We Keep Coding

VS 2019 C# intellisense not suggesting references from within solution

I'm new to programming so I might not be using some of the correct terminology. I'm running into an issue with InteliSense when calling a C# class from another project within the same solution. It's not suggesting a using statement and is instead trying to get me to create a new class inside of the current project which is not what I want. I am having to go in and add a refernce to the project and then add the using statement in order to get access to the class.
I looked at some of the documentation online and nothing has helped so far. InteliSense appears to configured correctly to suggest using statements. It provide suggestions just fine. I've been able to create lists and then use it to add the proper using statement along with some other things. Just doesn't want to work with anything inside the solution. I've been following a couple of different tutorials including, .net core 2.1 and 3.1, inside MVC and Razor page projects along with a couple just straight C# console apps. It doesn't work in any of them when I start adding multiple projects to the solution and try using classes from outside the current project.

I am having to go in and add a refernce to the project and then add the using statement in order to get access to the class.
That is the correct behavior. In order for ProjectB to use classes defined in ProjectA, you must first add a reference to ProjectA. Just having the projects in the same solution is not sufficient.
The purpose of having multiple projects in the same solution is simply for grouping related code. The projects may or may not actually depend on each other. For example, a web application may have a separate projects for the actual web UI (the pages themselves), a data access layer, unit tests, maybe some class libraries for shared code used by multiple projects, and maybe even console applications (or some other project type) for performing backend administrative tasks. In this scenario, the web UI and console applications may have references to the data access layer project and/or the class libraries. The unit test project will have a reference to the web UI project, and so on. The dependencies are one-way - you may not have circular references (the unit test project has the web UI project as a dependency, but not the other way).

Is it possible to import class file without converting it to a dll in c#?

There are a lot of variables and methods in my program and I want to seperate some of them in other class files. But as the program grows the methods and functions can change.
I searched on the net but many people generally speaking for dll files. Without making a dll file, how can I arrange my code and split into small class files?

Yes, just split it out in to a separate file in a new class but still inside the same project. The term for what you are doing is called Code Refactoring. There are some tools built in to Visual Studio to make it easier to do, and there are some 3rd party tools that add even more features to make it easier to do.
But all it boils down to is just making new classes in the same project and referencing those new classes from where you took the code out from.

You can add folders to your solution. Classes are by default a namespaceprovider, so that classes in this folder have a different namespace.
For example if your default-namespace is MyNameSpace and you create a folder called Entity then all classes in this folder have the namespace MyNameSpace.Entity
And all Items in a project are compiled to one single dll or exe

Just add more classes to the project and put the data and behavior (methods) into the appropriate classes. The project will still build into a single exe or dll.
Generally, it's better to add a second project under the same solution call it "CommonLib" or something like that. Then you add it as a reference to the main application and set up the project so that the applications build depends on the libraries build. Add a using statement for the common lib where ever you want to use those objects. This is definitely better for large scale or enterprise applications. There's a pretty decent chance that somewhere down the line you'll want to reuse some of this code, if everything builds into a single exe that won't be an option.

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

DLL - Using main project classes in library class

I've never tried separating down my projects into DLLs but something I'm working on at the moment calls for a more modular approach. What I want to be able to do is switch out versions of a number of my classes easily to push updates to users without them having to re install my entire application.
These classes create instances of other classes which are defined within the application and shouldn't be part of the modules which are switched in and out - they're central. Obviously in the class Library project these classes are undefined as they are only defined in the main application. How do I go about enabling the class library to use these main application classes?
All help is much appreciated.

How to force a C# root namespace throughout project files for when none is coded?

I want to force a root namespace on the contents of any .cs source files that don't have their contents wrapped in an explicit namespace. In other words I want to keep classes and other namespace-level structures out of the default namespace.
(Working inside a Windows .NET environment with Visual Studio)
In the following example, I want to force the Car class into the MotorIndustry namespace by default even though it has no explicit namespace coded.
Vehicle.cs (has namespace)
namespace MotorIndustry {
public class Vehicle {
// ...
}
}
Car.cs (no namespace/default)
public class Car : Vehicle {
//...
}
Is there a way to accomplish this "root namespace" modification behaviour through project settings in Visual Studio, AssemblyInfo.cs file or some other means?
My understanding is VB.NET has a feature like this but C# acts differently?
Some context about why am I asking this: I have hundreds of classes and the programmer forgot to wrap the namespace around some. When I reference the assembly from other projects it's polluting the default namespace with classes that end up causing some ambiguous situations.

Use ReSharper to move the classes into the proper namespace. In fact, version 5.0 (still in Beta) allows you to correct namespaces globally.
The other way to do it is to make the other developer fix the code.

It sounds like you are trying to replicate VB.Net's project namespace feature in C#. This is not a supported feature of the C# compiler or IDE. You will not be able to create one by modifying a project file. You will need to add the namespace to every file in the project either manually or via a tool.