connect and read and insert data in sql database using c#

connect to sql database using c#
add these library first
using System.Data;
using System.Data.SqlClient;


create string to connect with database
string  str = @"Data Source=(local);Initial Catalog=database;Persist Security Info=True;User ID=username;Password=password";



 SqlConnection con = new SqlConnection(); //create object to connect sql
    SqlCommand cmd = new SqlCommand(); // create object to send command in sql
    DataTable dt = new DataTable();  //create object to store data in temprary
      con.ConnectionString = str; //it open connection for database
        cmd.Connection = con; // it make connection between connection and command
        SqlDataAdapter da = new SqlDataAdapter(cmd); //adatapter object to talk database
            con.Open();//open connection
            cmd.CommandText = "insert into table(comuln1,column2)values('a','b')"; // specify command
            da.Fill(dt);//fire command
con.close();//close connection

//note: if id column is auto then no need to specify id column name to insert


now read database from sql database
create connection as above

System.Data.SqlClient.SqlDataReader SqlDR; // create object to read database
       
  con.Open();
            cmd.CommandText = "SELECT column1,column2 FROM table";
            System.Data.SqlClient.SqlDataReader SqlDR2;
            SqlDR = cmd.ExecuteReader();

            while (SqlDR.Read())
            {

                complainno = SqlDR["column1"].ToString();

            }
            con.Close();