C#•4y 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
    }
}

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?

phaseshift•12/16/22, 2:26 PM

What?

the_sea_jayOP•12/16/22, 2:28 PM

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?

the_sea_jayOP•12/16/22, 2:29 PM

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•12/16/22, 2:29 PM

Presume it's down to an override of Equals

phaseshift•12/16/22, 2:29 PM

In that case it's the same as any other dictionary

phaseshift•12/16/22, 2:31 PM

Not really following your question. If you have two items with the same hash, they will go in the same bucket

phaseshift•12/16/22, 2:32 PM

But this whole question smacks of premature optimisation

Anton•12/16/22, 3:36 PM

It depends if

Enum

Enum

overrides

GetHashCode

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

Enum

field that does the default behavior of

GetHashCode

GetHashCode

, so

(int) _enum xor _enum.GetType().GetHashCode()

(int) _enum xor _enum.GetType().GetHashCode()

if you happen to need that.

Anton•12/16/22, 3:38 PM

(can't type caret on phone lol)

Tthe_sea_jay I could also do 2 Dictionary with Tee1 as a key and in the other Tee2, my questi...

Aaron•12/16/22, 9:08 PM

yes, using Enum means that you have to box the values (aka put them on the heap)

Aaron•12/16/22, 9:09 PM

when it's boxed, it is boxed with the information that the type of the value is Tee1

Aaron•12/16/22, 9:09 PM

that is how it knows the difference

AAaron yes, using Enum means that you have to box the values (aka put them on the heap)

Anton•12/16/22, 9:15 PM

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

Aaron•12/16/22, 9:16 PM

what

Anton•12/16/22, 9:16 PM

well

Aaron•12/16/22, 9:16 PM

(Enum)Tee1.None

(Enum)Tee1.None

is a boxing conversion

Anton•12/16/22, 9:16 PM

at least they should reuse the same reference

Aaron•12/16/22, 9:16 PM

I don't think they do

MODiX•12/16/22, 9:17 PM

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•12/16/22, 9:17 PM

they do not

Anton•12/16/22, 9:17 PM

hm alright

wetweezil•12/17/22, 6:57 AM

Answer.cs1.99KB

Aaron•12/17/22, 6:59 AM

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

wetweezil•12/17/22, 7:03 AM

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

Aaron•12/17/22, 7:06 AM

I mean yes you can make a dictionary of the types

Aaron•12/17/22, 7:06 AM

but that doesn't help you when it comes to values

MODiX•12/17/22, 7:07 AM

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•12/17/22, 7:07 AM

see how both E.A and E.B get the same value

Aaron•12/17/22, 7:07 AM

because they both have the type

AAnton It depends if `Enum` overrides `GetHashCode` to just return the integer value. I...

canton7•12/17/22, 12:12 PM

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

Accord•12/18/22, 12:27 PM

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.

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

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