Dapper in .Net Core

一、前言

　　关于什么是Dapper，在此不做赘述；本文仅对Dapper在.Net Core中的使用作扼要说明，所陈代码以示例讲解为主，乃抛砖引玉，开发者可根据自身需要进行扩展和调整；其中如有疏漏之处，望不吝斧正。

二、Dapper环境搭建

当前以.Net Core WebAPI或MVC项目为例，框架版本为.NET 5.0，相关NuGet包引用如下：

Install-Package Dapper

Install-Package Dapper.Contrib

Install-Package Dapper.SqlBuilder

Install-Package System.Data.SqlClient

其中Dapper.Contrib和Dapper.SqlBuilder为Dapper的扩展，当然，Dapper的扩展还有如Dapper.Rainbow等其他包，根据自身需要引用，对相关引用作下说明：

• Dapper：不言而喻；
• Dapper.Contrib：可使用对象进行数据表的增删改查，免却SQL语句的编写；
• Dapper.SqlBuilder：可以方便动态构建SQL语句，如Join、SELECT、Where、OrderBy等等；
• System.Data.SqlClient：由于示例数据库为Sql Server，如MySql则引用MySql.Data；

对于Dapper.Contrib实体配置选项，以Product类为例，作扼要说明如下：

[Table("Product")]
public class Product
{
    [Key]  
    public int Id { get; set; }
    public string Name{ get; set; }
    public string Description { get; set; }
    public decimal Price { get; set; }
    public DateTime CreateTime { get; set; }
}

对于实体配置项，有如下几个主要项：

• Table：指定数据库表名，可忽略；
• Key：指定为自动增长主键；
• ExplicitKey：指定非自动增长主键，如guid；
• Computed：计算列属性，Insert、Update操作将忽略此列；
• Write：是否可写入，true/false，如[Write(false)]，false时Insert、Update操作将忽略此列，比如可扩展局部类作数据表额外查询字段使用；

对于数据表对象实体，可结合T4模板生成即可。

三、Dapper封装

作修改调整封装如下：

定义DapperDBContext类

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以上代码涵盖了Dapper访问数据库的基本操作，分同步和异步，其中大部分不作赘述，着重说下分页部分；

异步分页构建（PageAsync）

分页这里为方便调用，只需传入要查询的Sql语句（如：SELECT * FROM Table，必须带Order BY）、页索引、页大小即可；

至于具体如何构建的，这里参照某小型ORM工具PetaPoco，抽取相关代码如下，有兴趣的同学也可以自行改造：

public class Page<T>
    {
        /// <summary>
        /// The current page number contained in this page of result set 
        /// </summary>
        public long CurrentPage { get; set; }

        /// <summary>
        /// The total number of pages in the full result set
        /// </summary>
        public long TotalPages { get; set; }

        /// <summary>
        /// The total number of records in the full result set
        /// </summary>
        public long TotalItems { get; set; }

        /// <summary>
        /// The number of items per page
        /// </summary>
        public long ItemsPerPage { get; set; }

        /// <summary>
        /// The actual records on this page
        /// </summary>
        public IEnumerable<T> Items { get; set; }
        //public List<T> Items { get; set; }
    }
    public class DapperPage
    {
        public static void BuildPageQueries(long skip, long take, string sql, out string sqlCount, out string sqlPage)
        {
            // Split the SQL
            if (!PagingHelper.SplitSQL(sql, out PagingHelper.SQLParts parts))
                throw new Exception("Unable to parse SQL statement for paged query");

            sqlPage = BuildPageSql.BuildPageQuery(skip, take, parts);
            sqlCount = parts.sqlCount;
        }
    }

    static class BuildPageSql
    {
        public static string BuildPageQuery(long skip, long take, PagingHelper.SQLParts parts)
        {
            parts.sqlSelectRemoved = PagingHelper.rxOrderBy.Replace(parts.sqlSelectRemoved, "", 1);
            if (PagingHelper.rxDistinct.IsMatch(parts.sqlSelectRemoved))
            {
                parts.sqlSelectRemoved = "peta_inner.* FROM (SELECT " + parts.sqlSelectRemoved + ") peta_inner";
            }
            var sqlPage = string.Format("SELECT * FROM (SELECT ROW_NUMBER() OVER ({0}) peta_rn, {1}) peta_paged WHERE peta_rn>{2} AND peta_rn<={3}",
                                    parts.sqlOrderBy ?? "ORDER BY (SELECT NULL)", parts.sqlSelectRemoved, skip, skip + take);
            //args = args.Concat(new object[] { skip, skip + take }).ToArray();

            return sqlPage;
        }

        //SqlServer 2012及以上
        public static string BuildPageQuery2(long skip, long take, PagingHelper.SQLParts parts)
        {
            parts.sqlSelectRemoved = PagingHelper.rxOrderBy.Replace(parts.sqlSelectRemoved, "", 1);
            if (PagingHelper.rxDistinct.IsMatch(parts.sqlSelectRemoved))
            {
                parts.sqlSelectRemoved = "peta_inner.* FROM (SELECT " + parts.sqlSelectRemoved + ") peta_inner";
            }    

            var sqlOrderBy = parts.sqlOrderBy ?? "ORDER BY (SELECT NULL)";
            var sqlPage = $"SELECT {parts.sqlSelectRemoved} {sqlOrderBy} OFFSET {skip} ROWS FETCH NEXT {take} ROWS ONLY";
            return sqlPage;
        }
    }

    static class PagingHelper
    {
        public struct SQLParts
        {
            public string sql;
            public string sqlCount;
            public string sqlSelectRemoved;
            public string sqlOrderBy;
        }

        public static bool SplitSQL(string sql, out SQLParts parts)
        {
            parts.sql = sql;
            parts.sqlSelectRemoved = null;
            parts.sqlCount = null;
            parts.sqlOrderBy = null;

            // Extract the columns from "SELECT <whatever> FROM"
            var m = rxColumns.Match(sql);
            if (!m.Success)
                return false;

            // Save column list and replace with COUNT(*)
            Group g = m.Groups[1];
            parts.sqlSelectRemoved = sql.Substring(g.Index);

            if (rxDistinct.IsMatch(parts.sqlSelectRemoved))
                parts.sqlCount = sql.Substring(0, g.Index) + "COUNT(" + m.Groups[1].ToString().Trim() + ") " + sql.Substring(g.Index + g.Length);
            else
                parts.sqlCount = sql.Substring(0, g.Index) + "COUNT(*) " + sql.Substring(g.Index + g.Length);


            // Look for the last "ORDER BY <whatever>" clause not part of a ROW_NUMBER expression
            m = rxOrderBy.Match(parts.sqlCount);
            if (!m.Success)
            {
                parts.sqlOrderBy = null;
            }
            else
            {
                g = m.Groups[0];
                parts.sqlOrderBy = g.ToString();
                parts.sqlCount = parts.sqlCount.Substring(0, g.Index) + parts.sqlCount.Substring(g.Index + g.Length);
            }

            return true;
        }

        public static Regex rxColumns = new Regex(@"\A\s*SELECT\s+((?:\((?>\((?<depth>)|\)(?<-depth>)|.?)*(?(depth)(?!))\)|.)*?)(?<!,\s+)\bFROM\b", RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.Compiled);
        public static Regex rxOrderBy = new Regex(@"\bORDER\s+BY\s+(?!.*?(?:\)|\s+)AS\s)(?:\((?>\((?<depth>)|\)(?<-depth>)|.?)*(?(depth)(?!))\)|[\w\(\)\.])+(?:\s+(?:ASC|DESC))?(?:\s*,\s*(?:\((?>\((?<depth>)|\)(?<-depth>)|.?)*(?(depth)(?!))\)|[\w\(\)\.])+(?:\s+(?:ASC|DESC))?)*", RegexOptions.RightToLeft | RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.Compiled);
        public static Regex rxDistinct = new Regex(@"\ADISTINCT\s", RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.Compiled);
    }

对于构建分页语句，分别示例BuildPageQuery和BuildPageQuery2，前者为通过ROW_NUMBER进行分页（针对SqlServer2005、2008），后者通过OFFSET、FETCH分页（针对SqlServer2012及以上版本），相关辅助操作类一览便知，如果使用MySql数据库，可酌情自行封装； 

至于Where查询的进一步封装，有兴趣的也可兑Dapper lamada查询进行扩展。

定义工作单元与事务

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定义数据仓储

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根据自身需要进行调整或扩展，一般借助T4模板生成

数据库连接

通过Ioptions模式读取配置文件appsettings中连接字符串
 

public class MyDBContext : DapperDBContext
    {
        public MyDBContext(IOptions<DapperDBContextOptions> optionsAccessor) : base(optionsAccessor)
        {
        }

        protected override IDbConnection CreateConnection(string connectionString)
        {
            IDbConnection conn = new SqlConnection(connectionString);
            return conn;
        }
    }

四、Dapper使用

Startup.cs注入并读取数据库连接字符串

{
  "SQLConnString": "Data Source=(local);Initial Catalog=database;Persist Security Info=True;User ID=sa;Password=123456;MultipleActiveResultSets=True;",  
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

services.AddDapperDBContext<MyDBContext>(options =>
            {
                options.Configuration = Configuration["SQLConnString"];
            });

简单示例WebAPI或Net Core MVC下的调用示例：

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