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Unable to use Custom .dll (C#) from C++/CLI

Thanks In advance for you time and Help.
Goal: I am trying to use my C# library within a C++ project
What I did:
Created a simple .dll file with my C# project. named: ClassLibrary1.dll
Created a C++ console application named: CppClr (Common Language RunTime Support Changed to: /clr)
Copied ClassLibrary1.dll to the root off CppClr project
Following is my ClassLibrary1(C#) code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ClassLibrary1
{
public class Class1
{
public static void output()
{
String mystr;
mystr = "Helloo World"; // create a new string
Console.WriteLine(mystr);
Console.WriteLine("From C++ Managed Code!");
Console.ReadLine();
}
}
}
The Following is my C++/CLI code:
#include "CppClr.h"
#using <ClassLibrary1.dll>
int main()
{
ClassLibrary1::Class1::output();
}
Problem: The error I get when I run my C++/CLI code :
An unhandled exception of type 'System.IO.FileNotFoundException' occurred in Unknown Module.Additional information: Could not load file or assembly 'ClassLibrary1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.

Your problem is simply an issue of your C++ executable not being able find/load the C# DLL at runtime.
There are a few different places that the .NET runtime looks to load assemblies. The simplest of these is from the same directory that your compiled application is running from. It is not enough to have ClassLibrary1.dll in the root of your C++ project - it needs to be in same directory that your project is running from.
As a quick and dirty test, you should be able to verify this by copying ClassLibrary1.dll into the C++ project's output directory. If you're running in the Debug configuration, this will probably be something like C:\Projects\CppClr\Debug\ or wherever your C++ project is. Verify that this is the directory that contains the compiled CppClr.exe file. Once you copy the DLL here and run your project, the runtime should be able to find the DLL and all should be good.
However, this could be kind of a pain if you're frequently updating your C# project, since you'll continually need to copy it to the C++ output folder every time something changes. The solution to this is to add a reference in the C++ project to your C# library.
In Visual Studio, open up your C++ project. From the solution explorer, right-click on references and select Add reference... Next, click on the Browse... button at the bottom of the dialog and browse to the location of ClassLibrary1.dll on your disk. Click on Add and then OK.
Now, whenever you build the C++ project, it will copy the ClassLibrary1.dll assembly into the project's output folder. You should also be able to remove the #using <ClassLibrary1.dll> directive from the top of your C++ file, since adding the DLL as a project reference should perform the same function.

The msdn link shared by Chad is of the case when you want to add a dll created from C++ project.
What you need to do is to create a strong name for your C# library.
To create a strong name for your class library, type the following command at the Visual Studio .NET command prompt:
sn.exe -k MyKeyFile.SNK
Copy the MyKeyFile.SNK file to your project folder.
Double-click the AssemblyInfo.cs file to open the file in Solution Explorer.
Replace the following lines of code in the AssemblyInfo.cs file
[assembly: ComVisible(false)]
[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile("")]
with
[assembly: ComVisible(true)]
[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile("..\\..\\MyKeyFile.SNK")]
Press CTRL+SHIFT+B to generate the managed DLL.
Now register the managed DLL for use with COM or with Native C++
Call the Managed DLL from Native C++ Code or using it.
The link here is close to what the OP is trying to do.
https://support.microsoft.com/en-us/kb/828736

I was able to solve this problem by specifying the absolute path of the dll file.
before:
#using "ClassLibrary1.dll"
after:
#using "C:\user\ClassLibrary1\ClassLibrary1\bin\Debug\ClassLibrary1.dll"
if this works for you, you need to go to Properties -> C/C++ -> General -> "Resolve #using References" and add the path here. then you can use #using "ClassLibrary1.dll" again in your could without the complete path.
Answer was found here

Adding namespaces to Visual Studio project

I am creating a Visual Studio solution and am trying to add namespaces Kafka.Client, Kafka.Client.Producers.Parititioning, Kafka.Client.IntegrationTests, etc. to a program file I've created with a Main() method. I have Kafka.Client and Kafka.Client.IntegrationTests in the References of this program file as per Solution Explorer. The code is as follows:
using Kafka.Client;
using Kafka.Client.IntegrationTests;
using Kafka.Client.Producers.Partitioning;
using Kafka.Client.Utils;
using Kafka.Client.ZooKeeperIntegration;
namespace ConsoleApplication2 {
class Program {
static void Main(string[] args) {
//code here
}
}
}
The problem is that when I try to "Rebuild solution" or "debug solution", the aforementioned "using" lines give the error "The type or namespace name 'Kafka' could not be found (are you missing a using directive or an assembly reference?)" I've spent quite a long time on this and would appreciate some pointers about what I would need to do.

using doesn't actually "include" anything into your project. It just makes it so you don't always have to type out the full namespace. So the error is clearly in referencing the other project.
The Kafka project has to be built first. So first make sure you can successfully build the Kafka project.
if the Kafka is not in the same project, make sure you've add the the reference to the dll, and make sure "copy local" is true
to add the dll as a reference, right-click ConsoleApplication2 in the solution explorer, click add reference, then browse to and locate the actual dll output by the kafka project.

Sounds like these are separate class libraries or other projects. If that's the case, add a project reference from your main project and the using statements will then work.
The reason you want to add them as a project reference, and not a dll reference is that you'll likely switch back and forth from debug/release mode and you might end up with an outdated reference.

Thanks for trying to answer the questions. My problem was that the "Target application" was not the same for all the projects in my solution. Right-clicking on the referenced projects and selecting "Properties" enabled me to change the target application. Then I needed to rebuild the .dll files to run the program properly.

Using lib in dll for c#

I have some problems with dll. When I make dll without using of lib files (which I need), everything is fine. But when I'm trying to use some functions in dll that uses functions in lib then some exception appears:
System.DllNotFoundException, cant load a dll module (exception from HRESULT:0x8007007E).
dll file is in correct place.
P.S. using Visual Studio 2010.
What could be wrong?

More detail in the question will get you better answers. But with current information, the most likely cause is that the lib file you are referencing or one of its dependencies is unavailable. This could be because it is not in your GAC or your runtime location, a file format conflict, or a number of other things.
I'm assuming that everything compiles without errors, of course. Again, please add detail if this is not what you need.

if you are making a new class you should reference the default dlls.
in example when you make a new class library and want to use a messagebox in your code ,
you should first reference that required dll in your program(i dont mean your dll,i mean dot net default dll like system.windows.forms) and then add using something; in the top of your class.
example : we want use messagebox in a class library then :
1. first from solution explorer right click the project > add reference ,now reference manager opens now from left tabs click assemblies then framework and then find and select system.windows.forms
2. now its time to use it in our program first add this line in very top of your class file
using System.Windows.Forms; //add this line in top of your class
after that we can use messagebox without any compiler errors.
keep in mind any other dll files should be referenced this way but in windows form applications default lib files are referenced by default

C# - using library error

I have a .dll file which is in another folder from where my project is. I add the reference to this library into my project and then when I try to use(import) it using QRlib; I get the following: The type or namespace name 'QRlib' could not be found(are you missing a using directive or an assembly reference?)
I was given the library file, so it's not me who implemented it, but I have to use it (and I am pretty sure there are no problems with it). What might be the problem ?

In solution explorer window of visual studio, expand nodes like this
project node=>"reference" node
Double click the dll name you added. This will open the object browser window where you will find all namespaces, class and methods.

Look at the lib in decompiler (dotPeek for example) and check what are the namespaces. Maybe you are using wrong namespace?

microsoft.visualbasic.fileio does not exist

I am on .NET Framework 4.0, building a C# web application in VisualStudio 2012. I have Microsoft.VisualBasic added as a reference to the project. I am having trouble with the following line of code:
using Microsoft.VisualBasic.FileIO;
Building the solution returns the error: The type or namespace name 'FileIO' does not exist in the namespace 'Microsoft.VisualBasic' (are you missing an assembly reference?)
I have removed and re-added the reference to the assembly Microsoft.VisualBasic, but still get the error. Microsoft.VisualBasic is in the GAC, as well as Microsoft.VisualBasic.Compatibility, Microsoft.VisualBasic.Compatibility, Microsoft.VisualBasic.PowerPacks.Vs, and Microsoft.VisualBasic.Vsa.
Please let me know how to get VS2012 to recognize the FileIO namespace.

Right-click on your project and select Add Reference...
In the Reference Manager, expand Assemblies and select Framework. Then check the box for Microsoft.VisualBasic and click OK.

I had similar issue, fixed by change TargetFramework (in .csproj) from netstandard2.0 to netcoreapp3.0.

Application references are not available to uncompiled files in your application (aspx, ashx). There are two solutions to this issue as follows:
1) Move your code to a compiled part of the application (cs/vb file)
or
2) Add the reference to the web config
My reference was in an ashx file. I simply copied the code from the ashx file to the clipboard, deleted the file from the project, added a new Generic Handler (right click in Visual Studio > Add > Generic Handler), entered the same name as before, and pasted the code from the clipboard into the cs file editor that Visual Studio opened. I now have a cs file that will compile with the project and use the project reference. The file name is the same, so there is no need to update anything else -- just rebuild and deploy.