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    <p>More context is really required to answer the question properly:</p> <p>In a <strong>public API</strong>, you should try to use abstract collection types, so that you can change the internal implementation later if you need to.</p> <ul> <li>If the collection should not be changed by the outside world, use <code>IEnumerable&lt;T&gt;</code>.</li> <li>If the collection <em>will</em> be changed by the outside world, use <code>ICollection&lt;T&gt;</code>.</li> <li>If indexed access is required, use <code>IList&lt;T&gt;</code>.</li> </ul> <p>In a <strong>private implementation</strong>, it's not as important to use the abstract types:</p> <ul> <li>If you need indexed access and know the final size, use <code>T[]</code> or <code>List&lt;T&gt;</code>.</li> <li>If you need indexed access and don't know the final size, use <code>List&lt;T&gt;</code>.</li> <li>If you plan to access elements in a LIFO pattern, use <code>Stack&lt;T&gt;</code>.</li> <li>If you plan to access elements in a FIFO pattern, use <code>Queue&lt;T&gt;</code>.</li> <li>If you need to access elements at the beginning and end of the list, but not in the middle, use <code>LinkedList&lt;T&gt;</code>.</li> <li>If you don't want duplicates, use <code>HashSet&lt;T&gt;</code>.</li> </ul> <p>In .NET 4.0 you have a few more choices, but those are the basics.</p>
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    1. POWhich is better to use array or List<>?
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    1. CO"If the collection should not be changed by the outside world use IEnumerable<T>" - I disagree with this characterisation. Returning IEnumerable<T> won't help if the object referenced by the return value is a mutable list - the caller can always cast. You should return IEnumerable<T> if the caller should only be able to enumerate, ICollection<T> if he also needs a Count, and IList<T> if he will need indexed access. In **all** of the above cases, if the collection should not be changed by the outside world, wrap it in a readonly collection (e.g. using List<T>.AsReadOnly).
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    2. CO@Joe: I'm not a fan of that argument. The caller can always try to cast **anything** you give it to *anything* else - it's simply poor design to do so. You can wrap all results in a `ReadOnlyCollection<T>` if you want, but that won't protect you against *hostile* callers; they can just use Reflection to get at the internal mutable list. If this is ultra-secure code then the only way to guarantee this particular invariant is return a *new*, generated `IEnumerable<T>` instance, i.e. by returning `originalEnumerable.Select(x => x).ToArray()`.
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    3. COBut in any event, the recommendations are from a discoverability and code-maintenance perspective (writing self-documenting code), not a security perspective; if you're in the latter camp then you have *many* other things to worry about. It's also the reason why I used the phrase "should not" as opposed to "must not" - it's not a guarantee, just a very strong hint. `ReadOnlyCollection<T>` will give a *runtime* error if a modification is attempted; you want to catch such errors at *compile-time* whenever possible.
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