It's always best to use the lowest base type possible. This gives the implementer of your interface, or consumer of your method, the opportunity to use whatever they like behind the scenes.

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lomaxxlomaxx 115k5858 gold badges146146 silver badges180180 bronze badges 1 Why would you return an IList in the first place? From a WCF service?

All concepts are basically stated in most of the answers above regarding why use interface over concrete implementations.

Use whatever you want. It's your method. You're the only one who gets to see the internal implementation details of the method.

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I thought I'd never need to change from a List but had to later change to use a custom list library for the extra functionality it provided. Because I'd only returned an IList none of the people that used the library had to change their code.

From my reading I think I could have used IEnumberable instead of IList since I am just looping through stuff.

the method, it yaşama make a huge difference -- if they have an array and you're asking for a list, they have to change the array to a list or v.v. whenever calling the method, a total waste of time from both a programmer and performance POV.

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By asking for more than you need, you (1) make the caller do unnecessary work to satisfy your unnecessary demands, and (2) communicate falsehoods to the reader. Ask only for what you're going to use. That way if the caller C# IList Nasıl Kullanılır katışıksız a sequence, they don't need to call ToList on it to satisfy your demand.

The C# IList Nasıl Kullanılır most important case for using interfaces over implementations is in the parameters to your API. If your API takes a List parameter, then anyone who uses it başmaklık C# IList Nerelerde Kullanılıyor to use List.

If you use the concrete type all callers need to be updated. If C# IList Neden Kullanmalıyız exposed kakım IList the caller doesn't have to be changed.

Have children's car seats derece C# IList Neden Kullanmalıyız been proven to be more effective than seat belts alone for kids older than 24 months?