Problems learning C# (different operating systems, database servers and framework versions)?

Question

.NET

Previously practiced in C and C++ Have some experience with scripting languages ​​and MySQL, exclusive to NIX environment. The task is to learn C#, taking into account the support of outdated solutions, purely theoretically I want to go from VS 2010/15, XP/10, so far I can’t say anything about the database and MS SQL Server versions, but it’s scary to even imagine) There is information that the program is written in VS 2015 will not run on XP and that under XP a maximum of NET 4.5 can be installed, and modern 6th C # is already under version 4.6 Or the OS does not matter at all what version and what bitness of the processor (for example x64), but only the version of the framework matters. It is also not clear with the database, the local version is compiled along with the application and can be used on a separate machine, or you need to install a local server on the machine, if a corporate application then you need to use a database server in the local network. How to deal with all these features, or just start with XP + VS 2010 + .NET 3.5 + SQL Server Express and continue to increase the degree?)))

Answer 1

Infa runs that a program written in VS 2015 will not run on XP and that under XP a maximum of NET 4.5 can be installed

I don’t know where it “runs” there, but under XP, the maximum framework 4.0 is installed. All this is on MSDN without problems, it is tested on a virtual machine for reliability.
Now I will show you the trick. Create a console project in C#. We use a couple of features from C# 6.0:

namespace ConsoleApplication1
{
  class A
  {
    public int Test { get; set; } = 0;
  }

  class Program
  {
    static void Main(string[] args)
    {
      Console.WriteLine(nameof(Main));
    }
  }
}

Next, we retarget the project on the .Net Framework 2.0:
Remove assemblies and usings that are not available in the second framework (for the second, these are LINQ and TPL).
We compile, run, and rejoice.
Conclusions:
1) under XP all dotnets up to 4.0 inclusive are available
2) the version of the framework determines the features available in the "standard library", not the features of the language. Note that 4.0 has both LINQ and Tasks;
3) bitness matters if your program or dependent libraries are NOT compiled in AnyCPU. Otherwise, there is no difference.
4) SQL server is a different story altogether, I don't know what .Net has to do with it. You probably have such an opinion about the MS stack that everything is tightly tied together and does not untie. This is not true. Listen less to stupid colleagues, read MSDN more. Believe me, after 3 or more years of development under dotnet, you will tell all of the above by heart, even if you wake up at 3 am.
Of course, the Express version is for you as an example, so that you can create and run, for example, a web application. Read about ADO.NET, this is a subsystem for working with relational DBMS, and you will understand everything.

Answer 2

Если надо, чтобы работало на ХР+ - лучше всего просто взять максимально возможный стек для этой ОС, более-менее изучить, и разрабатывать проект без оглядки на современный средства. Документация по старым .NET, C#, MSSQL Server есть на MSDN в достаточном количестве.
По поводу БД - любой из перечисленных вариантов, в зависимости от требований к приложению, я лично проблем не вижу, вижу только специфику каждого из подходов(есть MSSQL, SQLite, MySQL, etc., и со всем этим можно работать из .NET).
А самый главный вопрос: зачем? Какая задача и какие требования? От этого уже можно плясать и думать, может быть корректировать требования под себя.