当在表之间连接时，Dapper会出错。

I'm new to dapper i get error when i retrieve data from joined table

当我从已连接的表中检索数据时，我对dapper有了新的错误。

var qry = @"SELECT Cities.Id,
                           Cities.Name,
                           Cities.Sort,
                           Countries.Name
                           FROM[dbo].[Cities]
                            JOIN Countries ON Countries.Id = Cities.CountryId";

var result = con.Query<Cities, Countries>(qry);

this is the error message

这是错误消息。

'SqlConnection' does not contain a definition for 'Query' and no extension method 'Query' accepting a first argument of type 'SqlConnection' could be found (are you missing a using directive or an assembly reference?)

“SqlConnection”不包含“查询”的定义，也不包含接受“SqlConnection”类型的第一个参数的扩展方法“查询”(您是否缺少使用指令或程序集引用?)

Error Image

错误图片

1 个解决方案

#1

The error is caused by invalid invocation - there is no overload that accepts two generic arguments.

错误是由无效调用引起的——没有重载接受两个通用参数。

Event though Dapper supports multimapping, the call actually requires three generic arguments, TFirst, TSecond and TReturn (where TReturn can be one of the two or yet another type).

虽然Dapper支持multimapping，但是调用实际上需要三个通用参数，TFirst、TSecond和TReturn(在其中TReturn可以是两种或另一种类型)。

https://github.com/StackExchange/dapper-dot-net#multi-mapping

https://github.com/StackExchange/dapper-dot-net multi-mapping

An example valid call from the docs

来自文档的一个有效调用示例。

var data = connection.Query<Post, User, Post>(sql, (post, user) => { post.Owner = user; return post;});

An invalid invocation with only two generic arguments yields exactly the error message you get.

只有两个通用参数的无效调用会产生您所得到的错误消息。

My guess is you either wanted the 3rd overload that accepts a single generic argument

我的猜测是，您要么想要第三个重载，它只接受一个一般性的参数。

 Query<TResult>( string sql, Type[] types, Func<object[], TResult> map, ... );

or the 4th one that accepts three

或者第4个接受3个。

 Query<TFirst, TSecond, TResult>( string sql, Func<TFirst, TSecond, TResult> map, ... );

In both cases you still need a map function.

在这两种情况下，您仍然需要一个map函数。

#1