关于 IQueryable.GetEnumerator 的一些令人困惑的事情
public class Query<T> : IQueryable<T> ...
{
...
public IEnumerator<T> GetEnumerator()
{
return((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
}
}
Query<string> someQuery = new Query<string>();
var results1 = someQuery.Select(...).Where(...);
string[] initialSet = { };
var results2 = initialSet.Select(...).Where(...);
在
initialSet上操作时，Linq-to-object 的Where返回WhereEnumerableIterator，因此results2< /code> 的类型为WhereEnumerableIterator。但是，当对someQuery进行操作时，Where运算符是否将通过调用someQuery.GetEnumerator检索到的实例分配给results1> 还是它还返回一些自定义类？如果是后者，
Where和Select运算符何时调用someQuery.GetEnumerator？
public class Query<T> : IQueryable<T> ...
{
...
public IEnumerator<T> GetEnumerator()
{
return((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
}
}
Query<string> someQuery = new Query<string>();
var results1 = someQuery.Select(...).Where(...);
string[] initialSet = { };
var results2 = initialSet.Select(...).Where(...);
When operating on
initialSet, Linq-to-object'sWhere<T>returnsWhereEnumerableIterator<T>and thusresults2is of typeWhereEnumerableIterator<T>. But when operating onsomeQuery, doesWhere<T>operator assign toresults1an instance retrieved by callingsomeQuery.GetEnumeratoror does it also return some custom class?If the latter, when exactly is
someQuery.GetEnumeratorcalled byWhereandSelectoperators?
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results2的类型只是Enumerable-results2值实际的实现类型执行时所指的恰好是WhereEnumerableIterator，仅此而已。在
someQuery上操作时，这取决于您用它做什么 -results1变量的类型是IQueryable，因此您可以对其使用更多Queryable调用。someQuery.GetEnumerator()可能永远被调用 - 由查询提供程序实现来确定如何准确地表示查询；它不需要像 LINQ to Objects 通常那样调用GetEnumerator。至于
Queryable.Where返回的对象类型 - 同样，这取决于查询提供程序的实现 - 区别在于，而知识被烘焙到Enumerable.Where中，并且无法替换，Queryable.Where会将调用链接到查询提供程序。The type of
results2is justEnumerable<T>- the type of the implementation that the value ofresults2actually refers to at execution time happens to beWhereEnumerableIterator<T>, that's all.When operating on
someQuery, it depends what you do with it - the type of theresults1variable isIQueryable<T>, so you can use moreQueryablecalls on it.someQuery.GetEnumerator()may never be called - it's up to the query provider implementation to work out exactly how to represent the query; it doesn't need to callGetEnumeratorall the way up the chain like LINQ to Objects typically does.As for the type of object returned by
Queryable.Where- again, that's up to the query provider implementation - the difference is that whereas the knowledge is baked intoEnumerable.Whereand can't be replaced,Queryable.Wherewill chain the call through to the query provider.当查询被枚举时。因此得名。
这看起来是错误的。您可以使用“筛选位置”和“选择”来投影结果。你似乎把它搞反了。
When the query is enumerated. Hence comes the name.
This looks wrong. You use the Where to filter, and the Select to project the result. You appear to have it backwards.