AsEnumerable（）与“作为IEnumerable” <T> ”

Question

What is the difference between

(db.Records as IEnumerable<Record>).Where(...)

and

(db.Records.AsEnumerable()).Where(...).

Is AsEnumerable() a shortcut for source as IEnumerable<T> only?

I've read https://msdn.microsoft.com/en-us/library/bb335435(v=vs.90).aspx , but did't find any info about it.

Answer 1

The difference is that you can call AsEnumerable on an IEnumerable<T> where T is an anonymous type, and you couldn't perform such a cast on such a sequence, since you can't type out the name of a type that has no name. This ability of AsEnumerable to infer the generic argument is the reason for it to exist.

(It's also arguably cleaner syntax, but outside of anonymous types that's not a functional difference, merely a personal preference.)

Answer 2

There are two relevant sections on that article.

First,

The AsEnumerable(IEnumerable) method has no effect other than to change the compile-time type of source from a type that implements IEnumerable to IEnumerable itself.

And second,

If remote execution is not desired, for example because the predicate invokes a local method, the AsEnumerable method can be used to hide the custom methods and instead make the standard query operators available.

In other words, yes, it looks like a shortcut. However, it is meant to enforce in memory operations of Where and other IEnumerable<T> methods versus the actual implementations of the underlying type.