I am getting an error.a namespace cannot directly contain members such as fields or methods

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C#

Create a project named BankAccountTester. Create a class named BankAccount. A bank account has fields for a balance. The bank account balance should be initialized to 0. The bank account class should include a deposit which adds an amount passed to it to the balance and a withdraw which subtracts the amount.

What I have tried:

using System;


namespace Banking
{
    public static void Main(string []args)
    { }

 //A bank account has a balance
    //that can be changed with 
    // withdraw or deposit.
    public class BankAccount
    {
        private double balance;
        //constructs a bank account 
        // with zero balance
        public BankAccount()
        {
            balance = 0;
        }
        //Constructs a bank account
        //with a given balance 
        //(initalBalance)
        public BankAccount(double initialBalance)
        {
            balance = initialBalance;
        }
        //Deposits money into the bank account.
        //(amount the amount to deposit)
        public void Deposit(double amount)
        {
            double newBalance = balance + amount;
            balance = newBalance;
        }
        //Withdraws money from the bank account
        //(the amount to withdraw)
        public void Withdraw(double amount)
        {
            double newBalance = balance - amount;
            balance = newBalance;
        }
        //Gets the current balance of the bank account.
        //(returns the current balance)
        public double GetBalance()
        {
            Console.ReadLine();
            return balance;
        }
    }
}