Using GetHashCode for getting Enum int value

问题:

I have an enum

public enum INFLOW_SEARCH_ON
{
  ON_ENTITY_HANDLE = 0,         
  ON_LABEL = 1,                 
  ON_NODE_HANDLE = 2            
} // enum INFLOW_SEARCH_ON

I have to use this enum for seraching in a grid columns

To get the column index I am using

  MyEnumVariable.GetHashCode() 

Which works ok, or should I use

  (short)MyEnumVariable

I am bit confused over using GetHashCode(). Is there any problem using that.

回答1:

Using GetHashCode() is incorrect. You should cast to int. Using it the way you do is asking for raptors(or Raymond) to come and eat you.

That GetHashCode() happens to return the integer value of the enum is an implementation detail and may change in future versions of .net.

GetHashCode() guarantees that if two values are equal their hash codes are equal too. The other way round is not guaranteed.

My rule of thumb is that if GetHashCode were to return a constant value your program should still work correctly (but potentially be much slower) since a constant GetHashCode trivially fulfills the contract, but has bad distribution properties.

回答2:

You should use (int)MyEnumVariable for getting the literal value... you can also convert the other way like (INFLOW_SEARCH_ON)int

回答3:

You should use (int) MyEnumVariable. All enums by default are inherited from int.

回答4:

When I did profiling, Enum.GetHashCode took a lot of cycles. I was able to improve it by using (int)Enum where possible.

回答5:

Others have said why casting to int is better.

Another reason to not use GetHashCode is performance. It causes boxing. In my quick tests, GetHashCode was about 50 times slower (if it matters).

回答6:

I see some comments that describe the disadvantages of using GetHashCode() for many reasons.

There are some cases where (int)INFLOW_SEARCH_ON.ON_LABEL doesn't work.

A good way to get the int value from Enum is using GeTTypeCode() method. This method works regardlesss of the underlying type. In this case it is int.

public enum INFLOW_SEARCH_ON
{
 ON_ENTITY_HANDLE = 0,         
 ON_LABEL = 1,                 
 ON_NODE_HANDLE = 2            
} // enum INFLOW_SEARCH_ON

INFLOW_SEARCH_ON selectedType = INFLOW_SEARCH_ON.ON_LABEL;

object val = Convert.ChangeType(selectedType, selectedType.GetTypeCode());
Console.WriteLine(val);

Output in this case is '1'.

Reference: Get int value from enum

回答7:

You also have the option methods provided by the Enum static class. For instance:

Enum.TryParse<TEnum>(string ValueType, out TEnum Result)

If all you are trying to do is get the value of the Enum. You can get the string ValueType, e.g. "ON_ENTITY_HANDLE" with Enum.GetName(...).

Otherwise, casting to (int) would definitely be preferred over GetHashCode().