how to avoid switch-case
You can create your own Attribute to decorate all of your classes (e.g., Plus, ConsoleHelp, Minus, Divide, and even Exit).  In the sample below, the attribute is called SystemCommandAttribute.  It would require to specify what the name of the command would be as a constructor parameter (what the user would have to type on the command line).  

The next step would be to have each of your command classes implement the same interface.  In the example code I used the name ISystemCommand.  That interface should have a single method:  Execute().  That way we know what to call when we want to invoke the command.

Then you can use reflection to search for all classes that have that attribute and load them into a dictionary of string to ISystemCommand.  The GetCommandsFromAssemblies() method handles instantiating each instance of ISystemCommand for you.  

From there you can use the dictionary instead of the switch statement. 

public class Program
{
    static void Main(string[] args)
    {
        Dictionary&lt;string, ISystemCommand&gt; commandToType = GetCommandsFromAssemblies();
        RunCommandLoop(commandToType);
    }

    /// &lt;summary&gt;
    /// Loop over all types currently loaded in the AppDomain looking for ISystemCommand
    /// &lt;/summary&gt;
    /// &lt;returns&gt;A dictionary from command name to instance of command&lt;/returns&gt;
    private static Dictionary&lt;string, ISystemCommand&gt; GetCommandsFromAssemblies()
    {
        var typesWithMyAttribute =
            from a in AppDomain.CurrentDomain.GetAssemblies()
            from t in a.GetTypes()
            let attributes = t.GetCustomAttributes(typeof(SystemCommandAttribute), true)
            where attributes != null &amp;&amp; attributes.Length &gt; 0
            select new
            {
                Instance = (ISystemCommand)Activator.CreateInstance(t),
                CommandName = attributes.Cast&lt;SystemCommandAttribute&gt;().Single().CommandName
            };

        Dictionary&lt;string, ISystemCommand&gt; commandToType = typesWithMyAttribute.ToDictionary(t =&gt; t.CommandName, t =&gt; t.Instance);

        return commandToType;
    }

    private static void RunCommandLoop(Dictionary&lt;string, ISystemCommand&gt; actions)
    {
        while (true)
        {
            Console.Write("command&gt; ");
            string com = Console.ReadLine();
            if (com != null)
            {
                ISystemCommand action;

                if (actions.TryGetValue(com, out action))
                {
                    action.Execute();
                }
                else
                {
                    Console.WriteLine("The command is not supported. Enter one of the valid commands.");
                    break;
                }
            }
        }
        Console.ReadLine();
    }
}

public interface ISystemCommand
{
    void Execute();
}

public class SystemCommandAttribute : Attribute
{
    public SystemCommandAttribute(string commandName)
    {
        CommandName = commandName;
    }

    public string CommandName { get; set; }
}

[SystemCommand("plus")]
public class Plus : ISystemCommand
{
    public void Execute()
    {
        Sum();
    }

    private void Sum()
    {
        // your existing Sum logic here
        Console.WriteLine("Sum called");
    }
}


[SystemCommand("?")]
public class ConsoleHelp : ISystemCommand
{
    public void Execute()
    {
        Helper();
    }

    private void Helper()
    {
        // your existing Helper logic here
        Console.WriteLine("Helper called");
    }
}

[SystemCommand("c")]
public class ExitCommand : ISystemCommand
{
    public void Execute()
    {
        Environment.Exit(0);
    }
}


To add a new command in the future, all you would need to do would be to 


create your command class
Inherit from ISystmeCommand
Add the SystmCommandAttribute


Your main program would not need to change at all.