A common platform for concurrent bags, stacks and queues in .NET

We’ve looked at the available concurrent collections in .NET before:

• ConcurrentStack
• ConcurrentBag
• ConcurrentQueue
• ConcurrentDictionary

3 of these objects implement the same interface. Can you guess which three are similar in some sense? Stacks, bags and queues differ from dictionaries in that elements in those collections cannot be retrieved by an index of any sort. You can take/pop/dequeue the elements one by one but you cannot get to element #3 without first removing all elements before that.

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Using a thread-safe dictionary in .NET C# Part 1: introduction

In this post we saw how to use the thread-safe counterpart of the Queue object, i.e. the ConcurrentQueue of T. The standard Dictionary class also has a thread-safe counterpart and it’s called ConcurrentDictionary which resides int the System.Collections.Concurrent namespace.

The ConcurrentDictionary is definitely a dictionary type but it can mimic other collection types if you need a thread-safe collection that doesn’t have a built-in concurrent counterpart such as a List. ConcurrentDictionary is more difficult to use than a standard dictionary so its usage cannot really be summarised in a single short post. Therefore we’ll go through the basics in a mini-series instead.

It implements the IDictionary interface just like Dictionary but some methods are hidden:

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Thread-safe bags in .NET

Bags are very similar to Stacks and Queues. We saw that both stacks and queues order their elements in a well defined way: last-in-first-out and first-in-first-out respectively. Bags on the other hand are unordered collections. There’s no guarantee on how, i.e. in which order the elements will be retrieved.

Unlike stacks and queues, bags have no one-to-one single-threaded implementation in .NET. They are however implemented as thread-safe ConcurrentBag objects in the System.Collections.Concurrent namespace.

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Thread safe queues in .NET

We saw how standard Queues work in .NET in this post. Queues are first-in-first-out collections where the first element added to the collection will the first to be removed. Queues are ideal to model consumer-producer scenarios where a producer adds items to a queue which are then processed by a consumer in the order they came in.

If you’d like to have a Queue that is shared among multiple threads in a multi-threaded application then the “normal” Queue of T object won’t be enough. You can never be sure of the current state of the queue in the very moment when a certain thread tries to dequeue an item from it. The queue may have been modified by another thread just a millisecond before and then the Dequeue method will fail. In general you should be very careful with how you share the resources among different threads.

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Thread-safe stacks in .NET

We saw how Stacks work in .NET in this post. Stacks are last-in-first-out collections where the last element added to the collection will the first to be removed.

If you’d like to have a Stack that is shared among multiple threads in a multi-threaded application then the “normal” Stack of T object won’t be enough. You can never be sure of the current state of the stack in the very moment when a certain thread tries to pop an item from it. The stack may have been modified by another thread just a millisecond before and then the Pop method will fail. In general you should be very careful with how you share the resources among different threads.

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