C#

the Connection property has not been initialized.

using (SqlConnection con = new SqlConnection(strConnect))
    {
    con.Open();
    using (SqlCommand cmd = new SqlCommand("SELECT iD, description FROM myTable", con))
        {
        using (SqlDataReader reader = cmd.ExecuteReader())
            {
            while (reader.Read())
                {
                int id = (int) reader["iD"];
                string desc = (string) reader["description"];
                Console.WriteLine("ID: {0}\n    {1}", iD, desc);
                }
            }
        }
    }

private void textBox1_KeyUp(object sender, KeyEventArgs e)
        {
            connection.Open();
            //Assignes la propriété Connection de ta SqlCommand à ta SqlConnection (connection) avant de l'ouvrir.
            
            SqlCommand cmd = new SqlCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "Select informix.thisent.etb from informix.thisent";
            
            cmd.ExecuteNonQuery();
            DataTable dt = new DataTable();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt);
            dataGridView1.DataSource = dt;
            connection.Close();

cmd.Connection = connection;

private void textBox1_KeyUp(object sender, KeyEventArgs e)
{
    DataTable dt = new DataTable();
    SqlDataAdapter da = new SqlDataAdapter("SELECT informix.thisent.etb FROM informix.thisent", connection);
    da.Fill(dt);
    dataGridView1.DataSource = dt;
}

SELECT TOP 100 informix.thisent.etb FROM informix.thisent