C# "Cannot convert" hatası

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.Data.Common;

namespace HatsaneGirisUygıulamasi
{
    public partial class Sekreterdetay : Form
    {
        public Sekreterdetay()
        {
            InitializeComponent();
        }

        private void groupBox1_Enter(object sender, EventArgs e)
        {

        }

        private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {

        }
        SQLbaglantisi bgl = new SQLbaglantisi();
        public string tc;
        private void Sekreterdetay_Load(object sender, EventArgs e)
        {
            lbltc.Text = tc;
            //Ad Soyad
            SqlCommand komut1 = new SqlCommand("Select SekreterAdSoyad From Table_Secretery Where SekreterTC=@p1", bgl.baglanti());
            komut1.Parameters.AddWithValue("@p1", lbltc.Text);
            SqlDataReader dr1 = komut1.ExecuteReader();
            while (dr1.Read())
            {
                lbladsoyad.Text = dr1[0].ToString();
            }
            bgl.baglanti().Close();

            //Branşları Datagridview'e aktarma

            DataTable dt1 = new DataTable();
            DataAdapter da = new SqlDataAdapter("Select * from Table_Branslar", bgl.baglanti());
            da.Fill(dt1);
            dataGridView1.DataSource = dt1;

            //Doktorlar

            DataTable dt2 = new DataTable();
            DataAdapter da2 = new SqlDataAdapter("Select * from Table_Doktorlar ",bgl.baglanti());
            da2.Fill(dt2);
            dataGridView2.DataSource = dt2;
        }

        private void lbltc_Click(object sender, EventArgs e)
        {

        }
    }
}