private void button1_Click(object sender, EventArgs e)
       {
            ConexionDB cnx = new ConexionDB();
            textBoxNombre.Text = cnx.Select("clientes",textBoxCodigo.Text);
       }

public string Select(string strFrom, string strWhere)
        {
            string query = "SELECT nombre FROM "+strFrom+" WHERE numero ='"+strWhere+"' AND TIPO = 'C'";
            string result="";


            if(this.OpenConnection())
            {
                OdbcCommand cmd = new OdbcCommand(query,conexion);
                OdbcDataReader dr = cmd.ExecuteReader();

                while (dr.Read())
                {
                    result = dr["nombre"].ToString();
                }
            }

            this.CloseConnection();

            return result;
        }

private void button1_Click(object sender, EventArgs e)
        {
            ConexionDB cnx = new ConexionDB();
            textBoxNombre.Text = cnx.Select("nombre","clientes",textBoxCodigo.Text);
        }

public string Select(string strSelect, string strFrom, string strWhere)
        {
            string query = "SELECT "+strSelect+ " FROM "+strFrom+" WHERE numero ='"+strWhere+"' AND TIPO = 'C'";
            string result="";


            if(this.OpenConnection())
            {
                OdbcCommand cmd = new OdbcCommand(query,conexion);
                OdbcDataReader dr = cmd.ExecuteReader();

                while (dr.Read())
                {
                    result = dr["strSelect"].ToString();
                }
            }

            this.CloseConnection();

            return result;
        }

while (dr.Read())
  {
      result = dr["strSelect"].ToString();
  }