C#•3y ago

❔ Entry API for dictionaries?

Rust has this nifty API called

Entry

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#?

mtreit•9/18/23, 6:02 PM

I don't think so.

Mmtreit I don't think so.

__dil__OP•9/18/23, 6:04 PM

What would the equivalent idiomatic C# code look like?

mtreit•9/18/23, 6:07 PM

If I understand the code correctly, something like this?

mtreit•9/18/23, 6:08 PM

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

Pobiega•9/18/23, 6:11 PM

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

Sossenbinder•9/18/23, 6:18 PM

I think there's only a native AddOrUpdate for ConcurrentDictionaries

Sossenbinder•9/18/23, 6:19 PM

But not for regular Dictionaries IIRC

Sossenbinder•9/18/23, 6:20 PM

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

SSossenbinder I think there's only a native AddOrUpdate for ConcurrentDictionaries

mtreit•9/18/23, 6:57 PM

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

Mmtreit Just out of curiosity I tried both: As usual, the penalty for using the thread-s...

Sossenbinder•9/18/23, 6:58 PM

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

SSossenbinder I wonder why there is no out of the box AddOrUpdate for the regular Dictionary, ...

mtreit•9/18/23, 7:02 PM

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•9/18/23, 7:04 PM

This issue talks about DictionarySlim.

mtreit•9/18/23, 7:04 PM

I've never heard of that.

Sossenbinder•9/18/23, 7:04 PM

Me neither

mtreit•9/18/23, 7:05 PM

Doesn't seem like anything that ever actually shipped

Sossenbinder•9/18/23, 7:07 PM

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

viktor11k•9/18/23, 8:10 PM

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•9/18/23, 8:21 PM

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

reflectronic•9/18/23, 9:45 PM

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•9/18/23, 9:46 PM

which you can use to efficiently implement an method yourself

reflectronic•9/18/23, 9:46 PM

it is "unsafe" so you do need to be mindful when using it

reflectronic•9/18/23, 9:51 PM

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•9/19/23, 9:16 AM

Ohh right I remember that CollectionsMarshal did receive that, indeed

Sossenbinder•9/19/23, 9:16 AM

Thanks for reminding me too, I completely forgot

Accord•9/20/23, 9:19 AM

Was this issue resolved? If so, run

/close

/close

otherwise I will mark this as stale and this post will be archived until there is new activity.