使用通用委托的 [英] Use of Generic Delegates
本文介绍了使用通用委托的的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我们知道，采取行动，Func键和predicate是pre定义泛型委托。因此，作为代表，他们可以指向与指定的签名功能。
我有以下的数据访问方案，其中 Func键＆LT; T，R＆GT; 有助于避免foreach循环在调用方法。该方法2不具有循环。在这里， Func键＆LT; T，R＆GT; 有助于避免环路
什么是其他方案的通用名代表，它可以节省大量的code？
行参考
- 动态创作前pression predicates
- 高级C＃
- <一个href="http://geekswithblogs.net/BlackRabbit$c$cr/archive/2011/11/24/c.net-little-wonders-the-$p$pdicate-comparison-and-converter-generic-delegates.aspx"相对=nofollow> C＃/。NET小奇迹：在predicate，比较和转换泛型委托
- Func键与行动与predicate
- 什么是Func键，如何以及什么时候使用它
- <一个href="http://stackoverflow.com/questions/12578965/how-can-i-pass-in-a-func-with-a-generic-type-parameter">How我可以通过在FUNC了泛型类型参数？
code
办法1
公共类MyCommonDAL
{
公共静态的IEnumerable＆LT; IDataRecord＆GT; ExecuteQueryWithTextCommandType（字符串CommandText中，列表与LT;的SqlParameter＆GT; commandParameters）
{
使用（SqlConnection的连接=新的SqlConnection（的connectionString））
{
使用（SqlCommand的命令=新的SqlCommand（））
{
command.Connection =连接;
command.CommandType = CommandType.Text;
command.CommandText =的CommandText;
command.CommandTimeout = 0;
command.Parameters.AddRange（commandParameters.ToArray（））;
connection.Open（）;
使用（VAR RDR = Command.ExecuteReader却（））
{
而（rdr.Read（））
{
得到的回报RDR;
}
rdr.Close（）;
}
}
}
}
}
公共类MyLogDAL
{
公开名单＆LT; LogSeverityType＆GT; GetLogSeveritiesFirstApproach（LogSeverityType logSeverityType）
{
名单＆LT;的SqlParameter＆GT; commandParameters =新的名单，其中，SqlParameter的＆GT;（）
{
新的SqlParameter {参数名称=@CreatedDateTime
值= logSeverityType.CreatedDateTime，
的SqlDbType = SqlDbType.DateTime}
};
字符串的CommandText = @SELECT * FROM dbo.LogSeverityType WHERE CreatedDateTime＆GT; @CreatedDateTime;
VAR的结果= MyCommonDAL.ExecuteQueryWithTextCommandType（CommandText中，commandParameters）;
名单＆LT; LogSeverityType＆GT; logSeverities =新的名单，其中，LogSeverityType＆GT;（）;
//循环
的foreach（IDataRecord REC的结果）
{
LogSeverityType objLogSeverityType = LogSeverityType.LogSeverityTypeFactory（REC）;
logSeverities.Add（objLogSeverityType）;
}
返回logSeverities;
}
}
办法2
公共类MyCommonDAL
{
公共静态的IEnumerable＆LT; T＆GT; ExecuteQueryGenericApproach＆LT; T＆GT;（字符串CommandText中，列表与LT;的SqlParameter＆GT; commandParameters，Func键＆LT; IDataRecord，T＆GT; factoryMethod）
{
//操作，FUNC键，predicate是pre定义泛型委托。
//因此，作为代表，他们可以指向函数指定的签名。
使用（SqlConnection的连接=新的SqlConnection（的connectionString））
{
使用（SqlCommand的命令=新的SqlCommand（））
{
command.Connection =连接;
command.CommandType = CommandType.Text;
command.CommandText =的CommandText;
command.CommandTimeout = 0;
command.Parameters.AddRange（commandParameters.ToArray（））;
connection.Open（）;
使用（VAR RDR = Command.ExecuteReader却（））
{
而（rdr.Read（））
{
收益回报factoryMethod（RDR）;
}
rdr.Close（）;
}
}
}
}
}
公共类MyLogDAL
{
公开名单＆LT; LogSeverityType＆GT; GetLogSeveritiesSecondApproach（LogSeverityType logSeverityType）
{
名单＆LT;的SqlParameter＆GT; commandParameters =新的名单，其中，SqlParameter的＆GT;（）
{
新的SqlParameter {参数名称=@CreatedDateTime
值= logSeverityType.CreatedDateTime，
的SqlDbType = SqlDbType.DateTime}
};
字符串的CommandText = @SELECT * FROM dbo.LogSeverityType WHERE CreatedDateTime＆GT; @CreatedDateTime;
// VAR的结果= MyCommonDAL.ExecuteQueryWithTextCommandType（CommandText中，commandParameters）;
IEnumerable的＆LT; LogSeverityType＆GT; logSeverities = MyCommonDAL.ExecuteQueryGenericApproach＆LT; LogSeverityType＆GT;（CommandText中，commandParameters，LogSeverityType.LogSeverityTypeFactory）;
//的foreach（IDataRecord REC的结果）
// {
// LogSeverityType objLogSeverityType = LogSeverityType.LogSeverityTypeFactory（REC）;
// logSeverities.Add（objLogSeverityType）;
//}
返回logSeverities.ToList（）;
}
}
其他$ C C必填$
公共类LogSeverityType
{
公众诠释LogSeverityTypeID {获得;组; }
公共字符串名称{;组; }
公共字符串描述{获得;组; }
公开日期时间CreatedDateTime {获得;组; }
公共静态LogSeverityType LogSeverityTypeFactory（IDataRecord记录）
{
返回新LogSeverityType
{
LogSeverityTypeID =（int）的记录[0]，
名称=（字符串）记录[1]，
说明=（字符串）记录[2]
CreatedDateTime =（DateTime的）记录[3]
};
}
}
静态无效的主要（字串[] args）
{
MyLogDAL logDAL =新MyLogDAL（）;
LogSeverityType logSeverityType =新LogSeverityType（）;
logSeverityType.CreatedDateTime = Convert.ToDateTime（1/1/2000）;
名单＆LT; LogSeverityType＆GT; logSeverities = logDAL.GetLogSeveritiesSecondApproach（logSeverityType）;
}
解决方案
我在解析时/发现在XML / HTML文档中的节点，并在分配值属性使用泛型委托。我写它的一篇博客文章，这说明重构code到通过在一个通用的委托，多少code去除。
We know that "Action, Func and Predicate are pre-defined Generic delegates. So as delegate they can point to functions with specified signature."
I have following data-access scenario in which Func<T,R> helps in avoiding a foreach loop in the calling method. The approach 2 doesn’t have looping. Here Func<T,R> helped to avoid loop.
What are the other scenarios for generic delegates in which it can save lots of lines of code?
REFERENCES
- Dynamically Composing Expression Predicates
- Advanced C#
- C#/.NET Little Wonders: The Predicate, Comparison, and Converter Generic Delegates
- Func vs. Action vs. Predicate
- What is Func, how and when is it used
- How can I pass in a func with a generic type parameter?
CODE
Approach 1
public class MyCommonDAL
{
public static IEnumerable<IDataRecord> ExecuteQueryWithTextCommandType(string commandText, List<SqlParameter> commandParameters)
{
using (SqlConnection connection = new SqlConnection(connectionString))
{
using (SqlCommand command = new SqlCommand())
{
command.Connection = connection;
command.CommandType = CommandType.Text;
command.CommandText = commandText;
command.CommandTimeout = 0;
command.Parameters.AddRange(commandParameters.ToArray());
connection.Open();
using (var rdr = command.ExecuteReader())
{
while (rdr.Read())
{
yield return rdr;
}
rdr.Close();
}
}
}
}
}
public class MyLogDAL
{
public List<LogSeverityType> GetLogSeveritiesFirstApproach(LogSeverityType logSeverityType)
{
List<SqlParameter> commandParameters = new List<SqlParameter>()
{
new SqlParameter {ParameterName = "@CreatedDateTime",
Value = logSeverityType.CreatedDateTime,
SqlDbType = SqlDbType.DateTime}
};
string commandText = @"SELECT * FROM dbo.LogSeverityType WHERE CreatedDateTime > @CreatedDateTime";
var results = MyCommonDAL.ExecuteQueryWithTextCommandType(commandText, commandParameters);
List<LogSeverityType> logSeverities = new List<LogSeverityType>();
//LOOP
foreach (IDataRecord rec in results)
{
LogSeverityType objLogSeverityType = LogSeverityType.LogSeverityTypeFactory(rec);
logSeverities.Add(objLogSeverityType);
}
return logSeverities;
}
}
Approach 2
public class MyCommonDAL
{
public static IEnumerable<T> ExecuteQueryGenericApproach<T>(string commandText, List<SqlParameter> commandParameters, Func<IDataRecord, T> factoryMethod)
{
//Action, Func and Predicate are pre-defined Generic delegates.
//So as delegate they can point to functions with specified signature.
using (SqlConnection connection = new SqlConnection(connectionString))
{
using (SqlCommand command = new SqlCommand())
{
command.Connection = connection;
command.CommandType = CommandType.Text;
command.CommandText = commandText;
command.CommandTimeout = 0;
command.Parameters.AddRange(commandParameters.ToArray());
connection.Open();
using (var rdr = command.ExecuteReader())
{
while (rdr.Read())
{
yield return factoryMethod(rdr);
}
rdr.Close();
}
}
}
}
}
public class MyLogDAL
{
public List<LogSeverityType> GetLogSeveritiesSecondApproach(LogSeverityType logSeverityType)
{
List<SqlParameter> commandParameters = new List<SqlParameter>()
{
new SqlParameter {ParameterName = "@CreatedDateTime",
Value = logSeverityType.CreatedDateTime,
SqlDbType = SqlDbType.DateTime}
};
string commandText = @"SELECT * FROM dbo.LogSeverityType WHERE CreatedDateTime > @CreatedDateTime";
//var results = MyCommonDAL.ExecuteQueryWithTextCommandType(commandText, commandParameters);
IEnumerable<LogSeverityType> logSeverities = MyCommonDAL.ExecuteQueryGenericApproach<LogSeverityType>(commandText, commandParameters, LogSeverityType.LogSeverityTypeFactory);
//foreach (IDataRecord rec in results)
//{
// LogSeverityType objLogSeverityType = LogSeverityType.LogSeverityTypeFactory(rec);
// logSeverities.Add(objLogSeverityType);
//}
return logSeverities.ToList();
}
}
Other Code Required
public class LogSeverityType
{
public int LogSeverityTypeID { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public DateTime CreatedDateTime { get; set; }
public static LogSeverityType LogSeverityTypeFactory(IDataRecord record)
{
return new LogSeverityType
{
LogSeverityTypeID = (int)record[0],
Name = (string) record[1],
Description = (string)record[2],
CreatedDateTime = (DateTime) record[3]
};
}
}
static void Main(string[] args)
{
MyLogDAL logDAL = new MyLogDAL();
LogSeverityType logSeverityType = new LogSeverityType();
logSeverityType.CreatedDateTime = Convert.ToDateTime("1/1/2000");
List<LogSeverityType> logSeverities = logDAL.GetLogSeveritiesSecondApproach(logSeverityType);
}
解决方案
I use generic delegates when parsing / finding nodes in XML / HTML documents, and assigning the values to properties. I wrote a blog post about it, which shows refactoring the code to pass in a generic delegate, and how much code was removed.
这篇关于使用通用委托的的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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