Is it safe for structs to implement interfaces?

Question

Since no one else explicitly provided this answer I will add the following:

Implementing an interface on a struct has no negative consequences whatsoever.

Any variable of the interface type used to hold a struct will result in a boxed value of that struct being used. If the struct is immutable (a good thing) then this is at worst a performance issue unless you are:

• using the resulting object for locking purposes (an immensely bad idea any way)
• using reference equality semantics and expecting it to work for two boxed values from the same struct.

Both of these would be unlikely, instead you are likely to be doing one of the following:

Generics

Perhaps many reasonable reasons for structs implementing interfaces is so that they can be used within a generic context with constraints. When used in this fashion the variable like so:

class Foo<T> : IEquatable<Foo<T>> where T : IEquatable<T>
{
    private readonly T a;

    public bool Equals(Foo<T> other)
    {
         return this.a.Equals(other.a);
    }
}

1. Enable the use of the struct as a type parameter
   • so long as no other constraint like new() or class is used.
2. Allow the avoidance of boxing on structs used in this way.

Then this.a is NOT an interface reference thus it does not cause a box of whatever is placed into it. Further when the c# compiler compiles the generic classes and needs to insert invocations of the instance methods defined on instances of the Type parameter T it can use the constrained opcode:

If thisType is a value type and thisType implements method then ptr is passed unmodified as the 'this' pointer to a call method instruction, for the implementation of method by thisType.

This avoids the boxing and since the value type is implementing the interface is must implement the method, thus no boxing will occur. In the above example the Equals() invocation is done with no box on this.a¹.

Low friction APIs

Most structs should have primitive-like semantics where bitwise identical values are considered equal². The runtime will supply such behaviour in the implicit Equals() but this can be slow. Also this implicit equality is not exposed as an implementation of IEquatable<T> and thus prevents structs being used easily as keys for Dictionaries unless they explicitly implement it themselves. It is therefore common for many public struct types to declare that they implement IEquatable<T> (where T is them self) to make this easier and better performing as well as consistent with the behaviour of many existing value types within the CLR BCL.

All the primitives in the BCL implement at a minimum:

• IComparable
• IConvertible
• IComparable<T>
• IEquatable<T> (And thus IEquatable)

Many also implement IFormattable, further many of the System defined value types like DateTime, TimeSpan and Guid implement many or all of these as well. If you are implementing a similarly 'widely useful' type like a complex number struct or some fixed width textual values then implementing many of these common interfaces (correctly) will make your struct more useful and usable.

Exclusions

Obviously if the interface strongly implies mutability (such as ICollection) then implementing it is a bad idea as it would mean tat you either made the struct mutable (leading to the sorts of errors described already where the modifications occur on the boxed value rather than the original) or you confuse users by ignoring the implications of the methods like Add() or throwing exceptions.

Many interfaces do NOT imply mutability (such as IFormattable) and serve as the idiomatic way to expose certain functionality in a consistent fashion. Often the user of the struct will not care about any boxing overhead for such behaviour.

Summary

When done sensibly, on immutable value types, implementation of useful interfaces is a good idea

---

Notes:

1: Note that the compiler may use this when invoking virtual methods on variables which are known to be of a specific struct type but in which it is required to invoke a virtual method. For example:

List<int> l = new List<int>();
foreach(var x in l)
    ;//no-op

The enumerator returned by the List is a struct, an optimization to avoid an allocation when enumerating the list (With some interesting consequences). However the semantics of foreach specify that if the enumerator implements IDisposable then Dispose() will be called once the iteration is completed. Obviously having this occur through a boxed call would eliminate any benefit of the enumerator being a struct (in fact it would be worse). Worse, if dispose call modifies the state of the enumerator in some way then this would happen on the boxed instance and many subtle bugs might be introduced in complex cases. Therefore the IL emitted in this sort of situation is:

IL_0001:  newobj      System.Collections.Generic.List..ctor
IL_0006:  stloc.0     
IL_0007:  nop         
IL_0008:  ldloc.0     
IL_0009:  callvirt    System.Collections.Generic.List.GetEnumerator
IL_000E:  stloc.2     
IL_000F:  br.s        IL_0019
IL_0011:  ldloca.s    02 
IL_0013:  call        System.Collections.Generic.List.get_Current
IL_0018:  stloc.1     
IL_0019:  ldloca.s    02 
IL_001B:  call        System.Collections.Generic.List.MoveNext
IL_0020:  stloc.3     
IL_0021:  ldloc.3     
IL_0022:  brtrue.s    IL_0011
IL_0024:  leave.s     IL_0035
IL_0026:  ldloca.s    02 
IL_0028:  constrained. System.Collections.Generic.List.Enumerator
IL_002E:  callvirt    System.IDisposable.Dispose
IL_0033:  nop         
IL_0034:  endfinally  

Thus the implementation of IDisposable does not cause any performance issues and the (regrettable) mutable aspect of the enumerator is preserved should the Dispose method actually do anything!

2: double and float are exceptions to this rule where NaN values are not considered equal.

There are several things going on in this question...

It is possible for a struct to implement an interface, but there are concerns that come about with casting, mutability, and performance. See this post for more details: https://docs.microsoft.com/en-us/archive/blogs/abhinaba/c-structs-and-interface

In general, structs should be used for objects that have value-type semantics. By implementing an interface on a struct you can run into boxing concerns as the struct is cast back and forth between the struct and the interface. As a result of the boxing, operations that change the internal state of the struct may not behave properly.

In some cases it may be good for a struct to implement an interface (if it was never useful, it's doubtful the creators of .net would have provided for it). If a struct implements a read-only interface like IEquatable<T>, storing the struct in a storage location (variable, parameter, array element, etc.) of type IEquatable<T> will require that it be boxed (each struct type actually defines two kinds of things: a storage location type which behaves as a value type and a heap-object type which behaves as a class type; the first is implicitly convertible to the second--"boxing"--and the second may be converted to the first via explicit cast--"unboxing"). It is possible to exploit a structure's implementation of an interface without boxing, however, using what are called constrained generics.

For example, if one had a method CompareTwoThings<T>(T thing1, T thing2) where T:IComparable<T>, such a method could call thing1.Compare(thing2) without having to box thing1 or thing2. If thing1 happens to be, e.g., an Int32, the run-time will know that when it generates the code for CompareTwoThings<Int32>(Int32 thing1, Int32 thing2). Since it will know the exact type of both the thing hosting the method and the thing that's being passed as a parameter, it won't have to box either of them.

The biggest problem with structs that implement interfaces is that a struct which gets stored in a location of interface type, Object, or ValueType (as opposed to a location of its own type) will behave as a class object. For read-only interfaces this is not generally a problem, but for a mutating interface like IEnumerator<T> it can yield some strange semantics.

Consider, for example, the following code:

List<String> myList = [list containing a bunch of strings]
var enumerator1 = myList.GetEnumerator();  // Struct of type List<String>.IEnumerator
enumerator1.MoveNext(); // 1
var enumerator2 = enumerator1;
enumerator2.MoveNext(); // 2
IEnumerator<string> enumerator3 = enumerator2;
enumerator3.MoveNext(); // 3
IEnumerator<string> enumerator4 = enumerator3;
enumerator4.MoveNext(); // 4

Marked statement #1 will prime enumerator1 to read the first element. The state of that enumerator will be copied to enumerator2. Marked statement #2 will advance that copy to read the second element, but will not affect enumerator1. The state of that second enumerator will then be copied to enumerator3, which will be advanced by marked statement #3. Then, because enumerator3 and enumerator4 are both reference types, a REFERENCE to enumerator3 will then be copied to enumerator4, so marked statement will effectively advance both enumerator3 and enumerator4.

Some people try to pretend that value types and reference types are both kinds of Object, but that's not really true. Real value types are convertible to Object, but are not instances of it. An instance of List<String>.Enumerator which is stored in a location of that type is a value-type and behaves as a value type; copying it to a location of type IEnumerator<String> will convert it to a reference type, and it will behave as a reference type. The latter is a kind of Object, but the former is not.

BTW, a couple more notes: (1) In general, mutable class types should have their Equals methods test reference equality, but there is no decent way for a boxed struct to do so; (2) despite its name, ValueType is a class type, not a value type; all types derived from System.Enum are value types, as are all types which derive from ValueType with the exception of System.Enum, but both ValueType and System.Enum are class types.

Structs are implemented as value types and classes are reference types. If you have a variable of type Foo, and you store an instance of Fubar in it, it will "Box it" up into a reference type, thus defeating the advantage of using a struct in the first place.

The only reason I see to use a struct instead of a class is because it will be a value type and not a reference type, but the struct can't inherit from a class. If you have the struct inherit an interface, and you pass around interfaces, you lose that value type nature of the struct. Might as well just make it a class if you need interfaces.

(Well got nothing major to add but don't have editing prowess yet so here goes..)
Perfectly Safe. Nothing illegal with implementing interfaces on structs. However you should question why you'd want to do it.

However obtaining an interface reference to a struct will BOX it. So performance penalty and so on.

The only valid scenario which I can think of right now is illustrated in my post here. When you want to modify a struct's state stored in a collection, you'd have to do it via an additional interface exposed on the struct.

I think the problem is that it causes boxing because structs are value types so there is a slight performance penalty.

This link suggests there might be other issues with it...

http://blogs.msdn.com/abhinaba/archive/2005/10/05/477238.aspx