using System.Configuration;
using System.Data;
using System.Data.SqlClient;
public class SQLHelper
{
 private static readonly string ConnectionString = ConfigurationManager.ConnectionStrings["Default"].ConnectionString;
 public static DataSet GetDataSet(string sql)
 {
  using (SqlConnection conn =new SqlConnection(ConnectionString))
  {
   SqlCommand cmd = new SqlCommand(sql, conn);
   SqlDataAdapter adapter = new SqlDataAdapter(cmd);
   DataSet ds = new DataSet();
   conn.Open();
   adapter.Fill(ds);
   return ds;
  }
 }
}

<"1.0" encoding="utf-8" "Default" connectionString="Data Source=.;Initial Catalog=EFDb;Integrated Security=true"/>
 </connectionStrings>
</configuration>

using System.Configuration;
using System.Data;
using System.Data.SqlClient;
public class SQLHelper
{
 private static readonly string ConnectionString = ConfigurationManager.ConnectionStrings["Default"].ConnectionString;
 public static DataSet GetDataSet(string sql)
 {
  using (SqlConnection conn =new SqlConnection(ConnectionString))
  {
   SqlCommand cmd = new SqlCommand(sql, conn);
   SqlDataAdapter adapter = new SqlDataAdapter(cmd);
   adapter.MissingSchemaAction = MissingSchemaAction.AddWithKey;
   DataSet ds = new DataSet();
   conn.Open();
   adapter.Fill(ds);
   return ds;
  }
 }
}