Filling Textbox, when combobox Selected Index Changed

Question

In the code below, i just want to fill my textbox based on the selected combo box changed. but i get the following error.

'Conversion failed when converting the varchar value 'System.Data.DataRowViewConvert.ToString()' to data type int.'

i'd appreciate it if you help me.

SqlConnection objConnection = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\UniversityDataBase.mdf;Integrated Security=True");
        private void comboBox1_Click(object sender, EventArgs e)
        {
            string query = "SELECT *FROM TutorTable";
            SqlDataAdapter SDA = new SqlDataAdapter(query, objConnection);
            DataTable dt = new DataTable();
            SDA.Fill(dt);
            comboBox1.DataSource = dt;
            comboBox1.DisplayMember = "Tid";
            objConnection.Close();
        }

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string sqlQuery = "SELECT *FROM TutorTable where Tid = '"+comboBox1.Text+ "Convert.ToString()'";
        SqlCommand objCommand = new SqlCommand(sqlQuery, objConnection);
        objConnection.Open();
        objCommand.ExecuteNonQuery();
        SqlDataReader dr;
        dr = objCommand.ExecuteReader();
        while (dr.Read())
        {
            string Tname = (string)dr["Tname"].ToString();
            textBox1.Text = Tname;
        }

    }

Answer 1

Why are you using the Convert.ToString() in this piece of code:

"SELECT * FROM TutorTable where Tid = '"+comboBox1.Text+ "Convert.ToString()'"

I think the correct way wolud be:

"SELECT * FROM TutorTable where Tid = '"+comboBox1.Text+ "'"

But consider using a store procedure to prevent sql injection or using an ORM.

I think that comboBox1.Text is already returning a string value. So if that, it is not necessary to put the Convert.ToString(comboBox.Text), just put comboBox1.Text

According to documentation, the Text property is a string https://msdn.microsoft.com/en-us/library/system.windows.forms.combobox.text(v=vs.110).aspx