使用IDbConnection.Query()返回IEnumerable的AsList()是否优于ToList()? [英] Is AsList() better than ToList() with IDbConnection.Query() which returns IEnumerable?

问题描述

我从Marc Gravell(@MarcGravell)阅读了以下答案: https://stackoverflow.com/a/47790712/5779732

I read this answer from Marc Gravell (@MarcGravell): https://stackoverflow.com/a/47790712/5779732

最后一行说:

作为代码的次要优化:最好使用AsList()而不是ToList()以避免创建副本.

As a minor optimization to your code: prefer AsList() to ToList() to avoid creating a copy.

该语句与QueryMultiple()有关，该语句返回GridReader.

That statement is about QueryMultiple() which returns GridReader.

据我了解，System.Linq提供了扩展方法IEnumerable.ToList().以下是来自 Microsoft 关于ToList().

In my understanding, System.Linq provides an extension method IEnumerable.ToList(). Following is from Microsoft about ToList().

ToList(IEnumerable)方法强制立即进行查询评估，并返回包含查询结果的List.您可以将此方法附加到查询中，以获得查询结果的缓存副本.

The ToList(IEnumerable) method forces immediate query evaluation and returns a List that contains the query results. You can append this method to your query in order to obtain a cached copy of the query results.

IDbConnection.Query()将始终返回IEnumerable或null.可以在调用代码中轻松进行空检查.那么AsList有什么不同?

IDbConnection.Query() will ALWAYS return IEnumerable or null. Null-check could be easily done in calling code. What difference does AsList makes then?

如果我的理解正确，那么AsList始终会内部调用ToList来创建副本.

If my understanding is correct, AsList will always internally call ToList which will create a copy.

考虑到这一点，AsList()是否优于返回IEnumerable的IDbConnection.Query()的ToList()?如是;为什么?

Considering this, is AsList() better than ToList() with IDbConnection.Query() which returns IEnumerable? If yes; why?

AsList()在内部执行什么操作使其在这种情况下成为更好的选择?

What is that AsList() does internally that makes it a better choice in this case?

推荐答案

AsList是自定义的Dapper扩展方法.它所做的只是检查您传递给它的IEnumerable<T>是否真的是List<T>.如果是-则将其返回，仅强制转换为List<T>.如果不是，则调用常规ToList.关键是-ToList()始终创建一个副本，即使传递给它的已经是列表. AsList()方法避免执行此复制，因此在不需要此复制时非常有用.

AsList is a custom Dapper extension method. All it does is checks if IEnumerable<T> you pass to it is really List<T>. If it is - it returns it back, just casts to List<T>. If it's not - it calls regular ToList. The point is - ToList() always creates a copy, even if what you pass to it is already a list. AsList() method avoids doing this copy, and so is useful if such copy is unnecessary.

在这种特定情况下，您具有以下代码:

In this specific scenario, you have the following code:

multipleresult.Read<MerchantProduct>()

，其中multipleresult是GridReader. Read具有buffered参数，默认情况下为true.当它为true时，Read会真正返回List<T>，因此通过调用ToList，您将在没有太多理由的情况下再次复制该列表.

where multipleresult is GridReader. Read has buffered argument which is true by default. When its true - Read will really return List<T>, so by calling ToList you will copy that list again without much reason.

对于IDbConnection.Query()来说也是如此-也具有buffered参数，默认情况下为true，因此默认情况下也会返回List<T>.

The same is true for IDbConnection.Query() - is also has buffered parameter, which is true by default, so it will also by default return List<T>.

如果您更喜欢使用ToList()-可以将buffered: false传递给Query()或Read()以避免创建该额外副本.

If you prefer to use ToList() - you can pass buffered: false to Query() or Read() to avoid creating that additional copy.

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