CODEWITHSUNDEEP
c#.netconcurrencyconcurrentdictionary
Martin Ingvar Kofoed Jensen
Martin Ingvar Kofoed Jensen

Reputation: 4311

When will ConcurrentDictionary TryRemove return false

Will it only return false if the dictionary does not contain a value for the given key or will it also return false due to thread race conditions, like another thread adds/updates something?

Question in code:

ConcurrentDictionary<int, string> cd = new ConcurrentDictionary<int, string>();

// This might fail if another thread is adding with key value of 1.
cd.TryAdd(1, "one"); 

// Will this ever fail if no other thread ever removes with the key value of 1?
cd.TryRemove(1);

Edit: I think that it only will return false if it does not contain a value for the given key, but want to be absolutely sure.

Upvotes: 91

Views: 24083

Answers (3)

Drew Noakes
Drew Noakes

Reputation: 310977

One other point to make:

// This might fail if another thread is adding with key value of 1.
cd.TryAdd(1, "one");

This comment is incorrect and possibly suffers from the same misconception about what it means to 'try'. It's not about a concurrent attempt to add, it's whether a value has already been added with key 1.

Consider a standard Dictionary<TKey,TValue>. The equivalent code would be:

if (!d.Contains(1))
    d.Add(1, "one");

This requires two operations. There's no way to design such an API to be threadsafe, as cd might have a value with key 1 added between the call to Contains and Add, which would then result in Add throwing.

The concurrent collections have APIs that logically bundle these test-and-do pairs into single atomic operations, behind a single API.

Upvotes: 4

Dan Tao
Dan Tao

Reputation: 128307

While Mitch is right that a ConcurrentDictionary is not vulnerable to race conditions, I think the answer to the question you are asking is that yes, if the key is present, TryRemove will work and will return true.

In the code you posted, there's no way that TryRemove would return false since cd is a local variable not accessed anywhere else. But if some code elsewhere were given a reference to this ConcurrentDictionary object and were removing keys on a separate thread, then it's possible that TryRemove could return false, even here -- but only because the key was already removed, not because some other action is being performed on the dictionary and the key is somehow "stuck" there.

Upvotes: 102

Mitch Wheat
Mitch Wheat

Reputation: 300559

The ConcurrentDictionary does not suffer from race conditions. That's why you use it.

Return Value

true if an object was removed successfully; otherwise, false.

Upvotes: 5

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