SQLite Insert语句未执行命令

Question

在使用我的代码之前，我一直遇到问题，但是仍然无法将我的语句插入并保存到数据库中。当我在数据库中输入问题时，它会回读说“由于对象的当前状态，操作无效”

    private void SubmitBttn_Click_1(object sender, EventArgs e)
    {
        try
        {
            //Reading User Question
            string userQuestionTxt = UsrQstnTxtBox.Text.ToString();
            string userQuestionID = QstnIDTextBx.Text.ToString();
            string userCourse = CourseDropBox.ToString();
            string userAnswr = User_Answr.ToString();

            //IF the question exists first 
            string SQLstr = "SELECT * FROM UserQuestion where QID = \'" + userQuestionID + "\'userQuestion = \'" + userQuestionTxt + "\'userCourse = \'" + userCourse + "\'";
            int returnCode = 0;
            MyQuestion.Clear();
            MyQuestion = dbMgr.GetData(SQLstr, out returnCode);

            //check for debug flag
            if (GlobalData.isDebug) { System.Windows.Forms.MessageBox.Show(SQLstr); }

            if (returnCode > 0)
            {
                string suggestion = "The question already exits in the database.";
                System.Windows.Forms.MessageBox.Show(suggestion, "Add Question");
                return;
            }
            //Insert Query
             SQLstr = "INSERT INTO UserQuestion (Course, QID , userQuestion, AnswrID, UserAnswr) " +
                        "VALUES ('" + userCourse + "," + userQuestionID + "," + userQuestionTxt + "," + User_Answr + "')";

            //Checking for debug flag
            if (GlobalData.isDebug) { System.Windows.Forms.MessageBox.Show(SQLstr); }

            int numRowsModified = 0;
            numRowsModified = dbMgr.PutData(SQLstr);

            if (numRowsModified > 0)
            {
                string greeting = "Question Posted.";
                System.Windows.Forms.MessageBox.Show(greeting, "Add Question");
            }
            else
            {
                System.Windows.Forms.MessageBox.Show("Question has not been added.");
            }
        }

        catch (Exception ex)
        {
            String msg = ex.Message.ToString() + ex.StackTrace.ToString();
            System.Windows.Forms.MessageBox.Show(msg);
        }

    }