C# SQLite 和 SQL 命令指定的强制转换无效

Question

我正在使用 SQlite 和 sql 命令。我正在尝试制作一个测验程序，我有一个循环，它从我的数据库中读取问题和答案并将它们添加到列表中。我还有一个布尔值，用于定义从数据库中选取的答案是正确还是错误。

我的问题是，当我的循环第一次执行代码并将 true 和 false 添加到我的布尔数组时，所有这些工作正常，但第二次我的循环执行时它会抛出异常：SPECIFIED CAST NOT VALID。失败的方法如下所示：我在代码失败的地方做了评论：

    public void GetQuestion(int categoryRef)
    {
        Console.Clear();

        int arrayIndex = 0;

        int qListIndex = 0;

        int idListIndex = 0;

        List<string> qList = new List<string>();

        List<int> idList = new List<int>();

        int ansNr = 1;

        bool[] isTrue = new bool[3];


        SQLiteDataReader sqReader;

        SQLiteCommand sqCommand = new SQLiteCommand(sqConnection);

        try
        {
            sqCommand.CommandText = "SELECT Question, ID FROM Questions WHERE CategoryRef=" + categoryRef.ToString();
            sqCommand.Connection.Open();
            sqReader = sqCommand.ExecuteReader();
            foreach (var item in sqReader)
            {
                qList.Add(sqReader.GetString(0));
                idList.Add(sqReader.GetInt32(1));
            }
            sqReader.Close();
        }
        finally
        {
            sqConnection.Close();    
        }

        for (int i = 0; i < qList.Count; i++)
        {   
            try
            {
                sqCommand.CommandText = "SELECT Answer FROM Answers WHERE QuestionRef=" + idList[idListIndex].ToString();
                sqConnection.Open();
                sqReader = sqCommand.ExecuteReader();
                Console.WriteLine(qList[qListIndex]);
                foreach (var answer in sqReader)
                {
                    Console.WriteLine(ansNr + ":" + sqReader.GetString(0));

                    ansNr++;
                }
                sqReader.Close();
            }
            finally
            {
                sqConnection.Close();
            }

            try
            {   
                //THIS CODE FAILS 2'nd TIME IT LOOPS THROUGH
                sqCommand.CommandText = "SELECT IsTrue FROM Answers WHERE QuestionRef=" + idList[idListIndex].ToString();
                sqConnection.Open();
                sqReader = sqCommand.ExecuteReader();
                foreach (var item in sqReader)
                {

                    isTrue[arrayIndex] = sqReader.GetBoolean(0); //<-- Specified cast is not valid.
                    arrayIndex++;


                }
                sqReader.Close();
            }
            finally
            {

                sqConnection.Close();
            }

            string input = Console.ReadLine();
            int number = Convert.ToInt32(input);

            switch (number)
            {
                case 1:
                    if (isTrue[0] == true)
                    {
                        Console.WriteLine("Correct");
                    }
                    if (isTrue[0] == false)
                    {
                        Console.WriteLine("False");
                    }
                    break;

                case 2:
                    if (isTrue[1] == true)
                    {
                        Console.WriteLine("Correct");
                    }
                    if (isTrue[1] == false)
                    {
                        Console.WriteLine("False");
                    }
                    break;

                case 3:
                    if (isTrue[2] == true)
                    {
                        Console.WriteLine("Correct");
                    }
                    if (isTrue[2] == false)
                    {
                        Console.WriteLine("False");
                    }
                    break;
            }
            Console.ReadLine();
            idListIndex++;
            qListIndex++;
            arrayIndex = 0;
            ansNr = 1;
        }
    }

Answer 1

最有可能的是，您从数据库中读取了 DbNull，它不能转换为 bool。