C#中do while循环返回false仍无法退出的问题排查

开发的C#程序里，外层do-while循环无法正常退出，会停在空白行等待输入，只有输入以Y开头的内容才会继续执行，输入N的时候反而一直卡在等待状态。程序代码如下：

using System;
namespace Assignment2
{
    class FunFeatures
    {
        private string name = "";
        private string email = "";

        static void Main(string[] args)
        {
            Console.Title = "Strings, selection and interation in C#";
            FunFeatures funObj = new FunFeatures();
            funObj.Start();

            ContinueToNextPart(); //call the method below

        }

        private static void ContinueToNextPart()
        {
            Console.WriteLine("\nPress enter to continue to the next part");
            Console.ReadLine();
            Console.Clear();
        }
        
        public void Start()
        {
            Introduce();

            bool done = false;
            do
            {
                //Call method to read a number 1 to 7 and display
                //name of the day (1 = Monday, 7 = Sunday with a comment
                PredictTheDay();

                //Calculate the length of a given text
                CalculateStrengthLength();

                //Run again or exit
                done = RunAgain();
            } while (!done);
            Console.WriteLine("Welcome back, " + name);
        }


        public void CalculateStrengthLength()
        {
            Console.WriteLine("\nLength of text: Write a text with a number of characters and press Enter");
            Console.WriteLine("It will then calculate the number of chars included  in the text");
            Console.WriteLine("Give me a text of any length, or press Enter to exit!");
            string str = Console.ReadLine();
            int strLength = str.Length;
            Console.WriteLine("\n"+ str.ToUpper());
            Console.WriteLine("Number of chars = " + strLength);
        }

        private void Introduce()
        {
            Console.WriteLine("\nLet me know about yourself!");
            ReadName();
            Console.Write("Give me your email please: ");
            email = Console.ReadLine();
            Console.WriteLine("\nHere is your full name and your email.");

            //split first name and last name
            var names = name.Split(" ");
            string fName = names[0];
            string lName = names[1];

            Console.WriteLine(lName + ", " + fName + " " + email);
        }

        public void PredictTheDay()
        {
            Console.WriteLine(); // blank row
            Console.WriteLine("\nI am a fortune teller.");
            Console.Write("Select a number between 1 and 7: ");
            string str = Console.ReadLine();
            int day = int.Parse(str);


            switch (day)
            {
                case 1:
                    // Monday
                    Console.WriteLine("Monday: Keep calm my friend! You can fall apart!");
                    break;
                case 2:
                    //Tuesday
                    Console.WriteLine("Tuesday and Wednesday break your heart!");
                    break;
                case 3:
                    //Wednesday
                    Console.WriteLine("Tuesday and Wednesday break your heart!");
                    break;
                case 4:
                    //Thursday
                    Console.WriteLine("Thursday, OMG, still one day to Friday!");
                    break;
                case 5:
                    //Friday
                    Console.WriteLine("It's Friday! You are in love!");
                    break;
                case 6:
                    //Saturday
                    Console.WriteLine("Saturday, do nothing and do plenty of it!");
                    break;
                case 7:
                    //Sunday
                    Console.WriteLine("And Sunday always comes too soon!");
                    break;

                default: // if user gives a number out of range
                    Console.WriteLine("Not in a good mode? This is not a valid date!");
                    break;
            }
        }

        public void ReadName()
        {
            Console.WriteLine("Your first name please: ");
            string firstname = Console.ReadLine();
            Console.Write("Your last name please: ");
            string lastname = Console.ReadLine().ToUpper();
            name = firstname + " " + lastname;
            Console.WriteLine("Nice to meet you " + firstname + "!");
        }

        //Ask user to whether to continue, 
        //return true if the user answers Y, y or any word beginning
        //with these, or N, n or any word beginning with these. 
        //Otherwise, return false.
        private bool RunAgain()
        {
            bool done = false; //true = y, false = n

            Console.WriteLine(); //blankline
            Console.Write("Run again? (y/n) ");

            do
            {
                string str = Console.ReadLine();
                //change str to uppercase letters to make comparison 
                //easier
                str = str.ToUpper();

                if (str[0] == 'Y')   //str[0]is the first letter in the string
                {
                    // continue wit
                    done = true;
                }
                else if (str[0] == 'N')
                {
                    // do not continue with calculation 
                    done = false;
                }
            } while (!done);   //if (!done) is same as: if (done == false)
            return done;
        }
    }
}

问题成因 #

问题出在RunAgain方法的内部do-while循环逻辑：

• 当用户输入以N开头的内容时，代码将done设为false，但循环条件是while (!done)，也就是只要done为false，循环就会继续执行，导致程序一直停在等待输入的状态，无法退出该方法。
• 只有当done被设为true（输入以Y开头的内容）时，内部循环才会退出，方法返回true，外层循环继续执行。

解决办法 #

修改RunAgain方法，确保无论用户输入Y还是N，内部循环都能退出，同时返回正确的布尔值来控制外层循环：

private bool RunAgain()
{
    bool shouldContinue = false;
    bool isValidInput = false;

    Console.WriteLine(); //blankline
    Console.Write("Run again? (y/n) ");

    do
    {
        string str = Console.ReadLine();
        if (string.IsNullOrEmpty(str))
        {
            Console.Write("Please enter y or n: ");
            continue;
        }
        str = str.ToUpper();

        if (str[0] == 'Y')
        {
            shouldContinue = true;
            isValidInput = true;
        }
        else if (str[0] == 'N')
        {
            shouldContinue = false;
            isValidInput = true;
        }
        else
        {
            Console.Write("Invalid input, please enter y or n: ");
        }
    } while (!isValidInput);
    
    return shouldContinue;
}

修改说明 #

1. 新增isValidInput变量，专门用来控制内部循环的退出条件，只要输入了Y/N就标记为有效输入，退出循环。
2. shouldContinue变量用来存储用户的选择，返回给外层循环决定是否继续执行。
3. 增加了空输入和无效输入的提示，提升用户体验。

内容的提问来源于stack exchange，提问作者NotSmart