Should I always return IEnumerable<T> instead of IList<T>?

Question

When I'm writing my DAL or other code that returns a set of items, should I always make my return statement:

public IEnumerable<FooBar> GetRecentItems()

or

public IList<FooBar> GetRecentItems()

Currently, in my code I have been trying to use IEnumerable as much as possible but I'm not sure if this is best practice? It seemed right because I was returning the most generic datatype while still being descriptive of what it does, but perhaps this isn't correct to do.

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Answer 1

It really depends on why you are using that specific interface.

For example, IList<T> has several methods that aren't present in IEnumerable<T>:

IndexOf(T item)
Insert(int index, T item)
RemoveAt(int index)

and Properties:

T this[int index] { get; set; }

If you need these methods in any way, then by all means return IList<T>.

~~Also, if the method that consumes your IEnumerable<T> result is expecting an IList<T>, it will save the CLR from considering any conversions required, thus optimizing the compiled code.~~

Should I always return IEnumerable<T> instead of IList<T>?

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