I am working on a C# project where I make heavy use of interfaces, and the System.Collections.Immutable library. I wish to sort implementations of one of my interfaces in an immutable set, ImmutableSortedSet<IMyInterface>.
In Java this is a straightforward matter of implementing Comparable<IMyInterface> and overriding the equals and hash code functions. I found a similar interface in .net IComparable<IMyInterface> but it warns implementers that if they choose to implement the interface, then they should also override the comparison operators (<,>,<=,>=), as well as implement IEquatable<IMyInterface>. IEquatable<T> warns implementers that they should override the equals and hash code functions, as well as the '==' and '!=' operators.
Now I'm having second thoughts about implementing IComparable<T>, I'm not creating a new primitive type here, I just want to provide a convenient sorting algorithm for a complex reference type. Furthermore, there seems to be a certain problem in C# with overriding operators at the interface level, I am therefore leaning towards using a separate IComparer<IMyInterface> implementation.
What really raised my eyebrows though was hearing this:
The IEquatable<T> interface is used by generic collection objects such as Dictionary<TKey, TValue>, List<T>, and LinkedList<T> when testing for equality in such methods as Contains, IndexOf, LastIndexOf, and Remove. It should be implemented for any object that might be stored in a generic collection.
Does this combined with
If you implement IEquatable<T>, you should also override the base class implementations of Object.Equals(Object) and GetHashCode so that their behavior is consistent with that of the IEquatable<T>.Equals method. If you do override Object.Equals(Object), your overridden implementation is also called in calls to the static Equals(System.Object, System.Object) method on your class. In addition, you should overload the op_Equality and op_Inequality operators. This ensures that all tests for equality return consistent results.
Mean that I am expected to override both '==' and '!=' operators for any type that I want to store inside a generic collection??
IComparable<T> is the preferred mechanism to provide comparison support for sorting. The advice of implementing the comparison operators doesn't make a lot of sense for most types, and they could not be utilized by generic collections anyway. You should also implement IEquatable<T>, override GetHashCode(), and override object.Equals to delegate to IEquatable<T>.Equals.
In general, whenever I implement IComparable<T>, I also implement the non-generic IComparable, but I implement IComparable.CompareTo explicitly such that it is normally hidden.
As you already noticed, if you need sorting, you can pass IComparer<T> implementation and your type doesn't have to implement IComparable<T>. In some cases it's easier and in some cases it's the only way to go, e.g. when you want the same type to be sorted differently in different situations. You can just pass different IComparer<T> implementations in that case.
About IEquatable<T> related quotes. For most situations overriding Equals and GetHashCode is enough. It will make your type work with List<T>.Contains and similar methods. It will also allow you to use your type as key in Dictionary<TKey, TValue> and store it in HashSet<T>. That's because all these cases use EqualityComparer<T>.Default when comparer is not specified.
The way EqualityComparer<T>.Default works can be found here. As you can see it verifies that your type implements IEquatable<T>, but if it doesn't creates an instance of ObjectEqualityComparer<T>, which will just use Equals and GetHashCode methods to verify equality.
Related
Ok so I need a bit of help. I have a generic dictionary "cashdata". The keys of this dictionary are objects (Query objects, a class I have defined). Query objects have "Terms" field, which is a list of strings, and an "Operator" field, which is an enum (Either "All" or "Any").
cashdata.ContainsKey(a_query_object);
And have it yield true or false depending on if a_query_object and an object in the dictionary are identical in terms of their Terms and Operator. What is the best way to do this? A HashCode possibly? I would appreciate an example, thanks in advance.
EDIT: cashdata Dictionary is defined as such
Dictionary<Query,List<string> > cashData = new Dictionary<Query,List<string>>();
Make your object implement IEquatable Interface along with overriding Object.Equals and GetHashCode as mentioned in the remarks section in MSDN
If you implement IEquatable, you should also override the base
class implementations of Object.Equals(Object) and GetHashCode so that
their behavior is consistent with that of the IEquatable.Equals
method. If you do override Object.Equals(Object), your overridden
implementation is also called in calls to the static
Equals(System.Object, System.Object) method on your class. In
addition, you should overload the op_Equality and op_Inequality
operators. This ensures that all tests for equality return consistent
results.
In .NET, the fact that IEnumerable<T> extends IEnumerable often comes in handy. Frustratingly, though IEqualityComparer<T> and IComparer<T> do not extend their non-generic counterparts, despite the fact that the EqualityComparer<T> and Comparer<T> classes implement both interfaces. Is there a reason for this discrepancy?
If I have a sequence of strings I can use that to get a sequence of objects, since each string is also an object. This is true of any sequence; I can always get a sequence of objects when given any sequence.
If I have an object that can compare two strings I can't use it to compare two objects, since those two objects might not be strings.
The reason for this is because IEnumerable<T> is covariant, whereas IComparer and IEqualityComparer are not. (Not just in the C# sense, although that's true as well, but also in the conceptual computer science sense.)
As for why the concrete EqualityComparer<T> and Comparer classes implement both interfaces, rather than not just the generic versions, is a decision that can only really be explained by the employees who choose to create those types. I'd imagine that they did it because at the time enough people were using the non-generic versions of the interfaces that they wanted the types to be usable with all of the existing non-generic code.
Can someone please explain with example that I can understand about the difference between .Equals, IComparable and IComparer.
I was asked this in an interview.
Well first off, on the surface, Equals is a method (present in every object), while IComparable and IComparer are interfaces.
Equals is present in any class and can be overriden to provide equality testing depending on the context of the class (it's a good practice to override GetHashCode as well). By default it just tests if objects are equal in memory which is not very useful. Equals (and GetHashCode) are usually given a different implementation in the context of searching or hashing.
Implementing IComparable is a more fine-grain way of comparison, as it provides the CompareTo method, which is a greater-than/less-than comparison as opposed to Equals which is simply a is-equal-or-not comparison. For example a binary search tree structure could benefit from this method.
IComparer is similar to IComparable, except that it works from the outside. It allows you to define a "neutral" object that is used for comparing two other objects without modifying them directly, which you need to do with IComparable.
Equals is a method, when 2 other are interfaces. So look like the biggest difference.
More seriously - #ChrisSinclair gave you an answer in comments...
Equals returns true/false if the two objects are equal (or the same reference depending on your implementation) IComparable/IComparer: difference between IComparable and IComparer
.Equals() gives your class a way to test for equality against all other possible objects. This can be considered as the fallback for object equality. So this answers the question am I equivalent to the object passed in as a param.
IComparable provides for a way of comparing objects which can be ordered, possible uses include sorting. Implementing this interface puts the ordering logic into your class.
IComparer does pretty much the same as IComparable except the logic is contained in separate class.
1) Are the reasons why IEqualityComparer<T> was introduced:
a) so we would be able to compare objects (of particular type) for equality in as many different ways as needed
b) and by having a standard interface for implementing a custom equality comparison, chances are that much greater that third party classes will accept this interface as a parameter and by that allow us to inject into these classes equality comparison behavior via objects implementing IEqualityComparer<T>
2) I assume IEqualityComparer<T> should not be implemented on type T that we're trying to compare for equality, but instead we should implement it on helper class(es)?
Thank you
I'm doubtful that anyone here will be able to answer with any authority the reason that the interface was introduced (my guess--and that's all it is--would be to support one of the generic set types like Dictionary<TKey, TValue> or HashSet<T>), but its purpose is clear:
Defines methods to support the comparison of objects for equality.
If you combine this with the fact you can have multiple types implementing this interface (see StringComparer), then the answer to question a is yes.
The reason for this is threefold:
Operators (in this case, ==) are not polymorphic; if the type is upcasted to a higher level than where the type-specific comparison logic is defined, then you'll end up performing a reference comparison rather than using the logic within the == operator.
Equals() requires at least one valid reference and can provide different logic depending on whether it's called on the first or second value (one could be more derived and override the logic of the other).
Lastly and most importantly, the comparison logic provided by the type may not be what the user is after. For example, strings (in C#) are case sensitive when compared using == or Equals. This means that any container (like Dictionary<string, T> or HashSet<string>) would be case-sensitive. Allowing the user to provide another type that implements IEqualityComparer<string> means that the user can use whatever logic they like to determine if one string equals the other, including ignoring case.
As for question b, probably, though I wouldn't be surprised if this wasn't high on the list of priorities.
For your final question, I'd say that's generally true. While there's nothing stopping you from doing so, it is confusing to think that type T would provide custom comparison logic that is different from that provided on type T just because it's referenced as an IEqualiltyComparer<T>.
agreed on a and b
"should not be" is always a normative question and rarely a good metric. You do what works without getting into trouble. (Pragmatic Programmer). The fact that you can implement the interface statefull, stateless and in any which way, makes it possible to implement (alternative) comparers for all types, including value types, enums, sealed types, even abstract types; In essence it is a Strategy pattern
Sometimes there's a natural equality comparison for a type, in which case it should implement IEquatable<T>, not IEqualityComparer<T>. At other times, there are multiple possible ways of comparing objects for equality - so it makes sense to implement IEqualityComparer<T> then. It allows hash tables (and sets etc) to work in a flexible way.
IEquatable<T> could have been declared to be contravariant in T, since it only uses T in an input position (or, equivalently, U being a subtype of T should imply that IEquatable<T> is [a subtype of] IEquatable<U>).
So, why did the BCL team not annotate it (for C# 4.0) with the 'in' keyword, as they did with many other generic interfaces (like the entirely analogous IComparable)?
I think this is mainly for a philosophical reason rather than a technical limitation–as it's perfectly possible to simply annotate the interface. IEquatable<T> is meant to compare objects of the same type for exact equality. An instance of a superclass is not usually considered equal to an instance of a subclass. Equality in this sense implies type equality too. This is a bit different from IComparable<in T>. It can be sensible to define a relative sort order across different types.
To quote MSDN page on IEquatable<T>:
Notes to Implementers:
Replace the type parameter of the IEquatable<T> interface with the type that is implementing this interface.
This sentence further demonstrates the fact that IEquatable<T> is meant to work between instances of a single concrete type.
Inheritable types should generally not implement IEquatable<T>. If IEquatable<T> included a GetHashCode() method, one could define the semantics of IEquatable<T> to say that items should compare equal when examined as T's. Unfortunately, the fact that IEquatable<T> is bound to the same hash code as Object.Equals means that in general IEquatable<T> has to implement essentially the same semantics as Object.Equals.
Consequently, if an implementation of IEquatable<BaseClass> does anything other than call Object.Equals within it, a derived class which overrides Object.Equals and GetHashCode() and does not re-implement IEquatable<BaseClass> will end up with a broken implementation of that interface; an implementation of IEquatable<BaseClass> which simply calls Object.Equals will work just fine, even in that scenario, but will offer no real advantage over a class which doesn't implement IEquatable<T>.
Given that inheritable classes shouldn't be implementing IEquatable<T> in the first place, the notion of covariance is not relevant to proper implementations of the interface.