Implementing ICollection.CopyTo in C#: deep or shallow copy?

Question

I'm writing a custom class which implements IDictionary, and I'm not sure what to do about CopyTo. Should each element just be copied to the target array (shallow copy), or should I make a copy/clone of each element then place it in the target array (deep copy)?

Which `CopyTo` are you referring to? `IEnumerable` has no `CopyTo` method. — John Saunders, Jun 09 '15 at 02:29
@JohnSaunders `ICollection` does though, and since OP is implementing an `IDictionary`, I suspect they need to implement `ICollection.CopyTo`. — Asad Saeeduddin, Jun 09 '15 at 02:30
I suspect the same, but we should demand precision, since computers do precisely what we tell them to do. — John Saunders, Jun 09 '15 at 02:31
See https://msdn.microsoft.com/en-us/library/system.collections.dictionarybase.copyto.aspx. Better still, see https://msdn.microsoft.com/en-us/library/cc645053.aspx — John Saunders, Jun 09 '15 at 02:31
Also, are you really implementing `IDictionary` or the generic `IDictionary`? — Asad Saeeduddin, Jun 09 '15 at 02:33
@Asad Neither, actually: I'm implementing `IDictionary, MyClass>`. I figured the question applied regardless, though, so I left that detail out. — Eric Dand, Jun 09 '15 at 02:35
Always shallow for `IDictionary`. Since the elements are of type `object` you have no way of knowing how to do a deep copy. — Enigmativity, Jun 09 '15 at 02:36

Bas · Accepted Answer · 2015-06-09T02:41:57.433

The implementation is not defined by the contract, however all the generic collections appear to do a shallow copy (see http://referencesource.microsoft.com/#q=List.CopyTo). It makes sense, as there is no defined way to deep copy any object, which would be part of the CopyTo implementation.

It depends on your scenario, if you do not intend to use this method at all, consider not implementing it and throwing a NotSupportedException. If others will use your collection, implement a shallow copy to align with the standard .NET library. If you need a deep copy method, add it as a separate method, (potentially by implementing ICloneable).

Implementing ICollection.CopyTo in C#: deep or shallow copy?

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