HashCode as enum values

Question

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Using GetHashCode for getting Enum int value

Is it safe to use GetHasCode to get a enumeration value like the code below:

myList.SelectedIndex = myEnumValue.GetHashCode()

Safe in what sense? There *can't* be a guarantee that the values will be unique... after all, you could have an enum with an underlying type of `long`, with more than 2^32 values. — Jon Skeet, Jun 15 '11 at 20:46
Looks neither safe nor sensible. Consider `(int) myEnumValue` — H H, Jun 15 '11 at 20:46
Unless I misunderstand you: No, it isn't safe. The hashcode should provide a deterministic but unpredictable value in the entire range of int values, so you code seems very strange. What are you trying to achieve? — faester, Jun 15 '11 at 20:47
Should have notices this is a duplicate considering that I posted in that other thread -_- — CodesInChaos, Jun 15 '11 at 20:59

score 1 · Accepted Answer · answered Jun 15 '11 at 20:47

No. The result isn't guaranteed to be unique or equivalent to the integral value of that enum value.

I think it is currently implemented to be the same as a cast to int. But that's an implementation detail and may change at any time.

The correct way is to cast to the underlying integral type:

myList.SelectedIndex = (int)myEnumValue;

score 1 · Answer 2 · answered Jun 15 '11 at 20:49

In general, no. GetHashCode returns the underlying value for enums, most likely because enums are just integers under the covers. However, this is an implementation detail that might change in a later version of the framework.

Apart from that, remember that enums doesn't need to start from 0, and doesn't need to increment by exactly one for each value - in other words, there are a multitude of ways you could get into trouble with that code.

The correct way of getting the underlying value of an enum value is to cast it:

int underlyingValue = (int)enumValue;

Just remember that it still isn't ok to use that value for your SelectedIndex, unless you specifically know that the enum being used starts at 0 and increments by one.

HashCode as enum values

2 Answers2