IDictionary How to get the removed item value while removing

Question

I would like to know if it is possible to remove an IDictionary item by its key and in the same time get its actual value that has been removed?

Example

something like:

Dictionary<string,string> myDic = new Dictionary<string,string>();
myDic["key1"] = "value1";

string removed;
if (nameValues.Remove("key1", out removed)) //No overload for this...
{
    Console.WriteLine($"We have just remove {removed}");
}

Output

//We have just remove value1

@JonathonChase I know that, but what does the `out removed` then contain? `null`? `default(T)`? — Ron Beyer, Aug 28 '18 at 17:06
@RonBeyer I've updated my question, my current design is ok with `null`. — Shahar Shokrani, Aug 28 '18 at 17:06
@RonBeyer I'd expect `default(T)` as with the TryXYZ pattern, but you're right it needs to be defined. — Jonathon Chase, Aug 28 '18 at 17:07
Closing the question with duplicate link. The original question has an answer today (.NET Core 2.0 onwards). — nawfal, Jul 26 '20 at 07:39

Marie · Accepted Answer · 2018-08-28T17:31:27.860

Normal dictionaries don't have this functionality as an atomic operation but a ConcurrentDictionary<TKey,TValue> does.

ConcurrentDictionary<string,string> myDic = new ConcurrentDictionary<string,string>();
myDic["key1"] = "value1";

string removed;
if (myDic.TryRemove("key1", out removed))
{
    Console.WriteLine($"We have just remove {removed}");
}

You could write an extension method for a normal dictionary to implement this but if you are concerned about it being atomic a ConcurrentDictionary is probably more correct for your use case.

score 6 · Answer 2 · edited Aug 28 '18 at 17:47

6

You could write an extension method for this:

public static class DictionaryExtensions
{
    public static bool TryRemove<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, out TValue value)
    {
        if (dict.TryGetValue(key, out value))
            return dict.Remove(key);
        else
            return false;
    }
}

This will attempt to get the value and if it exists, will remove it. Otherwise you should use a ConcurrentDictionary as the other answer said.

edited Aug 28 '18 at 17:47

Marie

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answered Aug 28 '18 at 17:13

Ron Beyer

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3

`value = default(TValue);` – Rufus L Aug 28 '18 at 17:17
This is definitely a personal preference but it might be a little easier to follow if the default assignment happened before the return false since that is the only time it is ever used. – Marie Aug 28 '18 at 17:19
2

@RufusL 7.3 can infer it, it's not needed in this case. – Ron Beyer Aug 28 '18 at 17:27
@RufusL I was actually going to ask about that too. I am surprised i missed that feature, thank you for the heads up! – Marie Aug 28 '18 at 17:33
@RonBeyer Good to know, I guess I'm still a little old school.. :) – Rufus L Aug 28 '18 at 17:34
@RufusL You can set a value in an if and an else because both possible branches are covered. In this case you dont need it at all because no matter what the value is assigned by the `TryGetValue` – Marie Aug 28 '18 at 17:37
1

Cool, so the whole thing could be reduced to `return dict.TryGetValue(key, out value) && dict.Remove(key);` – Rufus L Aug 28 '18 at 18:04

IDictionary How to get the removed item value while removing

Example

Output

2 Answers2