C#•2y ago

❔ Will I get a performance hit if I use the Enum type as a key value for a Dictionary?

See Code below:

public enum Tee1
{
    None
}

public enum Tee2
{
    None
}

class Program
{
    static Dictionary<Enum, string> h = new Dictionary<Enum, string>();
    
    static void Main(string[] args)
    {
        h.Add(Tee1.None, "Tee1 string");
        h.Add(Tee2.None, "Tee2 string");

        System.Console.WriteLine(h[Tee1.None]); // Output : Tee1 string
        System.Console.WriteLine(h[Tee2.None]); // Output : Tee2 string
    }
}

public enum Tee1
{
    None
}

public enum Tee2
{
    None
}

class Program
{
    static Dictionary<Enum, string> h = new Dictionary<Enum, string>();
    
    static void Main(string[] args)
    {
        h.Add(Tee1.None, "Tee1 string");
        h.Add(Tee2.None, "Tee2 string");

        System.Console.WriteLine(h[Tee1.None]); // Output : Tee1 string
        System.Console.WriteLine(h[Tee2.None]); // Output : Tee2 string
    }
}

Does this also create a hashmap? Or does the compiler do something unexpected here which costs performance?

22 Replies

phaseshift•2y ago

What?

the_sea_jayOP•2y ago

Since Tee1.None and Tee2.None are both underlying int with the value 0, how does the compiler know the difference between Tee1.None and Tee2.None? I could also do 2 Dictionary with Tee1 as a key and in the other Tee2, my questing was just if I do it the way with Enum type as key , do i Get a performence hit

phaseshift•2y ago

Presume it's down to an override of Equals In that case it's the same as any other dictionary Not really following your question. If you have two items with the same hash, they will go in the same bucket But this whole question smacks of premature optimisation

Anton•2y ago

It depends if Enum overrides GetHashCode to just return the integer value. If it does, then yes, they'll end up in the same bucket. You could make a wrapper struct for the key type with that Enum field that does the default behavior of GetHashCode, so (int) _enum xor _enum.GetType().GetHashCode() if you happen to need that. (can't type caret on phone lol)

Aaron•2y ago

yes, using Enum means that you have to box the values (aka put them on the heap) when it's boxed, it is boxed with the information that the type of the value is Tee1 that is how it knows the difference

Anton•2y ago

in theory they shouldn't box if they are used as constants

Aaron•2y ago

what

Anton•2y ago

well

Aaron•2y ago

no (Enum)Tee1.None is a boxing conversion

Anton•2y ago

at least they should reuse the same reference

Aaron•2y ago

I don't think they do

MODiX•2y ago

Windows10CE#8553

REPL Result: Success

enum A { None }
object.ReferenceEquals((Enum)A.None, (Enum)A.None)

enum A { None }
object.ReferenceEquals((Enum)A.None, (Enum)A.None)

Result: bool

False

False

Compile: 449.481ms | Execution: 29.987ms | React with ❌ to remove this embed.

Aaron•2y ago

they do not

Anton•2y ago

hm alright

wetweezil•2y ago

Answer.cs

Aaron•2y ago

uh, I'm not sure what this is meant to show

wetweezil•2y ago

You should be able to click "Expand" otherwise, you will only see the comment at the top

Aaron•2y ago

I mean yes you can make a dictionary of the types but that doesn't help you when it comes to values

MODiX•2y ago

Windows10CE#8553

REPL Result: Success

enum E { A, B }
Dictionary<Type, string> dict = new();
dict[E.A.GetType()] = "test";
dict[E.B.GetType()]

enum E { A, B }
Dictionary<Type, string> dict = new();
dict[E.A.GetType()] = "test";
dict[E.B.GetType()]

Result: string

test

test

Compile: 500.192ms | Execution: 34.903ms | React with ❌ to remove this embed.

Aaron•2y ago

see how both E.A and E.B get the same value because they both have the type E

canton7•2y ago

It does. https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Enum.cs,f527a799d76cc18a

Accord•2y 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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❔ Will I get a performance hit if I use the Enum type as a key value for a Dictionary?