خلل في نموذج C# مربوط بقاعدة بيانات sql server عند الضغط على زر System.InvalidOperationException: 'ExecuteReader: لم تتم تهيئة الخاصية Connection.'

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;
namespace library
{
    public partial class Form4 : Form
    {
        SqlConnection conn = new SqlConnection();
        SqlCommand com = new SqlCommand();
        SqlDataReader dr;
        DataTable dt = new DataTable();
        public Form4()
        {
            conn.ConnectionString = "server=(local);database=library;integrated security=true;";

            try
            {
                conn.Open();
                MessageBox.Show("فُتح");
            }
            catch (Exception ex)
            { MessageBox.Show(ex.Message); }
            InitializeComponent();
        }

        private void Form4_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            int first_space_index = textBox1.Text.IndexOf(" ");
            string first_name=textBox1.Text.Substring(0,first_space_index);
            string last_name = textBox1.Text.Substring(first_space_index +1); 
           
            com.Connection = conn;
            try
            {
                if (!string.IsNullOrEmpty(textBox2.Text))
                {
                    MessageBox.Show("سيدخل رقم القسم تلقائياً");
                    textBox2.Text = null;
                }
                com.CommandText = "insert into authors (AfirstName,ALastName)values('" + first_name + "','" + last_name + "')";
                com.ExecuteNonQuery();
                MessageBox.Show("تم");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            com.Connection = conn;
            if (string.IsNullOrEmpty(textBox1.Text))
            {
                DialogResult resu = MessageBox.Show("هل أنت متأكد من حذف جميع البيانات", "تأكيد", MessageBoxButtons.OKCancel, MessageBoxIcon.Information);
                if (resu==DialogResult.OK)
                {
                    com.CommandText = "delete authors ";
                    com.ExecuteNonQuery();
                    MessageBox.Show("تم");
                }
            }
            else
            {
                com.CommandText = "delete authors where AID='" + textBox2.Text + "' ";
                com.ExecuteNonQuery();
                MessageBox.Show("تم");
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            com.Connection = conn;
            try
            {
                if (!string.IsNullOrEmpty(textBox1.Text))
                {
                    int first_space_index = textBox1.Text.IndexOf(" ");
                    string first_name = textBox1.Text.Substring(0, first_space_index);
                    string last_name = textBox1.Text.Substring(first_space_index + 1); ;
                    com.CommandText = "update authors set AfirstName= '" + first_name + "',ALastName='"+last_name+"'where DID='" + textBox2.Text + "'";
                    com.ExecuteNonQuery();
                }

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            MessageBox.Show("تم");
        }

        private void button5_Click(object sender, EventArgs e)
        {
            com.Connection = conn;
            if (string.IsNullOrEmpty(textBox2.Text))
            {
                com.CommandText = "select * from authors";
            }
            else
            { com.CommandText = "select * from authors where AID= '" + textBox2.Text + "'"; }
            dr = com.ExecuteReader();
            dt.Load(dr);
            dataGridView1.DataSource = dt;
        }

        private void button6_Click(object sender, EventArgs e)
        {
           
            com.CommandText = "select book.* from book,authors where AID= '" + textBox2.Text + "'";
            dr = com.ExecuteReader();
            dt.Load(dr);
            dataGridView1.DataSource = dt;
        }

        private void button4_Click(object sender, EventArgs e)
        {
            Form1 f = new Form1();
            f.Show();
            this.Hide();
        }
    }
}

هذا نموذج مربوط بقاعدة البيانات sql server لكنني واجهت مشكلة في زر عرض الكتب التي ألفها مؤلف 

 private void button6_Click(object sender, EventArgs e)
        {
           
            com.CommandText = "select book.* from book,authors where AID= '" + textBox2.Text + "'";
            dr = com.ExecuteReader();
            dt.Load(dr);
            dataGridView1.DataSource = dt;
        }

System.InvalidOperationException: 'ExecuteReader: لم تتم تهيئة الخاصية Connection.'

فهل فيه حد يعرف المشكلة