Which namespaces are supported in sololearn in C#??

Answer 1

Try

These are standard included when you start a empty project
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

If you want to use one that is not in the list, try to put in in the using list and look what happens.

What do you want to do ?

Answer 2

I want to run a message box on Solo Learn.. is it possible?

Answer 3

Unfortunately that is not possible.
The playground is base on a console application.

It does not give a compiler error but also does not give a messagebox either.
https://code.sololearn.com/cEott0iv0OeZ

What is the advantage of a messagebox above console.writeline ?

If you want to try and experiment with windows forms and messageboxes, download the community version of visual studio.