C# generic extension method for cloning ILists

Question

I am having trouble making an extension method for cloning a list.

This answer is very close, providing the following extension method :

static class Extensions
{
    public static IList<T> Clone<T>(this IList<T> listToClone) where T: ICloneable
    {
        return listToClone.Select(item => (T)item.Clone()).ToList();
    }
}

The problem here is that ToList() returns an object of type List which isn't necessarily what I used to call the function with. I would like it to return an object of the same type as the one I called Clone on.

The closest I've been able to come is this :

static private object Clone<T, K>(this T container)
    where T : IList<K>, new()
    where K : ICloneable
{
    T res = new T();

    int size = container.Count;
    for (int i = 0; i < size; ++i)
    {
        K obj = container[i];
        res.Add((K)obj.Clone());
    }

    return res;
}

but I keep getting a CS1061 error (does not contain a definition for 'Clone' and no extension method 'Clone' accepting a first argument of type [...] could be found...). Unless I specify :

List<CloneableType> clone = myList.Clone<List<CloneableType>, CloneableType>();

Is there a way to simply call Clone() without the specifications? Why can't the compiler deduce the types I want?

Note : Although the example uses the generic List, it's important for me that this works for non List implementations of IList as well.

Why are you using generic method, when deep cloning should be implemented in the List class itself?? — Euphoric, Apr 16 '14 at 06:44
I don't get the difference. Your first method is declared to return `IList`, `List` implements `IList`. What's the problem? — MarcinJuraszek, Apr 16 '14 at 06:54
See this: [Generics: Why can't the compiler infer the type arguments in this case?](http://stackoverflow.com/questions/3968834/generics-why-cant-the-compiler-infer-the-type-arguments-in-this-case) The crucial point in Eric Lippert's answer is: "The fundamental rule that you're missing is probably that constraints are not part of the signature. Type inference works off of the signature." — Ani, Apr 16 '14 at 06:55

score 3 · Accepted Answer · answered Apr 16 '14 at 06:58

The C# compiler infers generic types from parameters, but there is no parameter of type K in your method.

As an alternative, you could use the following method:

static class Extensions
{
    public static IList<T> Clone<T>(this IList<T> listToClone)
         where T : ICloneable
    {
        var list = (IList<T>)Activator.CreateInstance(listToClone.GetType());
        foreach (var item in listToClone)
        {
            list.Add((T)item.Clone());
        }
        return list;
    }
}

Note that:

This could fail at runtime (the list type doesn't have a public parameterless constructor - somewhat unlikely)
You'd still to cast the result to the original type

You might want to consider using serialization as an alternative method for deep-cloning objects.

This ended up being pretty much what I went for. I'm ok with casting the result since I wanted the behavior to mimic that of the ICloneable interface. Thanks! — Godfather, Apr 16 '14 at 15:52

C# generic extension method for cloning ILists

1 Answers1