字符串生成器的目的是什么？

Question

我只做过 C++ 和一些 C#，而且我是 MySQL 新手，我不明白这里的字符串生成器的目的。

此代码是用于从网页中提取数据并使用书签将其插入现有 Word 文件的程序的一部分。

我的代码

using System.Collections.Generic;
using System.Configuration;
using System.Text;
using MySql.Data.MySqlClient;

namespace ARExtractionLibrary
{
    public class DataExtractor
    {
        #region Attributes
        private string _databaseConfig = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        private const int AR_TRACKER_ID = 10;
        private const int AR_NUMBER_ID = 3;
        #endregion

        #region Methods
        public Dictionary<string, Field> GetFieldData(int TicketID)
        {
            MySqlConnection sqlConnection = new MySqlConnection(_databaseConfig);
            Dictionary<string, Field> fieldsDictionary = new Dictionary<string, Field>();

            StringBuilder queryDefaultFields = new StringBuilder();
            queryDefaultFields.Append("SELECT issues.id AS TicketID, issues.subject, issues.description, ");
            queryDefaultFields.Append(" CONCAT(users.firstname,' ', users.lastname) as Author");
            queryDefaultFields.Append(" FROM issues LEFT JOIN users ON users.id =issues.author_id");
            queryDefaultFields.Append("  WHERE issues.id = @TicketID");

            StringBuilder queryCustomFields = new StringBuilder();
            queryCustomFields.Append("SELECT custom_fields.name, custom_fields.field_format, CONCAT(users.firstname,' ',users.lastname) AS value");
            queryCustomFields.Append(" FROM custom_values");
            queryCustomFields.Append(" LEFT JOIN custom_fields ON custom_values.custom_field_id = custom_fields.id");
            queryCustomFields.Append(" JOIN users ON custom_values.value = users.id");
            queryCustomFields.Append(" WHERE custom_values.customized_id = @TicketId");
            queryCustomFields.Append(" AND field_format = 'user'");
            queryCustomFields.Append(" UNION");
            queryCustomFields.Append(" SELECT custom_fields.name, custom_fields.field_format, custom_values.value");
            queryCustomFields.Append(" FROM custom_values");
            queryCustomFields.Append(" LEFT JOIN custom_fields ON custom_values.custom_field_id = custom_fields.id");
            queryCustomFields.Append(" WHERE custom_values.customized_id = @TicketId");
            queryCustomFields.Append(" AND field_format <> 'user'");

            sqlConnection.Open();

            //First query
            MySqlCommand sqlCommand = null;
            sqlCommand = new MySqlCommand(queryDefaultFields.ToString(), sqlConnection);
            sqlCommand.Parameters.AddWithValue("@TicketId", TicketID);

            MySqlDataReader sqlDataReader = sqlCommand.ExecuteReader();
            while (sqlDataReader.Read())
            {
                Field subject = new Field("Subject", sqlDataReader["subject"].ToString());
                fieldsDictionary.Add(subject.Name, subject);

                Field description = new Field("Description", sqlDataReader["description"].ToString());
                fieldsDictionary.Add(description.Name, description);

                Field ticketID = new Field("TicketID", sqlDataReader["TicketID"].ToString());
                fieldsDictionary.Add(ticketID.Name, ticketID);

                Field author = new Field("Author", sqlDataReader["Author"].ToString());
                fieldsDictionary.Add(author.Name, author);
            }
            sqlDataReader.Close();

            //Second query
            sqlCommand = new MySqlCommand(queryCustomFields.ToString(), sqlConnection);
            sqlCommand.Parameters.AddWithValue("@TicketId", TicketID);
            sqlDataReader = sqlCommand.ExecuteReader();

            while (sqlDataReader.Read())
            {
                string fieldName = sqlDataReader["name"].ToString();
                string fieldValue = sqlDataReader["value"].ToString();
                string fieldFormat = sqlDataReader["field_format"].ToString();

                if (fieldsDictionary.ContainsKey(fieldName))
                {
                    fieldsDictionary[fieldName].Values.Add(fieldValue);
                }

                else
                {
                    Field localField = new Field(fieldName, fieldValue, fieldFormat);
                    fieldsDictionary.Add(localField.Name, localField);
                }
            }
            sqlDataReader.Close();
            sqlConnection.Close();

            return fieldsDictionary;
        }

        public Dictionary<int, string> GetARs()
        {
            Dictionary<int, string> ARDictionary = new Dictionary<int, string>();
            MySqlConnection sqlConnection = new MySqlConnection(_databaseConfig);
            StringBuilder sqlCommandIsAR = new StringBuilder();
            sqlCommandIsAR.Append("SELECT issues.id AS TicketID, issues.tracker_id AS Tracker, issues.subject AS Subject, custom_values.custom_field_id, custom_values.value");
            sqlCommandIsAR.Append(" FROM issues LEFT JOIN Custom_values ON custom_values.customized_id = issues.id ");
            sqlCommandIsAR.Append(" WHERE tracker_id = @IsAR AND custom_field_id = @IsARNumber ");

            sqlConnection.Open();

            MySqlCommand commandGetARs = new MySqlCommand(sqlCommandIsAR.ToString(), sqlConnection);
            commandGetARs.Parameters.AddWithValue("@IsAR", AR_TRACKER_ID);
            commandGetARs.Parameters.AddWithValue("@IsARNumber", AR_NUMBER_ID);
            MySqlDataReader sqlDataReader = null;
            sqlDataReader = commandGetARs.ExecuteReader();

            while (sqlDataReader.Read())
            {
                string DictionaryValue = sqlDataReader["TicketID"].ToString() + " - " + sqlDataReader["Subject"].ToString();

                if (sqlDataReader["value"].ToString().Length != 0)
                {
                    DictionaryValue += " [" + sqlDataReader["value"].ToString() + "]";
                }

                int key = int.Parse(sqlDataReader["TicketID"].ToString());
                ARDictionary.Add(key, DictionaryValue);
            }

            sqlDataReader.Close();
            sqlConnection.Close();

            return ARDictionary;
        }

        #endregion
    }
}

Answer 1

StringBuilder 避免每次添加字符串时都必须重新创建字符串。在 C# 中使用 String 类意味着您使用的是不可变对象，但 StringBuilder 在大多数情况下要快得多，因为它不必每次附加到它时都创建一个新字符串。

因此，一般而言，如果您多次附加到字符串，StringBuilder 将比仅执行 str += "text to append"（其中 str 是 String）更有效。

这里有一些注意事项需要注意，请参阅this 答案。

Answer 2

为了提高性能，您将添加大量字符串。众所周知，即使是 reference 类型 string 的行为也类似于 value 类型。

因此，在每次添加字符串时，都会分配一个 new 字符串对象。

string s = "hello"; 
s += " world"; //NEW STRING OBJECT ALLOCATED 

如果是StringBuilder，则没有任何新的分配，只是追加到它的末尾。

StringBuilder queryDefaultFields = new StringBuilder();
queryDefaultFields.Append("hello");   //THE SAME queryDefaultFields OBJECT
queryDefaultFields.Append(" world");  //THE SAME queryDefaultFields OBJECT

Answer 3

由于 StringBuilder 是可变的（与字符串相反），这意味着您可以在任何给定时间使用 Append（或 AppendLine）方法简单地向其添加数据，只要您有它的引用。

Answer 4

一般

StringBuilder 允许您构建字符串，而无需在每次变异操作时为字符串的内容重新分配缓冲区（始终使用相同的缓冲区；仅当字符串扩展超出当前容量时才会重新分配）。这使得构建大字符串的整个过程更加高效。

在这段代码中

在这种情况下，根本没有动态构建字符串；字符串实际上是硬编码的。这里StringBuilder 完全没有意义；如果字符串是这样的硬编码文字，实际上性能会更好：

var queryDefaultFields = @"SELECT issues.id AS TicketID, issues.subject, issues.description,
                           CONCAT(users.firstname,' ', users.lastname) as Author
                           FROM issues LEFT JOIN users ON users.id =issues.author_id
                           WHERE issues.id = @TicketID";

我的猜测是，这段代码的作者要么误用了StringBuilder，因为他们记得读过“这样更快”，但从未真正理解为什么，或者犯了疏忽复制粘贴的罪行。