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Adding references for code moved into a class library

From a solution, which I made in Visual Studio 2013, I want to transfer a method to a DLL (class library project).
When I paste the method in the DLL project, it doesn't recognize parts of the code and it's showing this error`:
"are you missing a using directive or an assembly reference?"
Because of that, the DLL can't be built. I am assuming, that a certain reference to the solution is required, but I am not sure how to proceed.
I know how to add a reference from a solution to a DLL, but I'm not sure how it's done the other way around or even if it's possible.

You can't add reference to EXE from class library (assuming EXE uses that class library) as it will introduce circular reference.
Usually you need to refactor all dependencies so class library either have them all or allow application to inject dependencies from EXE/other clients. In later case class library needs to define base classes/interfaces to allow such injection.

Yes, you need to restore the same references that the original project uses, if they are used in the code you want to move.
If you need to do this by hand (i.e. without tools like ReSharper):
Move the code to the new assembly.
For each namespace or type giving the error, find it in the Object Browser.
Locate the assembly containing that namespace and type, and add a reference to that assembly in your new project.
You may also have to add a Project Reference to the original project.

Bind several classes into one .dll file in C#

A beginner's question. It is c#.
Let's say I have three classes in one project named Employee, Department, Address. For some reason, I would like to have a .dll file (let's name it test.dll) to have all three classes included that I can call it from some other project using syntax like "test.Employee emp1 = new test.Employee();"
That is my idea. Is this possible? If yes, how should I do that? Do I have to create a class library project to do so? I know nothing about a class library project. So I may need further help with that.
If the answer is no, how do I add references to those classes from other solutions?
Thanks.

Create a class library project.
Give it a proper namespace
Write your library classes, then compile to a dll
Then add a reference to that dll in the other project you want to use it in
add a using statement to include the reference in your code files.
Pretty straightforward

Using vb.net classes in c#

How can I create a type in vb.net and use the type in c#?
I guess I need to compile the vb project and add reference to the dll but have no idea if this is the right way to do it, or the reference will be valid.
Doing this is not for fun, we have some vb code and we're considering this option.

Just create a Class Library project in VB, and then add a reference to that project from your C# project. (Or if it's in a different solution, add a reference to the DLL created by the VB project.) It should be absolutely fine - you'll have access to all the Public types from your class library.
To address the comment in your question, the projects can both default to the same namespace, but it's generally a better idea to make each project have its own namespace, so it's clear where any particular type comes from.

C# dll can be used in C++/CLI project easily?

I have one C++/CLI project, a GUI application, which is compiled in mixed mode (managed+unmanaged).
Now I want to write a custom user control using C# and compile it to become CSharpA.dll.
my question is: Can this dll be used by my C++/CLI project easily? How would I do that?

Yes. Just add a reference to it. You may find yourself wanting using namespace directives, which, like the C# using directive, will add classes in other namespaces into the search space.

You only have to compile your C# code into an assembly, and this assembly can be referenced from your C++/CLI App as any other assembly.

Program does not contain a static ‘Main’ method suitable for an entry point

What do I do if I just want to create a project which contains a bunch of library functions? In other words no Main method is required. It seemed to be compiling a minute ago and then I added another .cs file and now I am confronted with this error message.

Create a .NET Class Library project if you only want a library project. If this is a project that already exists, you can set the Project Output type to a DLL ("Class Library") instead of an Executable ("Windows Application"/"Console Application") in the project properties.

What type of project did you create? It sounds like you meant to create a class library but accidentally created an executable assembly. Ensure that you are in fact creating a class library assembly (i.e. ".dll" not ".exe").
If you aren't using Visual Studio and are compiling your code with csc.exe then make sure that you are specifying /target:library to compile your code into a library.

You want to make the project a Class Library type. I believe you can change the type of project in the project properties settings.

or you could use the tried-and-true empty main method

I have the solution. Really simple. You wrote the static void main with lower case. You should write it like this: static void Main()

This problem is occured when we deleted App.xaml file from our project after the required method written due to that please ensure that your App.xaml file is in correct format with respective namespace and references, if it is not, create it and add it in your project.