C#•15mo ago

❔ Entry API for dictionaries?

Rust has this nifty API called Entry which allows one to write code such as:

map.entry(c)
   .and_modify(|count_of_c| { *count_of_c += 1 })
   .or_insert(1);

map.entry(c)
   .and_modify(|count_of_c| { *count_of_c += 1 })
   .or_insert(1);

So, you can modify a value in a hashmap (dictionary) if the key exists, or insert a new entry if the key does not exist. Is there something similar in C#?

19 Replies

mtreit•15mo ago

I don't think so.

__dil__OP•15mo ago

What would the equivalent idiomatic C# code look like?

mtreit•15mo ago

If I understand the code correctly, something like this?

if (!dict.TryGetValue(key, out var val))
{
    dict.Add(key, 0);
}

dict[key] += 1;

if (!dict.TryGetValue(key, out var val))
{
    dict.Add(key, 0);
}

dict[key] += 1;

Actually since the value isn't used, something like this:

if (!dict.ContainsKey(key))
{
    dict.Add(key, 0);
}

dict[key] += 1;

if (!dict.ContainsKey(key))
{
    dict.Add(key, 0);
}

dict[key] += 1;

Pobiega•15mo ago

I very often end up with extension methods like IncrementOrInsert for numeric dictionaries

Sossenbinder•15mo ago

I think there's only a native AddOrUpdate for ConcurrentDictionaries But not for regular Dictionaries IIRC The closest thing I have in mind would be a TryAdd to have contains & add in one statement, but it does not allow modification of an existing entry

mtreit•15mo ago

Just out of curiosity I tried both: As usual, the penalty for using the thread-safe version is pretty high.

Sossenbinder•15mo ago

I wonder why there is no out of the box AddOrUpdate for the regular Dictionary, it's a welcome QoL addition and I think everyone would be aware that it is not an atomic threadsafe operation catthinking

mtreit•15mo ago

https://github.com/dotnet/runtime/issues/26848

GitHub

API Proposal: Add AddOrUpdate to Dictionary · Issue #26848 · dotnet...

Add the AddOrUpdate method that is on ConcurrentDictionary to Dictionary. This would improve performance in use cases such as counting keys. K-Nucleotide in the benchmarksgame does this and has an ...

mtreit•15mo ago

This issue talks about DictionarySlim. I've never heard of that.

Sossenbinder•15mo ago

Me neither

mtreit•15mo ago

Doesn't seem like anything that ever actually shipped

Sossenbinder•15mo ago

Can't find anything about that on source dot net as well

viktor11k•15mo ago

https://github.com/dotnet/corefxlab/blob/archive/src/Microsoft.Experimental.Collections/Microsoft/Collections/Extensions/DictionarySlim.cs Interesting

GitHub

corefxlab/src/Microsoft.Experimental.Collections/Microsoft/Collecti...

This repo is for experimentation and exploring new ideas that may or may not make it into the main corefx repo. - dotnet/corefxlab

mtreit•15mo ago

Since it's archived, feels like it didn't pan out.

reflectronic•15mo ago

there is https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.collectionsmarshal.getvaluereforadddefault?view=net-7.0

CollectionsMarshal.GetValueRefOrAddDefault Method (System.Runtime.I...

Gets a reference to a TValue in the specified dictionary, adding a new entry with a default value if the key does not exist.

reflectronic•15mo ago

which you can use to efficiently implement an AddOrUpdate method yourself it is "unsafe" so you do need to be mindful when using it

reflectronic•15mo ago

there are some related issues https://github.com/dotnet/runtime/issues/15059 and there seems to be little appetite to introduce convenience methods for these cases

GitHub

Propose Adding a GetOrAdd(TKey key, Func valueFactory) to Dictionar...

On ConcurrentDictionary<TKey, TValue> there is the very useful function TValue GetOrAdd(TKey key, Func<TKey, TValie> valueFactory). The pattern that the method represents is one that is...

Sossenbinder•15mo ago

Ohh right I remember that CollectionsMarshal did receive that, indeed Thanks for reminding me too, I completely forgot

Accord•15mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Entry API for dictionaries?