewTestFunc()
Now use , NewTestFunc()
I am attempting to access a Newlands scanner using Platform Invoke from a c# program.
It should be relatively straight forward.
I set up my P/Invoke and and call it.
[DllImport("NLcpfw.dll", EntryPoint = "cpfw_open", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall)]
public static extern IntPtr cpfw_open(string pwStrPort,
string pwStrParam,
int nMode);
public static void CallFromHere(){
IntPtr hDev = cpfw_open("udp", "", CPFW_OM_NORMAL);
//GCHandle handle = GCHandle.Alloc(HNLCPFW, GCHandleType.Pinned);
System.Diagnostics.Debug.Assert(hDev != null);
NewlandInterface.cpfw_close(hDev);
}
Some how it is not picking it up.
I get a BadImageFormatException was unhandled
Make sure it is a valid managed assembly
Make sure you have supplied a correct path for the assembly
The C++ header reads
__declspec(dllimport) HNLCPFW WINAPI cpfw_open(WCHAR *pwStrPort, WCHAR *pwStrParam, int nMode = CPFW_OM_NORMAL);
typedef struct
{
void* hDev;
int nMode;
PNLCPFW_PLUG_API DevAPI;
HINSTANCE hPlugDll;
void *exData;
}NLCPFW,*HNLCPFW;
I'm assuming it is ok just to take a IntPtr in my p\Invoke code.
Any ideas as to why this is happening would be much appreciated.
I have copied all of the dll's in with the exe. In this case these are
NLcpfw.dll, cpfw_udp.dll, cpfw_tcp.dll,cpfw_hidpos.dll etc
Thanks leppie I need to change the platform to X86
I have a dll which contains this function:
int __stdcall PrnText(char *printtext);
In Windows Forms i have this code to invoke the dll:
[DllImport("Printing.dll", EntryPoint = "PrnText", CharSet = CharSet.Ansi)]
public static extern int PrnText(char *printtext);
When i call the function in C# code i get an error like this : " cannot cast string to char*
PrnText("Hello World");
What parameter should i give to PrnText() to make it work?
Later edit:
Parameter: printtext
pointer to string containing text to be printed
The CLR knows how to convert a string to an unmanaged char* at runtime. You should use a signature which accepts a string, as such:
public static extern int PrnText(string printtext);
Note that this will work only if the parameter is input only.
I'm trying to use a C++ unmanaged dll in a C# project and I'm getting an error when trying to call a function that says that entry point cannot be found.
public class Program
{
static void Main(string[] args)
{
IntPtr testIntPtr = aaeonAPIOpen(0);
Console.WriteLine(testIntPtr.ToString());
}
[DllImport("aonAPI.dll")]
public static extern unsafe IntPtr aaeonAPIOpen(uint reserved);
}
Here is the dumpbin for the function:
5 4 00001020 ?aaeonAPIOpen##YAPAXK#Z
I changed the dll import to [DllImport("aonAPI.dll", EntryPoint="?aaeonAPIOpen")] and [DllImport("aonAPI.dll", EntryPoint="_aaeonAPIOpen")] and no luck.
Using the undname.exe utility, that symbol demangles to
void * __cdecl aaeonAPIOpen(unsigned long)
Which makes the proper declaration:
[DllImport("aonAPI.dll", EntryPoint="?aaeonAPIOpen##YAPAXK#Z",
ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr aaeonAPIOpen(uint reserved);
It looks like the function you're trying to call is compiled as a C++ function and hence has it's name mangled. PInvoke does not support mangled name. You need to add an extern "C" block around the function definition to prevent name mangling
extern "C" {
void* aaeonAPIOpen(uint reserved);
}