Why would I want to put my objects in a bag?

I just saw an SO question about the System.Collections.ConcurrentBag<T> class, and I've seen the ViewBag property of the Controller in ASP.NET MVC. In my experience, I've learned that it's easier to use people's code if you understa开发者_JAVA技巧nd what exactly they were getting at in writing it. I think its pretty intuitive as to what a List<T> or a Dictionary<TKey,TValue> or a ReadOnlyCollection<T> are meant to represent. A Bag on the other hand is not so intuitive.

So, my question is: What is this Bag metaphor meant to represent, specifically with respect to the .NET framework?

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ConcurrentBag<T> is a thread-safe, unordered sequence of items which can include duplicates.

So compared with some other collections:

• It's unordered, like a HashSet<T> but unlike a List<T>
• It can contain duplicates, like a List<T> but unlike a HashSet<T>
• It's just values rather than a map (unlike Dictionary<TKey, TValue>)
• It's thread-safe, unlike all the non-concurrent collections - so you can share an instance between multiple threads, all reading and writing.

Possible uses include a work queue where you don't care about the ordering (as otherwise you'd use ConcurrentQueue / ConcurrentStack) or a list of items where you'll always apply a sort order after fetching the data into another "local" collection.

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From the first line of the MSDN documentation:

Represents a thread-safe, unordered collection of objects.

I don't think that's any less intuitive than "List" or "ReadOnlyCollection", really - YMMV.

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ConcurrentBag is the same as ConcurrentStack and ConcurrentQueue except that it doesn't maintain ordering for performance reasons, this article explain it in more details http://www.emadomara.com/2011/08/what-is-concurrentbag.html