不能在同一个文件夹中使用类

我刚刚开始学习 C# 和 Microsoft Visual Studio。我刚刚创建了一个 C# 控制台应用程序项目，它有一个 program.cs 文件。我修改了它的内容是这样的：

using System;

namespace Calculator
{
    class Calculator
    {
        public static double DoOperation(double num1, double num2, string op)
        {
            double result = double.NaN; // Default value is "not-a-number" which we use if an operation, such as division, could result in an error

            // Use a switch statement to do the math
            switch (op)
            {
                case "a":
                    result = num1 + num2;
                    break;
                case "s":
                    result = num1 - num2;
                    break;
                case "m":
                    result = num1 * num2;
                    break;
                case "d":
                    // Ask the user to enter a non-zero divisor
                    if (num2 != 0)
                    {
                        result = num1 / num2;
                    }
                    break;
                // Return text for an incorrect option entry
                default:
                    break;
            }
            return result;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
         //   NewClass obj = NewClass(); //this line is causing problem
            bool endApp = false;
            // Display title as the C# console calculator app
            Console.WriteLine("Console Calculator in C#\r");
            Console.WriteLine("------------------------\n");

            while (!endApp)
            {
                // Declare variables and set to empty
                string numInput1 = "";
                string numInput2 = "";
                double result = 0;

                // Ask the user to type the first number
                Console.Write("Type a number, and then press Enter: ");
                numInput1 = Console.ReadLine();

                double cleanNum1 = 0;
                while (!double.TryParse(numInput1, out cleanNum1))
                {
                    Console.Write("This is not valid input. Please enter an integer value: ");
                    numInput1 = Console.ReadLine();
                }

                // Ask the user to type the second number
                Console.Write("Type another number, and then press Enter: ");
                numInput2 = Console.ReadLine();

                double cleanNum2 = 0;
                while (!double.TryParse(numInput2, out cleanNum2))
                {
                    Console.Write("This is not valid input. Please enter an integer value: ");
                    numInput2 = Console.ReadLine();
                }

                // Ask the user to choose an operator
                Console.WriteLine("Choose an operator from the following list:");
                Console.WriteLine("\ta - Add");
                Console.WriteLine("\ts - Subtract");
                Console.WriteLine("\tm - Multiply");
                Console.WriteLine("\td - Divide");
                Console.Write("Your option? ");

                string op = Console.ReadLine();

                try
                {
                    result = Calculator.DoOperation(cleanNum1, cleanNum2, op);
                    if (double.IsNaN(result))
                    {
                        Console.WriteLine("This operation will result in a mathematical error.\n");
                    }
                    else Console.WriteLine("Your result: {0:0.##}\n", result);
                }
                catch (Exception e)
                {
                    Console.WriteLine("Oh no! An exception occurred trying to do the math.\n - Details: " + e.Message);
                }

                Console.WriteLine("------------------------\n");

                // Wait for the user to respond before closing
                Console.Write("Press 'n' and Enter to close the app, or press any other key and Enter to continue: ");
                if (Console.ReadLine() == "n") endApp = true;

                Console.WriteLine("\n"); // Friendly linespacing
            }
            return;
        }
    }
}

现在，我想在此项目中添加另一个 C# 文件并在我的程序中使用该类，如下所示：

使用系统；

namespace Calculator
{
    public class NewClass
    {
        public NewClass()
        {
            Console.WriteLine("Hello World");
        }
    }
}

当我尝试在程序中使用 NewClass 时，它给出了一个错误。我已将其添加到 program.cs 所在的同一文件夹中，但仍未修复。我从这个链接得到了这个解决方案。

所以我的主要问题是：

1. 如何在c#中使用同一文件夹中的其他类（就像在java中一样，如果两个程序在同一个文件夹中，则认为它们在同一个包中，无需任何import语句即可使用。不能这样做在这种情况下？）
2. 不过，这与当前主题无关，但是我们可以在 Microsoft Visual Studio 中创建一个项目，该项目只是一个包含一些 C# 文件的文件夹吗？（更具体地说，我只想像在普通 IDE 中一样编写 C# 代码，我该怎么做？在不创建任何项目的情况下做到这一点？）

编辑：我应该发布确切的错误..我的错。因此，当我尝试运行该程序时，它显示：

1>------ Build started: Project: ConsoleApp1, Configuration: Debug|AnyCPU ------
C:\Users\<SystemName>\Source\Repos\<UserName>\C-sharp-Practice-Codes\ConsoleApp1\ConsoleApp1\Program.cs(42,28,42,36): error CS1955: Non-invocable member 'NewClass' cannot be used like a method.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

为什么 NewClass() 不可调用？