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Converting a C# class I created inside a project to a separate reusable class

I'm not new to C# programming, but I suppose I'm new to programing "the right way" in C#. I've worked in C on embedded devices for years and have written desktop apps to support them. First in VB6, then in C#.
I recently started making better use of classes for reusing code (and for instantiating more than one instance of the class in a program). For example, I "wrapped" a UART interface with some additional functionality so I can use the same code for multiple ports by creating an instance of the class for each one.
It is in a separate file, but still in the same program namespace, so when I want to reuse it, I have to copy the file and change the namespace to the new project.
I'm sure there's a way to create it such that I can just reference it like everything else with either a "using..." reference at the top of the program or with a "Project | References..." checkbox. But for the life of me I can't find a good learning journey for this.
Any direction would help.

You want to create your reuseable class in an assembly - this is the equivalent of a dll from your C experience.
To create an assembly, have a separate project of type assembly (instead of exe) . You can reference the assembly from other projects. If your project is in the same solution you can reference the project, otherwise you can reference the compiled assembly.
C# uses a packaging system called Nuget, so you can package your assemblies into "Nugets" which you host in a Nuget Server. You can then use tooling to discover and import these.

Please create a Class Library project and include your class into that project. Make sure your class is public. Once you build this project you'll get an assembly which can be referenced from other projects. See Tutorial: Create a .NET class library using Visual Studio
There are different ways of referencing it.
You can have the class library project in the same solution as the main project. In this case you should add a project reference.
You can copy the compiled *.dll file to some folder in your solution (e.g. Lib) and add an assembly reference.
If this assembly is to be used in multiple projects please consider creating a NuGet package with this library and pushing it to some repository. Then other projects can add a package reference to this package.
Details:
How to: Add or remove references by using the Reference Manager
Install and manage packages in Visual Studio using the NuGet Package Manager

It is in a separate file, but still in the same program namespace, so when I want to reuse it, I have to copy the file and change the namespace to the new project.
Well, it isn't the best practice but (unfortunatly) still a common behavior. So don't worry to much about it.
What you could do to improve it place the file (and other reusable parts) in a seperated csproj.
For example name the project of the type class library and name it VinDag.Tools. Within the project create a folder UART and place the wrapper there. The namespace of the wrapper would then be VinDag.Tools.UART.
From know on you can just reference the class library instead of renaming the file. It's not necessarily required to be the same namespace as the project.
From there you can start considering (private) nugets. This would prevent you from copying files/csproj around.

How to use class from other project in visual studio C#

I have a Xamarin.Forms project and a C# console app project. I want to use one class from the console app in my Xamarin.Forms project.
I added the console app project to the solutions explorer of the Xamarin.Forms project.
Unfortunately, I can't figure out how to use the class from the console app in one of the Xamarin files.
I always get the error message:
The name 'MyClass' does not exist in the current context.
I tried to press alt+enter to show potential fixes but it does not offer me the option of importing/using the class.
I also wrote manual using directives in various forms but it still does not seem to make the class accessible.
The only way I was able to use the class was by adding the class directly to the Xamarin project by adding it with add->existing item. The problem with this is that it imports a copy of the class. Since I'm still working on the class within the other project the added class is fast outdated and I have to manually copy its contents over.
How can I use a class from an external project without making a copy of the file?

Instead of access class from console app(its exe) try creating new reusable library add that class and use in both projects also you can write wrapper class in both projects

Try to add a reference to the second project in your first project. To do this, right-click on your project, select Add Reference then select the project in your solution. Once your main project references the second project, then you can access its public types.

Missing DLL's own dependencies in consuming projects

I created a class library project using C# and .Net.
In this project I used two external dependencies(to be more specific: Microsoft.Win32.Registry(4.6.0) and System.Data.SqlClient(4.7.0) Nuget packages).
After I build this project, I can see the generated DLL file under /bin/debug folder.
Now I want to import this generated DLL in another project and consume its methods. Once imported and I run this project, it complains about not being able to find those two external dependencies I had in class library project.
As a temporary fix, I can import these two missing references in this project and it will work fine and as expected. But this is not what I want(and I guess is not a clean solution as well).
I want to know why the dependencies of class library project is not reflected in generated dll file? And is there any way to fix this?
Many thanks for your help.

If your class library is in the same solution or source control repository as the app that's using it, you should use a project-to-project reference, rather than referencing the assembly directly. As the docs say, this way it automatically detects changes to the class library when you compile the app, but what the docs didn't say is that dependencies flow though as well.
Otherwise, as Lance Li wrote, you should create a NuGet package from your class library. Unfortunately there's a bit of a barrier to get started. Creating the package is easy, but then you need to publish the nupkg file somewhere. For early development (before the package is ready to be shared), the easiest option is to use a local file feed. You'll then need a nuget.config in the app that will use the package to add that local feed as a source, then you can install the package in your consuming project, which will bring dependencies.
As you can see, for development, this is slow and difficult because if your consuming app finds a bug in your package, or if you're trying to develop a new feature in both the consuming app and class library at the same time, it means every time you make code changes to class library, you need to increment the version number, pack a package, publish the package, then update the package version in the consuming project. It's far, far easier to use a ProjectReference which lets you simply edit code, compile, run. Nothing else to think about.

See this, the way you reference that assembly is not a recommended way when both the projects are in same machine.
You're using the file reference(Add reference => browse...). And that's why you have to import these two missing references in this project manually.
So I suggest you add the project reference, if both the two projects are in same solution, you can right-click current project=>add reference=>project tab find that assembly you need.(instead of browsing...)
If the referenced project is not in same solution. Right-click solution in solution explorer=>add existing project to import it. Then add project reference.

class name change in a dll project wont update?

In a dll solution, I changed the main public class (namebase) to have a better name EasyXML.Settings class has now becomme EasyXML.XMLconfig.
namespace EasyXML
{
public class XMLconfig // i changed this name and its constructor.
{
When compiling i did a clean solution first and then i compiled both debug and release versions for this .dll
Another project already used the older version, but needed to update to the new version. Thus I removed references to the old dll, and removed old dll, added the new version of the dll, and created a reference to it again.
However somehow that other project doesn't recognize the updated class name XMLconfig. (not on clean builds either), it behaves like if it is cached or so.
I'm not sure, do i need to do additional actions on the dll solution, or on the other project?, to refresh those names.

There must be a simple explanation for this:
make sure there aren't any errors in the second project (otherwise VS most likely fails to update the class names from the dll)
make sure by adding the reference of the dll to the second project you are referencing the right file (use solution tab instead of browse)
make sure your dll project was actually compiled. (There are several cases where the project didn't compile but the solution build was successful). Check the modified date of the dll project output and make sure it isn't an old one.
you can go to project->properties->build->output path and project->properties->build events and make sure the output for the current build configuration points to a right location
make sure both projects use the exact same version of .net framework
other than that:
make sure the class name (XMLConfig) is not exact match of a namespace (EasyXML) otherwise the class might get inaccessible to the VS user
make sure there aren't any partial classes in dll project with the same name.
make sure there aren't any other classes somewhere else with the same namespace and class name

Building class library project in C#

I am working on a existing c# class library project. I want to add few more class to it. This is then used in a new project. I have added the required classes to the existing typelib project, but when I refer the typelib in the next project and try to access the newly added methods it showing error
The type or namespace name 'abc' does not exist in the name space
xyz.TypeLib.core (are you missing assembly reference)
but I can access all those classes which existed previously in that project. I am having issue with newly added files.
Here are the steps I did,
added new classes to the existing project.
Build the project
Added dll to next project from the bin folder of the class library project
Also, as a test I also tried adding a new method in one of the classes which I can access, but this method is not avaiable!
How can I do this properly?

1.) Make sure you have added the correct DLL to the project. Therefore, clean the library project, build it, and use that DLL.
2.) (this will most likely be the problem): In Visual Studio, click on the DLL and in the properties, make sure that "Copy to output directory" is set to "Copy if newer".
3.) Re-add the reference to the file.
4.) Clean and then build again your new project that uses the library.

If the two projects are in the same Visual Studio Solution, the correct approach is to add the class library project directly as a reference. This will bypass any possible dll mismatching that can be caused by other approaches.