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Creating Unit Tests in VS only allowed in Public Classes?

I am trying to clarify this notification message when trying to create a unit test for my method. The message clearly states that creating a unit test is only supported on a NON-Test project and within a public class OR a public method. I clearly have a public method but the class isn't. So is this just an incorrectly typed error message? Does it actually mean that you need to have both a public class and method?
Note: It works when I try this in a public class, just testing the notification.
Picture of notification message:

To test your class/method, you have to create another project - Testing project. Look at this as it has been another application that uses your classes.
So for example you have 3 projects (for sake of simplicity):
Domain - project with models and domain services
Console application - application that uses domain project
Test... let's say "Testing application" that tests Domain project.
So it points that class that you want to test MUST be available from test project. So it has to be public. You can also use attribute InternalsVisibleTo to make this class available for test project.

Your unit test project must be able to see (public visibility) the method you want to test in order to call it and execute it.
Here is a sample on a WinForm application:
You can clone the whole solution here:
Visual Studio solution on GitHub
Note that you can unit test private methods but it is not a good practice:
Solution downloadable here:
visual Studio solution on GitHub

Is it possible to execute an arbitrary class in C# with a Main method, similar to that in Java?

I've recently had to migrate to the C# world. Coming from the Java land, I could add a public static void main(String[] args) method to any class and select to run that class from Eclipse/Netbeans for any code/logic that I wanted to quickly test.
Is there an equivalent of the same capability in C#.Net/Visual Studio? I've tried doing that and the best I can do is to execute it from the command prompt via csc.exe. However, for some reason, it complains about not finding the relevant DLLs - it seems to expect to run that class in complete isolation without any dependency on "external code" (i.e., code residing in that VS project/solution where the class resides).
Reason for this capability: All project files are marked as class libraries and sometimes I just wish to check if a particular set of methods/data/logic will work as expected with the current code base. In Java, I'd quickly write it in the main method and execute that class to see how it goes prior to committing it to version control. However, there seems to be no easy way to trigger the execution of "my class" with all dependencies correctly handled by csc.exe
Current Solution: Add this testing code to the unit test project and select to execute that particular "test" so as to check if the idea seems to work fine (it may fire DB calls or webservice class etc., and not be purely a logical flow of computation). This seems to work fine and is my current way of doing things. I was wondering if the Main method was even possible/recommended.
Question: Is this even possible with C#/VS or not recommended?
Update: I can't add a console project just to achieve this since the addition of projects is tightly controlled by the source control team. Hence the question of the Main method 'hack' for quick and dirty checks/tests.

Your project type needs to be Console Application for it to "recognize" a Program.Main method, not Class Library. The intent is for a Class Library to be an encapsulated grouping of functionality that can only be accessed by a project that is set up to allow for user input. Those can be a Console Application, Web project (MVC/API), or Desktop (WPF).
If you just want to execute a test against the code within a Class Library project, you can also create a Unit Test project, add a reference and execute very explicit tests against the functionality you're looking to achieve.
You can find out the differences between the different project types by examining the .csproj files in your favorite text editor.

In Visual Studio go New->Project then select Console Application (in Windows\Classic Desktop in VS2015). This gives you a basic console application with...
static void Main(string[] args)
{
}
setup and ready to go. However for simply trying out code you may find this cumbersome (creating a new project and folder just to test code) and for testing code (that doesn't rely on existing libraries) you could use something like .NET Fiddle...
https://dotnetfiddle.net/
Where you can quickly create and test code there and run it via the browser.

Block loading a static class in design mode in Visual Studio 2013

My C# solution (.Net 4.0) in VS2013 includes a project defining a static class. The constructor of this static class tries to connect to a database and does some manipulation with the data.
I'm facing the problem that every time I'm trying to build my solution or I'm referencing this static class in other projects (in the same solution), VS2013 is trying to call the constructor of the static class. After calling the constructor, VS2013 gets nonresponding. Definitely, as a temporary solution, I can comment the complicated part of the constructor, but eventually I'll still have to uncomment it and try to build.
Could you please tell me how to turn off in VS2013 this option of automatic loading of static classes in design mode? Tried to google, but no success.
Thanks!

You could add a test in your static constructor and only execute your code logic if you're not in design mode.
That being said, I wouldn't recommend you to run code that accesses a database in a static constructor. The main problem is you don't really know when it's called. See https://csharpindepth.com/articles/BeforeFieldInit

Can a single source of code be ran universally after each Selenium test or test run?

I am curious if there is a capability in Visual Studios 2012/2013 using C# and Selenium to have a universal bit of code run for a project or solution after each Test completes (or each batch of tests).
The opposite is useful as well, running code in the initialization phase of a test.
I know that each test class has the ability to run a [SetUp] or [TearDown] method, but I was curious if there was a repository elsewhere that runs this code for all tests without defining it in each class.

Make a base class for all of your tests and decorate it with the appropriate attributes for your testing framework of choice. Then have all of your test classes inherit that class.

How to test if class works properly in C#?

I've written a class and want to test if it works well. For now I think the best way to do it is to create new console application referencing main project, then make new instance of my class and mess with it. This approach unlike others enables IntelliSense, using keywords (no full names for classes) and Debugging.
Anyone knows how to do it in more convenient way to do this without making new console app?

Using a console app to test your class is what I would call a "poor man's unit test."
You are on the right track in wanting to do this sort of testing and I (and most others on SO) would suggest using a unit testing framework of some sort to help you out. (Mocking may be important and useful to you as well.)
Here's the thing though. Regardless of what framework you use, or if you go with a console app to test your code, you do have to create a separate project, or a separate, significant chunk of code of some sort, to be able to execute tests properly and independently. That's just part of the process. It is an investment but don't let the extra work keep you from doing it. It will save a lot time, and your skin, a little while in the future. Maybe even next week.
Also, while you're looking up unit testing make sure to also study up on test-driven development (TDD.)

Unit testing is absolutely the way to go. Depending on what version of VS you are running, there may be unit testing functionality built in, or you may have to use an additional tool such as NUnit. Both options are good and will allow you to fully test your classes.
Bear in mind also, that a comprehensive suite of unit tests will make refactoring much easier in the long run as well. If you refactor and break your unit tests, you know you've made a boo-boo somewhere. :)

Unit testing is the way forward here> this is a good introductory article.
The basic concept of a unit test is that you isolate and invoke a specific portion of code and assert that the results are expected and within reason. For example lets say you have a simple method to return the square of a floating point number:
public float Square(float value)
{
return value * value;
}
A reasonable unit test would be that the method returns the correct value, handles negetive values etc:
Assert.AreEqual(25, Square(5));
Assert.AreEqual(100, Square(-10));
Unit tests are also a good way to see how your code handles edge cases:
Assert.Throws<OverflowException>(Square(float.MaxValue));

If you are using VS 2010, check out Pex and Moles...
http://research.microsoft.com/en-us/projects/pex/

The Console App approach is more of a test harness for your class, which is fine.
But you can also use a unit testing framework to test your class. Visual Studio has one built in or you can leverage something like NUnit.

Also, try the Object Test Bench in Visual Studio. It should allow you to create a new instance, modify and view properties, and call some methods. It usually only works with very simple apps, though.

If you use Visual Studio 2008 or higher you will be able to test your code using MSTest framework:
1.Open Test View window: Test/Windows/Test View;
2.Add new unit test project: right click on Solution in Solution Explorer/Add/New
Project/Test Project;
3.Remove all files apart from UnitTest.cs file in created test project;
4.Write your unit test in method under [TestMethod] attribute:
[TestClass]
public class UnitTest1
{
[TestMethod]
public void TestMethod1()
{
var ranges = new Ranges();
int sum = ranges.CountOccurrences(11);
Assert.AreEqual(128, sum);
}
}
5.Run your test from Test View window added in p.1
6.See test results in Test/Windows/Test Results window