为什么我必须在不必使用关键字static时使用？

Question

I've been trying simple experiments to learn C# methods. The below code simply calls playerSelection() which asks the user for a character and returns that character to Main(string[] args). Main prints that to the console. With the below code I get the following error "An object reference is required from a non-static field."

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using System.Threading;


namespace SimpleFunction
{
    class Program
    {
        static void Main(string[] args)
        {
        char cplayerChoice = playerSelection();
            Console.WriteLine(cplayerChoice);
        }

        char playerSelection()
        {
            Console.WriteLine("\nEnter a Character");
            char cplayerChoice = Console.ReadKey().KeyChar;
            return cplayerChoice;
        }
    }
}

Now if I add the word static like so:

static char playerSelection()

it compiles and works. I do understand static versus non...abstractly.

Ok, so here is where I'm confused and the question comes.

I'm learning C# from a book and in that book they go through the below example to illustrate using methods:

using System;

namespace GetinPaid
{
    class Program
    {
        static void Main(string[] args)
        {
            (new Program()).run();
        }

        void run()
        {
            double dailyRate = readDouble("Enter your daily rate:");
            int noOfDays = readInt("Enter the number of days: ");
            writeFee(calculateFee(dailyRate, noOfDays));
        }

        private void writeFee(double p)
        {
            Console.WriteLine("The consultant's fee is: {0}", p * 1.1);
        }

        private double calculateFee(double dailyRate, int noOfDays)
        {
            return dailyRate * noOfDays;
        }

        private int readInt(string p)
        {
            Console.Write(p);
            string line = Console.ReadLine();
            return int.Parse(line);
        }

        private double readDouble(string p)
        {
            Console.Write(p);
            string line = Console.ReadLine();
            return double.Parse(line);
        }
    }
}

Question:

Why in their example can they call methods without using the keyword static but I have to use it?

Thanks!

Answer 1

In their example, they're creating an instance of Program , and calling a method on that instance:

(new Program()).run();

This is more cleanly written as:

Program program = new Program();
program.run();

From within those instance methods, you can call other instance methods because you're implicitly calling them on this .

As an aside, if that really is sample code from the book, I suggest you get a different book: there are some very questionable aspects of style there. In particular:

• Method names should be CamelCased; certainly for public methods but typically private too
• The author is inconsistent in whether private is explicit or implicit
• Method names which refer to types should usually use the CLR name rather than the C# name, eg ReadInt32 instead of readInt . Again, it's not as important for private methods, but it's a bad habit to get into
• Using double for currency values is a really bad idea
• The parameter name of p gives no information (in various places, used for different meanings)
• For user input, you'd typically use TryParse rather than Parse , and check the return value then potentially reprompt on bad input