Why GetHashCode is not a property like HashCode in .NET?
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6 Answers
Probably because it requires computation, and exposing it as a propery might imply that the hashcode is already available for free.
Edit: Guidelines on this: Properties versus Methods
"The operation is expensive enough that you want to communicate to the user that they should consider caching the result."
Perhaps GetHashCode is expensive enough in some cases.
1 Comment
Joan Venge
Thanks, is there a guideline for not having properties that requires computation? The reason I ask is, I have seen lots that computes when you access the property.
I don't think there's any good reason. Any implemention of GetHashCode should be fast enought to put into a property. That said, there are plenty of design flaws in the .Net framework, some small, some serious. This seems like a small one.
Comments
Often it is not possible to define a HashCode for a class that makes since:
e.g. the objects of the class don’t have a well defined concept of identity.
Therefore it is common to make the GetHashCode() method throw a NotImplementedException. This would course all sort of problem if HashCode was a property, as most people (and debuggers) assume it is always valid to get the value of a property
3 Comments
supercat
Nearly all class objects should generally either have reference semantics or immutable value semantics. Objects with reference semantics have a clear concept of identity (object X and object Y are equal if and only if X and Y refer to the same object). Objects with immutable value semantics have a clear concept of identity (object X and object Y are equal if they hold equal values). What types of object wouldn't have a well-defined concept of identity?
Ian Ringrose
@supercat, Equals should only return true for object with the same HashCode, however often I see an equals method defined just for the use of unit tests on obejct that are not immutable. So to stop the warning from the compiler, GetHashCode() must be overridden, however know the object should never be used as a key. Without a usefull equals method on objects, the assets on collections in nUnit don't work.
supercat
That would be a definite abuse of Object.Equals(). Actually, I wish the method had been named Equivalent(), rather than Equals(), since it is the concept of equivalence, rather than the name, which is universally applicable, and in some cases it might be useful for some objects to have an Equals() method with looser semantics. For example, if I had my druthers, Equivalent(1.00m, 1.0m) would be false, but Equals(1.00m, 1.0m) would be true. Equals would be defined by spec as a transitory property, unlike Equivalent which would be (whenever implemented properly) immutable.
Besides that a property is nothing else than a getter and a setter method, from a design perspective a property should never contain any computations other than initializing or validation, eg:
private object _obj;
public object Obj
{
get
{
if(_obj == null)
{
_obj = new object();
}
return _obj;
}
set
{
if(value == badvalue)
{
throw new ArgumentException("value");
}
_obj = value;
}
}
GetHashCode() does not contain extensive computations, but it could contain such long running operations (just from the fact that it could compute the hashcode of an object in a complex manner), this is why its a method instead of a property.
Comments
properties should only be used if the computation behind them is really fast or cached
besides most of the time the only logic in properties should be validation
Comments
You have to remember that the .NET Framework is designed to be accessed by a wide variety of languages.
In theory you could create a compiler that is incapable of correctly overriding properties. While that would make for a pretty crappy compiler, it would not necessarily be illegal. (Remember properties are just methods with some meta data)
