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9.8 Simulating Covariance

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 section 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.

var names = new List<string>();
names.Add("John Smith");
names.Add("Mary Johnson");

Such generic data structures have many benefits over their nongeneric 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(){
   var 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, let's consider the following type:

public class CountedReference<T> {
   public CountedReference(T value);
   public T Value { get; }
   public int Count { get; }
   public void AddReference();
   public void ReleaseReference();
}

There is nothing wrong with casting a CountedReference<string> to CountedReference<object>, as in the following example.

var reference = new CountedReference<string>(...);
CountedReference<object> obj = reference; // this won't compile

In general, having a way to represent any instance of this generic type is very useful.

// what type should ??? be?
// CountedReference<object> would be nice but it won't work
static void PrintValue(??? anyCountedReference){
   Console.WriteLine(anyCountedReference.Value);
}

Unfortunately, unless CountedReference<T> implemented the Simulated Covariance Pattern described next, the only common representation of all CountedReference<T> instances would be System.Object. But System.Object is too limiting and would not allow the PrintValue method to access the Value property.

The reason that casting to CountedReference<object> is just fine, but casting to List<object> can cause all sorts of problems, is that in case of CountedReference<object>, the object appears only in the output position (the return type of Value property). In the 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 except the constructor
public class CountedReference<T> {
   public CountedReference(T value);
   public T Value { get; }
   public int Count { get; }
   public void AddReference();
   public void ReleaseReference();
}

// 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 CountedReference<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.

Consider a generic type (class or interface) and its dependencies described in the code fragment that follows.

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 a T 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<out 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 instantiations 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);
}

In the case of the simple CountedReference<T>, the code would look like the following:

public interface ICountedReference<out T> {
   T Value { get; }
   int Count { get; }
   void AddReference();
   void ReleaseReference();
}

public class CountedReference<T> : ICountedReference<object> {
   public CountedReference(T value) {...}
   public T Value { get { ... } }
   public int Count { get { ... } }
   public void AddReference(){...}
   public void ReleaseReference(){...}

   object ICountedReference<object>.Value { get { return Value; } }
}

check.jpg 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, because it results in additional types in the framework and can makes the existing types more complex.

check.jpg 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; } }
...
}

check.jpg CONSIDER using an abstract class instead of an interface to represent the root.

This might sometimes be a better option, because interfaces are more difficult to evolve (see section 4.3). On the other hand, there are some problems 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.

check.jpg CONSIDER using a nongeneric root type if such type is already available.

For example, List<T> implements IEnumerable for the purpose of simulating covariance.

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