Simulated Covariance for .NET Generics
I just wrote this pattern, but I am not sure if I should add it officially to the Framework Design Guidelines. It seems like a bit of a corner case scenario, though I do get questions about it from time to time. Anyway, let me know what you think.
Different constructed types don’t have a common root type. For example, there would not be a common representation of IEnumerable<string> and IEnumerable<object> if not for a pattern implemented by IEnumerable<T> called Simulated Covariance. This post describes the details of the pattern.
Generics is a very powerful type system feature added to the .NET Framework 2.0. It allows creation of so called parameterized types. For example, List<T> is such a type and it represents a list of objects of type T. The T is specified at the time when the instance of the list is created.
List<string> names = new List<string>();
names.Add(“John Smith”);
names.Add(“Mary Johnson”);
Such Generic data structures have many benefits over their non-Generic counterparts. But they also have some, sometimes surprising, limitations. For example, some users expect that a List<string> can be cast to List<object>, just as a String can be cast to Object. But unfortunately, the following code won’t even compile.
List<string> names = new List<string>();
List<object> objects = names; // this won’t compile
There is a very good reason for this limitation, and that is to allow for full strong typing. For example, if you could cast List<string> to a List<object> the following incorrect code would compile, but the program would fail at runtime.
static void Main(){
List<string> names = new List<string>();
// this of course does not compile, but if it did
// the whole program would compile, but would be incorrect as it
// attempts to add arbitrary objects to a list of strings.
AddObjects((List<object>)names);
string name = names[0]; // how could this work?
}
// this would (and does) compile just fine.
static void AddObjects(List<object> list){
list.Add(new object()); // it’s a list of strings, really. Should we throw?
list.Add(new Button());
}
Unfortunately this limitation can also be undesired in some scenarios. For example, there is nothing wrong with casting a List<string> to IEnumerable<object>, like in the following example.
List<string> names = new List<string>();
IEnumerable<object> objects = names; // this won’t compile
foreach(object obj in objects){
Console.WriteLine(obj.ToString());
}
In general, having a way to represent “any list” (or in general “any instance of this generic type”) is very useful.
// what type should ??? be?
static void PrintItems(??? anyList){
foreach(object obj in anyList){
Console.WriteLine(obj.ToString());
}
}
Unfortunately, unless List<T> implemented a pattern that will be described in a moment, the only common representation of all List<T> instances would be System.Object. But System.Object is too limiting and would not allow PrintItems method to enumerate items in the list.
The reason that casting to IEnumerable<object> is just fine, but casting to List<object> can cause all sorts of problems is that in case of IEnumerable<object>, the object appears only in the output position (the return type of GetEnumerator is IEnumerator<object>). In case of List<object>, the object represents both output and input types. For example, object is the type of the input to the Add method.
// T does not appear as input to any members or dependencies of this interface
public interface IEnumerable<T> {
IEnumerator<T> GetEnumerator();
}
public interface IEnumerator<T> {
T Current { get; }
bool MoveNext();
}
// T does appear as input to members of List<T>
public class List<T> {
public void Add(T item); // T is an input here
public T this[int index]{
get;
set; // T is actually an input here
}
}
In other words, we say that in IEnumerable<T>, the T is at covariant positions (outputs). In List<T>, the T is at covariant and contravariant (inputs) positions.
To solve the problem of not having a common type representing the root of all constructions of a generic type, you can implement what’s called the Simulated Covariance Pattern.
Given a generic type (class or interface) and its dependencies
public class Foo<T> {
public T Property1 { get; }
public T Property2 { set; }
public T Property3 { get; set; }
public void Method1(T arg1);
public T Method2();
public T Method3(T arg);
public Type1<T> GetMethod1();
public Type2<T> GetMethod2();
}
public class Type1<T> {
public T Property { get; }
}
public class Type2<T> {
public T Property { get; set; }
}
Create a new interface (Root Type) with all members containing Ts at contravariant positions removed. In addition, feel free to remove all members that might not make sense in the context of the trimmed down type.
public interface IFoo<T> {
T Property1 { get; }
T Property3 { get; } // setter removed
T Method2();
Type1<T> GetMethod1();
IType2<T> GetMethod2(); // note that the return type changed
}
public interface IType2<T> {
T Property { get; } // setter removed
}
The generic type should then implement the interface explicitly and “add back” the strongly typed members (using T instead of object) to its public API surface.
public class Foo<T> : IFoo<object> {
public T Property1 { get; }
public T Property2 { set; }
public T Property3 { get; set;}
public void Method1(T arg1);
public T Method2();
public T Method3(T arg);
public Type1<T> GetMethod1();
public Type2<T> GetMethod2();
object IFoo<object>.Property1 { get; }
object IFoo<object>.Property3 { get; }
object IFoo<object>.Method2() { return null; }
Type1<object> IFoo<object>.GetMethod1();
IType2<object> IFoo<object>.GetMethod2();
}
public class Type2<T> : IType2<object> {
public T Property { get; set; }
object IType2<object>.Property { get; }
}
Now, all constructed instantiation of Foo<T> have a common root type IFoo<object>.
var foos = new List<IFoo<object>>();
foos.Add(new Foo<int>());
foos.Add(new Foo<string>());
…
foreach(IFoo<object> foo in foos){
Console.WriteLine(foo.Property1);
Console.WriteLine(foo.GetMethod2().Property);
}
þ CONSIDER using the Simulated Covariance Pattern, if there is a need to have a representation for all instantiations of a generic type.
The pattern should not be used frivolously as it results in additional types in the library and can makes the existing types more complex.
þ DO ensure that the implementation of the root’s members is equivalent to the implementation of the corresponding generic type members.
There should not be an observable difference between calling a member on the root type and calling the corresponding member on the generic type. In many cases the members of the root are implemented by calling members on the generic type.
public class Foo<T> : IFoo<object> {
public T Property3 { get { ... } set { ... } }
object IFoo<object>.Property3 { get { return Property3; } }
…
}
þ CONSIDER using an abstract class instead of an interface to represent the root.
This might sometimes be a better option as interfaces are more difficult to evolve (see section X). On the other hand there are some problem with using abstract classes for the root. Abstract class members cannot be implemented explicitly and the subtypes need to use the new modifier. This makes it tricky to implement the root’s members by delegating to the generic type members.
þ CONSIDER using a non-generic root type, if such type is already available.
For example, List<T> implements IEnumerable for the purpose of simulating covariance.