ConfigurationManager

[reanimatik]

Почему вылазит ошибка "Имя ConfigurationManager не существует в текущем контексте" в строке:

string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;

полный код:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Configuration;

namespace WindowsFormsApp13
{
    public partial class Form1 : Form
    {
        static int index = 0;
        struct Question { public string question; }
        struct Option { public string option1, option2, option3, option4; }

        public Form1()
        {
            InitializeComponent();
            GenerateQuestionsOptions(index);
        }

        private void btnNext_Click(object sender, EventArgs e)
        {
            if (index < PopulateQuestions().Rows.Count - 1)
            {
                index++;
                GenerateQuestionsOptions(index);
            }
        }

        private void btnPrevious_Click(object sender, EventArgs e)
        {
            if (index > 0)
            {
                index--;
                GenerateQuestionsOptions(index);
            }
        }

        public void GenerateQuestionsOptions(int index)
        {
            Question question = new Question();
            question.question = PopulateQuestions().Rows[index]["QuestionDescription"].ToString();

            Option options = new Option();
            options.option1 = PopulateQuestions().Rows[index]["OptionOne"].ToString();
            options.option2 = PopulateQuestions().Rows[index]["OptionTwo"].ToString();
            options.option3 = PopulateQuestions().Rows[index]["OptionThree"].ToString();
            options.option4 = PopulateQuestions().Rows[index]["OptionFour"].ToString();

            List<string> optionsList = new List<string>();
            optionsList.Add(options.option1);
            optionsList.Add(options.option2);
            optionsList.Add(options.option3);
            optionsList.Add(options.option4);

            List<string> shuffledOptions = optionsList.OrderBy(a => Guid.NewGuid()).ToList();

            lblQuestion.Text = (index + 1) + " : " + question.question.ToUpper();
            rbOption1.Text = shuffledOptions[0];
            rbOption2.Text = shuffledOptions[1];
            rbOption3.Text = shuffledOptions[2];
            rbOption4.Text = shuffledOptions[3];
        }

        private DataTable PopulateQuestions()
        {
            DataTable dt = new DataTable();
            string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("SELECT * FROM QuestionTable", con))
                {
                    using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                    {
                        con.Open();
                        da.Fill(dt);
                        con.Close();
                    }
                }
            }
            return dt;
        }
    }
}

XML:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
    </startup>
<connectionStrings>
  <add name="conString" connectionString="Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\User\source\repos\WindowsFormsApp13\WindowsFormsApp13\Database1.mdf;Integrated Security=True" />
</connectionStrings>
</configuration>

Гуглил правильное написание, вроде все так же, как написал я.
Библиотека Configuration есть

[p51x]

Удалите референс на System.Configuration и снова добавьте.