.NetCore中Dapper的使⽤详解
Dapper 是⼀个轻量级ORM框架，在项⽬中如果对性能⽐较看中，Dapper是⼀个不错的选择。接下来我们就来看看如何在项⽬中使⽤Dapper.
1.安装Dapper
  这⾥直接使⽤Nuget安装。
安装完成之后，发现Nuget下已经有了Dapper。
2.创建DapperHelper
  接下来创建⼀个DapperHelper帮助类，来进⾏读取数据库连接字符串，打开数据库等操作。
代码：
public class DapperHelper
{
/// 数据库连接名
private static string _connection = string.Empty;
/// 获取连接名
private static string Connection
{
get { return _connection; }
//set { _connection = value; }
}
/// 返回连接实例
private static IDbConnection dbConnection = null;
/// 静态变量保存类的实例
private static DapperHelper uniqueInstance;
/
// 定义⼀个标识确保线程同步
private static readonly object locker = new object();
/// <summary>
/// 私有构造⽅法，使外界不能创建该类的实例，以便实现单例模式
/// </summary>
private DapperHelper()
{
// 这⾥为了⽅便演⽰直接写的字符串，实例项⽬中可以将连接字符串放在配置⽂件中，再进⾏读取。
_connection = @"server=.;uid=sa;pwd=sasasa;database=Dapper";
}
/// <summary>
/
// 获取实例，这⾥为单例模式，保证只存在⼀个实例
/// </summary>
/// <returns></returns>
public static DapperHelper GetInstance()
{
// 双重锁定实现单例模式，在外层加个判空条件主要是为了减少加锁、释放锁的不必要的损耗
if (uniqueInstance == null)
{
lock (locker)
{
if (uniqueInstance == null)
{
uniqueInstance = new DapperHelper();
}
}
}
return uniqueInstance;
}
/// <summary>
/// 创建数据库连接对象并打开链接
/// </summary>
/// <returns></returns>
public static IDbConnection OpenCurrentDbConnection()
{
if (dbConnection == null)
{
dbConnection = new SqlConnection(Connection);
}
//判断连接状态
if (dbConnection.State == ConnectionState.Closed)
{
dbConnection.Open();
}
return dbConnection;
}
}
　3.创建上下⽂ DbContext 作⽤是调⽤Dapper中操作数据库的⽅法，⾥⾯是我⾃⼰封的⼀些常⽤⽅法，当然了这⼀步可以略去，直接调⽤也⾏，这⾥只是提供⼀种思路。
public static class DbContext
{
// 获取开启数据库的连接
private static IDbConnection Db
{
get
{
//创建单⼀实例
DapperHelper.GetInstance();
return DapperHelper.OpenCurrentDbConnection();
}
}
/// <summary>
/// 查出⼀条记录的实体
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="sql"></param>
/// <returns></returns>
public static T QueryFirstOrDefault<T>(string sql, object param = null)
{
return Db.QueryFirstOrDefault<T>(sql, param);
}
public static Task<T> QueryFirstOrDefaultAsync<T>(string sql, object param = null)
{
return Db.QueryFirstOrDefaultAsync<T>(sql, param);
}
/// <summary>
/// 查出多条记录的实体泛型集合
/// </summary>
/
// <typeparam name="T">泛型T</typeparam>
/// <returns></returns>
public static IEnumerable<T> Query<T>(string sql, object param = null, IDbTransaction transaction = null, bool buffered = true, int? commandTimeout = null, CommandType? commandType = null)        {
return Db.Query<T>(sql, param, transaction, buffered, commandTimeout, commandType);
}
public static Task<IEnumerable<T>> QueryAsync<T>(string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
{
return Db.QueryAsync<T>(sql, param, transaction, commandTimeout, commandType);
}
public static int Execute(string sql, object param = null, IDbTransaction transaction = null, int? comman
dTimeout = null, CommandType? commandType = null)
{
return Db.Execute(sql, param, transaction, commandTimeout, commandType);
}
public static Task<int> ExecuteAsync(string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
{
return Db.ExecuteAsync(sql, param, transaction, commandTimeout, commandType);
}
public static T ExecuteScalar<T>(string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
{
return Db.ExecuteScalar<T>(sql, param, transaction, commandTimeout, commandType);
}
public static Task<T> ExecuteScalarAsync<T>(string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
{
return Db.ExecuteScalarAsync<T>(sql, param, transaction, commandTimeout, commandType);
java单例模式双重锁}
/// <summary>
/// 同时查询多张表数据（⾼级查询）
/// "select *from K_City;select *from K_Area";
/// </summary>
/// <param name="sql"></param>
/// <returns></returns>
public static SqlMapper.GridReader QueryMultiple(string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
{
return Db.QueryMultiple(sql, param, transaction, commandTimeout, commandType);
}
public static Task<SqlMapper.GridReader> QueryMultipleAsync(string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)        {
return Db.QueryMultipleAsync(sql, param, transaction, commandTimeout, commandType);
}
}
接下来就可以调⽤了，DbContext.Action()

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