Am getting error while connecting by db with visual studio 2019

Question

using System;

using System.Collections.Generic;

using System.Configuration;

using System.Data.SqlClient;

using System.Linq;

using System.Web;

namespace aspsnippets.Models

{

public class DBconnection

{

public List<asppiechart> DBconnect()

{

List<asppiechart> chartData = new List<asppiechart>();

string connectionString = ConfigurationManager.ConnectionStrings["conmydb"].ToString();

SqlConnection sqlConnection = new SqlConnection(connectionString);

string cmd1 = "select * from piechart";

SqlCommand sqlCommand = new SqlCommand(cmd1, sqlConnection);

SqlDataReader sqlDataReader = sqlCommand.ExecuteReader();

while(sqlDataReader.Read())

{

chartData.Add(new asppiechart

{

Month = sqlDataReader["Month"].ToString(),

Values = sqlDataReader["Values"].ToString()

});

}

return chartData;

}

}

}

 

 

AM GETTING THIS ERROR:

 

System.NullReferenceException: 'Object reference not set to an instance of an object.'

System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null.