Develop Reference

want to use c++ library in asp.net with dll

I want to use a C++ library for my asp.net website. I don't know how to make a .dll of the library and make it work. I tried making a dll and import it to asp.net. Do I need a to do something in the c++ code for the dll to work?
Source code: http://warp.povusers.org/FunctionParser/fparser.html

You could use:
[DllImport("dllname.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern double FunctionParser();

Unable to import c++ dll to c# project using DllImport

I'm trying to import a C++ Project Dll into a C# project. I found a lot of people talk about using DllImport. I tried using that and here is what I have-
CPP Code:
int __declspec(dllexport) beginCode(double reportId);
C# Code:
[DllImport("C:\\Users\\<my_user_id>\\Desktop\\ctxmix\\Release\\ctxmix.dll",CallingConvention =CallingConvention.Cdecl ,CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
public static extern int beginCode(double reportId);
int result = beginCode(reportId);
But when I run, I'm getting an exception - Exception thrown:
System.DllNotFoundException
Do I have to add any references for the CPP Dll in the project or do anything else apart from the code which I have on the top?
Edit: I'm trying to run my .exe using VS2015 and I get this exception on my local machine. Also, I don't see my CPP Dll in the Project->References section where as I see other references there.

The unmanaged DLL needs to be locateable by your managed process. Typically that means placing the DLL in the same directory as the executable file. But you gave used an absolute path which I presume you transcribed correctly.
You may also encounter this error if the DLL's dependencies cannot be located. That seems the likely explanation here. Most likely the MSVC runtime cannot be located when your DLL is loaded.
Using an absolute path isn't a great idea. That will break down when you distribute to another machine. Use just the DLL file name and place it in the same directory as the executable.
Your DllImport attribute seems fussy. No point specifying CharSet when there is no text. I doubt your function calls SetLastError. And do you really need ExactSpelling?

Third party DLL not found C# ASP.NET MVC

So I have this code:
[DllImport("ECDM400Drv", EntryPoint = "ECDM400_Open", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
public static extern bool Open(byte PortNo);
And a dll file called ECDM400Drv.dll, which I placed in the root folder, src folder, and even the windows system32 and syswow64 folders just to be safe. I did this because I have another third party dll that I am using, which I had set up the same way and it worked, but when I do the set up with this dll, I get a dll not found exception, telling me it can't locate this file. I tried adding it as a project reference but that did not work either. Any help would be appreciated!

It could be permissions related. The following article has some good solutions that might help you: DllImport generates System.DllNotFoundException

C++ DLL fails to load on some machines

I'm trying to figure out the reason why my DLL fails to load on certain machines.
My application:
A C# program deployed with a setup.msi installer. Inside the installer are the DLLs that get placed in the install directory of the application. Ex:
OUTDIR = c:\Program Files\MyApplicationName\%OUTDIR%\MyApplication.exe
dir %OUTDIR%\DLL_FOLDER\\*.dll
myDLL.dll
The C# application calls LoadLibrary specified by:
[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern IntPtr LoadLibrary(string libname);
So basically
intPtr dll_handle = LoadLibrary("myDll.dll");
Is called and we use the dll_handle to call the desired functions.
So far this has worked on 5/7 of the machines I've deployed it on... What are the obvious mistakes I'm making here? :-)
Grateful for any help!

The most likely explanations are:
The DLL cannot be found. Since you don't specify a full path, you rely on the Dynamic-Link Library Search Order to locate it. Placing the DLL in the same directory as the executable is the usual way to make sure you can find it, and find the right version.
The DLL has non-matching bitness. For instance, your have a 64 bit process and a 32 bit DLL, or vice versa.
The DLL is found, and has the right bitness, but the DLL's dependencies cannot be resolved. Usually this means that the appropriate MSVC runtime needs to be installed on the target machine.
The first diagnostics step to take is to check the return value of LoadLibrary and if it is NULL then call GetLastError to retrieve an error code. In p/invoke you do that like so:
IntPtr lib = LoadLibrary(...);
if (lib == IntPtr.Zero)
throw new Win32Exception();
You can use a tool like Dependency Walker to debug the problem further.

How to use a C++ dll in Unity3D?

I am aware of this similar question, but it does not respond to my problem.
I have written two .dlls using Visual Studio 2010. One is in C++, and communicates with an SDK that was written in C++. The other is a C# wrapper for that C++ library, so that it can be used in C# contexts.
My plan was that this would let me use my code in Unity3D, but apparently that is not the case. It seems like Unity3D does not allow me to import .dlls as Assets if they are not a .NET assembly. So I can add my C# wrapper, but not the C++ dll.
This results in a DllNotFoundException whenever I try to access the C++ library. I have tried simply copying the C++ library into the Assets/Plugins folder, but that gives the same results.
Is there a way to do this properly? This is a very vital part of my project setup.

The problem is that the DLL is not being found when the p/invoke runtime code calls LoadLibrary(YourNativeDllName).
You could resolve this by making sure that your DLL is on the DLL search path at the point where the first p/invoke call to it is made. For example by calling SetDllDirectory.
The solution that I personally prefer is for your managed code to p/invoke a call to LoadLibrary passing the full absolute path to the native DLL. That way when the subsequent p/invoke induced call to LoadLibrary(YourNativeDllName) is make, your native DLL is already in the process and so will be used.
internal static class NativeMethods
{
[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
internal static extern IntPtr LoadLibrary(
string lpFileName
);
}
And then somewhere in your code:
private static IntPtr lib;
....
public static void LoadNativeDll(string FileName)
{
if (lib != IntPtr.Zero)
{
return;
}
lib = NativeMethods.LoadLibrary(FileName);
if (lib == IntPtr.Zero)
{
throw new Win32Exception();
}
}
Just make sure that you call LoadNativeDll passing the full path to the native library, before you call any of the p/invokes to that native library.

Note that the DllNotFoundException can be caused by building your Unity DLL in Debug instead of Release!
A simple oversight that can cause a headache.

This also happens when Unity can find your DLL, but is not able to find it's dependencies. Obvious fix is to place dependency DLLs into /Plugins as well, or link your dependencies statically.
Less obvious reason is when your DLL depends on Visual Studio runtime library dynamically, i.e. is built with Properties -> C/C++ -> Code Generation -> /MD option. Change it to /MT to link with runtime statically.