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C/C++ DLL failing after .NET framework upgrade from 3.5 to 4.6

I have application called as Agent which have 2 projects :
C# project
C/C++ DLL
Whole application was running fine with .NET 3.5 which we use to build with vs2008.
Due to some requirement we upgraded the .Net Framework to 4.6 and vs 2019 to build this up.
Now my application service runs but looks like C/C++ dll exports are failing somewhere.
What I have tried:
C# side
TAgtLib.TAStartAgent(mServerPort, mName, mBinDir, mPath, mDebug,
mStdout, mDefaultLogDays, mEncryptOnly);
where TAgtLib is wrapper class which declares these C/C++ function like this
[DllImport("ccplusplus.dll", CharSet = CharSet.Ansi)]
public static extern int TAStartAgent(int port, string name, string bindir, string rundir, string debugOption, string stdoutOption, int mDefaultLogDays, int encryptonly);
C++ definition :
extern "C" int _stdcall TAStartAgent(int PortNum, char *Name, char *binDir, char *runDir, char *debugOption, char *stdoutOption, int mDefaultLogDays, int encryptonly);
Whenever debugging is hitting on this function from c# it goes into memory violation.
When we changed to binDir.ToArray() and changed the dllexport declaration as char[] binDir ,simmilary for runDir,it didnt gave access violation but it went inside the dll.
Though C dll still giving issues.
Can someone please guide me off what is that must be effecting the c/c++ dll after this migration from 3.5 to 4.6?
I am novice wrt C#.
Thanks

This is likely happening because .Net 4 changed the way that it corrects incorrect P/Invoke calling conventions.
From https://learn.microsoft.com/en-us/previous-versions/dotnet/netframework-4.0/ee941656(v=vs.100):
To improve performance in interoperability with unmanaged code,
incorrect calling conventions in a platform invoke now cause the
application to fail. In previous versions, the marshaling layer
resolved these errors up the stack.
To solve this, you must specify the correct calling convention in the P/Invoke declaration, e.g.
DllImport("ccplusplus.dll", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
public static extern int TAStartAgent(int port, string name, string bindir, string rundir, string debugOption, string stdoutOption, int mDefaultLogDays, int encryptonly);
Although you will of course need to specify the correct calling convention for your DLL.

Relative path to unmanaged DLL [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Specify the search path for DllImport in .NET
I have an unmanaged DLL. For clearance, its a C++ dll that I want to use in my c# code.
The problem is that its a windows app and user can install it in any directory of their choice. So I can't reference it with a static path and I couldn't find a way to give a relative path.
Here is the code I tried :
[DllImport("mydll.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern bool SetDllDirectory(string lpPathName);
public void SetDllDirectory(string lpPathName)
{
try
{
bool r = SetDllDirectory(lpPathName);
}
catch (Exception ex)
{
throw ex;
}
}
public void SetDirectoryPath()
{
try
{
DirectoryInfo directory = new DirectoryInfo(System.IO.Path.GetDirectoryName(System.Windows.Forms.Application.ExecutablePath));
if (directory.Exists)
SetDllDirectory(directory.ToString() + "\\mydll.dll");
}
catch (Exception ex)
{
throw ex;
}
}
And below is the error I am receiving.
Unable to find an entry point named 'SetDllDirectory' in DLL 'mydll.dll'.
This question could be a replica of
"C++ unmanaged DLL in c#"
"Relative Path to DLL in Platform Invoke Statement"
Sorry for that but I didn't find any solution in these references.

If you are trying to pinvoke into an unmanaged library that is in a non-standard location you need to add this location to the dll search path that windows uses to look for dll files.
If your users can tell the program where the c/c++ library file is, you can load it manually when they choose it.
public class UnsafeNativeMethods {
[DllImport("kernel32.dll", SetLastError = true)]
public static extern bool SetDllDirectory(string lpPathName);
[DllImport("kernel32.dll", SetLastError = true)]
public static extern int GetDllDirectory(int bufsize, StringBuilder buf);
[DllImport("kernel32", CharSet = CharSet.Unicode, SetLastError = true)]
public static extern IntPtr LoadLibrary(string librayName);
[DllImport("mylibrary")]
public static extern void InitMyLibrary();
}
You can call the above in some form of preload routine like so:
public void LoadDllFile( string dllfolder, string libname ) {
var currentpath = new StringBuilder(255);
UnsafeNativeMethods.GetDllDirectory( currentpath.Length, currentpath );
// use new path
UnsafeNativeMethods.SetDllDirectory( dllfolder );
UnsafeNativeMethods.LoadLibrary( libname );
// restore old path
UnsafeNativeMethods.SetDllDirectory( currentpath );
}
You might then call it like so:
LoadDllFile( "c:\whatever", "mylibrary.dll" );

This error message:
Unable to find an entry point named 'SetDllDirectory' in DLL 'mydll.dll'.
says nothing about not being able to find the DLL. It says that your P/Invoke definition is incorrect. There is no function named SetDllDirectory in your DLL (mydll.dll). And why should there be? The only functions that are exported from DLLs are those that you explicitly code and export. Since you didn't write code for a SetDllDirectory and indicate that it should be exported from mydll.dll, it doesn't exist in the DLL.
Beyond that, this question is unanswerable unless you update your question with real code. You need to post, at minimum, the signature for the function you're trying to call from the unmanaged DLL, along with the P/Invoke code that you've tried on the C# (managed) side.
The problem is that its a windows app and user can install it in any directory of their choice. So I can't reference it with a static path and I couldn't find a way to give a relative path.
This is not a real problem. Relative paths work just fine, and they work in exactly the way that your question indicates that you've already tried. You just include the name of the DLL in the P/Invoke definition. The default DLL search order takes care of everything else. If the managed EXE is in the same directory as the unmanaged DLL that you want to P/Invoke, everything will work seamlessly.
It doesn't matter where the user chooses to install the app, so long as the DLL and the EXE are located in the same folder. And that's the job of your installer.
And so long as you use relative paths, you absolutely do not need to use the SetDllDirectory function from the Win32 API (which is actually defined in kernel32.dll).

How to properly create an entry point on an external .dll resource written in C++ consumed in C#

Long time reader, first time poster. One day I hope to be answering questions on here...
So it's kind of similar to: "Unable to find an entry point named [function] in dll" (c++ to c# type conversion)
But I can't seem to apply the same solution...
Basically, I wrote a new Method:
Defined in the header file of the C++ project as :
extern "C" {
__declspec(dllexport) bool IsDataValid();
}
Defined in the source file of the C++ project as: (signiature only)
extern bool __cdecl IsDataValid() {
//function stuf......... returns a bool
}
Imported into a forms C# application within the C# Project as:
[DllImport("CarChipSDK_C_Sharp.dll", EntryPoint = "IsDataValid")]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool IsDataValid();
It is called from the same place within C# forms .cs file as:
bool isDataValid = IsDataValid();
It is returning an exception with the message:
"Unable to find an entry point
'IsDataValid()' named in DLL
'CarChipSDK_C_Sharp.dll'.
I have used dumpbin.exe and dependency walker on the .dll generated from the c++ code and it shows that it has the IsDataValid() entry point.
All help is much appreciated...
Problem Solved! Stupid me, this was the code from a previous co-op at my current company, turns out he was reading the .dll from the bin/release folder where as I was building to the bin/debug folder. Should have known. My sincere apologies.

You are encountering C++ name mangling. Declare the C++ functions as extern "C". So, in your C++ module...
extern "C" __declspec(dllexport) bool IsDataValid();
You really don't need the entry point specification attribute either. Your C# declaration will be:
[DllImport("CarChipSDK_C_Sharp.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool IsDataValid();
For future reference, dumpbin.exe is a very useful program for analyzing problems like this. If you run it on your DLL you will see what those functions are actually named by the time they are compiled.

Access Violation Exception

I am having some strange problem. I have written a small module in VC++ using OpenCV.
It works fine. The code aggregates feed from the CCTV camera connected to the USB port.
however, I had to write the rest of my application in C#, so I made a DLL of the VC++ code and called the VC++ method from C#.
Now, I have ended up getting an error
Attempted to read or write protected memory.
This is often an indication that other memory is corrupt.
Can anyone please suggest me any solution to this. Is there any Access Violation while accessing it in a managed code?

If TrackBlob returns a string, you should be able to define your dllimport as such:
[DllImport("Tracking.dll", EntryPoint = "TrackIt")]
public extern static string TrackBlob();
and skip trying to marshal it.
By returning it as an IntPtr, you're trying to get a pointer into memory owned by the unmanaged DLL... returning it as a string will return a copy of the string for you to work with.
Let me know if that works!
James
* Edit *
Try one of these:
[DllImport("Tracking.dll", EntryPoint = "TrackIt")]
public extern static [MarshalAs(UnmanagedType.BStr)] string TrackBlob();
or
[DllImport("Tracking.dll", EntryPoint = "TrackIt")]
public extern static [MarshalAs(UnmanagedType.AnsiBStr)] string TrackBlob();
Check out this MSDN link on string marshalling:
http://msdn.microsoft.com/en-us/library/s9ts558h.aspx

"Unable to find an entry point named [function] in dll" (c++ to c# type conversion)

I have a dll which comes from a third party, which was written in C++.
Here is some information that comes from the dll documentation:
//start documentation
RECO_DATA{
wchar_t Surname[200];
wchar_t Firstname[200];
}
Description:
Data structure for receiving the function result. All function result will be
stored as Unicode (UTF-8).
Method:
bool recoCHN_P_Name(char *imgPath,RECO_DATA *o_data);
Input:
char * imgPath
the full path of the image location for this
function to recognize
RECO_DATA * o_data
data object for receiving the function
result.
Function return:
True if Success, otherwise false will return.
//end documentation
I am trying to call the recoCHN_P_Name from my C# application. To this end, I came up with this code:
The code to import the dll:
public class cnOCRsdk
{
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct RECO_DATA{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst=200)]
public string FirstName;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 200)]
public string Surname;
}
[DllImport(#"cnOCRsdk.dll", EntryPoint="recoCHN_P_Name")]
public static extern bool recoCHN_P_Name(byte[] imgPath, RECO_DATA o_data);
}
The code to call the function:
cnOCRsdk.RECO_DATA recoData = new cnOCRsdk.RECO_DATA();
string path = #"C:\WINDOWS\twain_32\twainrgb.bmp";
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
byte[] bytes = encoding.GetBytes(path);
bool res = cnOCRsdk.recoCHN_P_Name(bytes, recoData);
And the error I'm getting is
""Unable to find an entry point named 'recoCHN_P_Name' in DLL 'cnOCRsdk.dll'."
I'm suspecting that I'm having an error in converting a type from C++ to C#. But where exactly ... ?

First make sure the function is actually exported:
In the Visual Studio Command Prompt, use dumpbin /exports whatever.dll

C# doesn't support C++ name mangling and you either need to declare the C++ functions with
extern "C" {...}
(may not an option if they're from a third party), or call the mangled name directly if you can get it to work. It may be easier to get the third party to provide a non-mangled interface to the functionality.

Solved - at least to the point where the program does not break and actually returns me a bool value.
The key, I guess, was to specify the entry point as the 'mangled' name
[DllImport(#"cnOCRsdk.dll", EntryPoint="?recoCHN_P_Name#CcnOCRsdk##QAE_NPADPAURECO_DATA###Z")]
public static extern bool recoCHN_P_Name(ref string imgPath, ref RECO_DATA o_data);
After that I got some other errors but the 'unable to find entry point' went away.

I solved the same problem in these steps:
step 1) If you program your custom DLL in C++ using Visual studio,then at the property page of your project set the Common Language Runtime Support (/clr)parameter to Common Language Runtime Support (/clr).
step 2) To function deceleration in .h file use __declspec(dllexport) keyword like below:
__declspec(dllexport) double Sum(int a,int b);
step 3) Build and export DLL file, then use the Dependency Walker software to get your function EntryPoint.
step4) Import DLL file In the C# project and set EntryPoint and CallingConvention variable like below:
[DllImport("custom.dll", EntryPoint = "?Sum##YAXHHHHHHNNN#Z", CallingConvention = CallingConvention.Cdecl)]
public static extern double Sum(int a,int b);

I'd write a wrapper using C++/CLI. This wrapper will be able to include the .h files and link to the .lib files you got from the third party vendor. Then it is both easy and safe to write a managed interface for your C# program.

Correct EntryPoint string could be found in ".lib" file that comes along with main unmanaged dll.

We had this problem when we want to access to DB and solved it by changing EF core to EF 6.4.4
It may be you have a problem like this and need to change or downgrade your version of EF (If you used EF)

We had this problem .we change EntityFramework.core to EntityFrameWork 6.4.4 and after that the program worked fine. you most change you're Framework Version.

you may get this error due to string marshalling mismatch between DLL and your application . for example, one is using ANSI and the other is unicode.
you can try something like this:
[DllImport("yourDLL.dll", CharSet = CharSet.Unicode )]
public static extern String YourFunction(String name);
checkout HERE for a list of other possible reasons.

You could try using the unmangled name while specifying a CallingConvention in the DllImport