CODEWITHSUNDEEP
c#polyvariadic
Lucina
Lucina

Reputation: 490

Specifying "any subclass" in a C# type constraint rather than "one particular subclass"

If I would like to write a method that takes a variable number of "TDerived" where TDerived is any subclass of a class "Base", is there any way to do this?

The following code only works with a single specific specified subclass:

void doStuff<TDerived>(params TDerived[] args) where TDerived : Base
{
    //stuff
}

ie if I have 

class Super { }
class Sub0 : Super { }
class Sub1 : Super { }

then I cannot do

Sub0 s0 = new Sub0();
Sub1 s1 = new Sub1();
doStuff(s0, s1);

since I get "best overloaded match... has some invalid arguments".

Regardless of how the compiler handles the type constraints and variadic functions, this seems (as far as I can tell) completely type-safe. I know I could cast, but if this is type safe why not allow it?

EDIT:

Perhaps a more convincing example:

void doStuff<TDerived>(params SomeReadOnlyCollection<TDerived>[] args) where TDerived : Base
{
    foreach(var list in args)
    {
        foreach(TDerived thing in list)
        {
            //stuff
        }
    }
}

Upvotes: 5

Views: 1068

Answers (4)

Branko Dimitrijevic
Branko Dimitrijevic

Reputation: 52107

One other alternative you could use is to simply specify the generic parameter explicitly. For example:

var s0 = new Sub0();
var s1 = new Sub1();

doStuff<Super>(s0, s1);

You should be able to apply the same principle on the case with SomeReadOnlyCollection, as long as it is covariant. For example, IEnumerable is such a collection:

static void doStuff2<TDerived>(params IEnumerable<TDerived>[] args) where TDerived : Super {
    // ...
}

// ...

var l0 = new List<Sub0>();
var l1 = new List<Sub1>();

doStuff2<Super>(l0, l1);

Upvotes: 0

Anthony Pegram
Anthony Pegram

Reputation: 126834

TDerived needs to be able to resolve to a single type. In your example, the only type it could resolve to would be Super, but the compiler is not going to make that leap. You can make that leap for the compiler.

doStuff(new Super[] { s0, s1 });
doStuff<Super>(s0, s1);

Regarding your update, consider (instead of a generic method) defining a method accepting IEnumerable<ISuper>, which will support derived types because IEnumerable<T> is covariant (as of .NET 4). IEnumerable<T> is also inherently readonly and forward-only, perfect if you have a foreach loop. Full working example:

class Program
{
    static void Main()
    {
        var sub0s = new Sub0[] { new Sub0() };
        var sub1s = new List<Sub1> { new Sub1() };
        doStuff(sub0s, sub1s);
    }

    static void doStuff(params IEnumerable<ISuper>[] args)
    {
        foreach (var sequence in args)
        {
            foreach (var obj in sequence)
            {
                Console.WriteLine(obj.GetType());
                // you have the ability to invoke any method or access 
                // any property defined on ISuper
            }
        }
    } 
}

interface ISuper { }
class Super : ISuper { }
class Sub0 : Super { }
class Sub1 : Super { }

IEnumerable<T> is implemented by BCL collections since .NET 2.0, including T[], List<T>, ReadOnlyCollection<T>, HashSet<T>, etc.

Upvotes: 6

Bojan Resnik
Bojan Resnik

Reputation: 7378

In your example, you are actually telling the compiler that all arguments to doStuff must be of the same type at compile time, and that this type has to be inherited from Base. If you want to allow the arguments to be of different types, then just don't use generics:

void doStuff(params Base[] args)
{}

EDIT

The same applies with your new example - instead of a specific SomeReadOnlyCollection you can use IEnumerable, as it is covariant:

void doStuff(params IEnumerable<Base>[] args)
{
    foreach (var list in args)
    {
        foreach (var thing in list)
        {
        }
    }
}

Upvotes: 6

Jonathan Henson
Jonathan Henson

Reputation: 8206

Well you could most certainly change 

Sub0 s0 = new Sub0();
Sub1 s1 = new Sub1();

To

Super s0 = new Sub0();
Super s1 = new Sub1();

and then it would work if Super is TDerived.

I may be misunderstanding you, but the only way to make a method take any subclass of a base class is to declare the method to take a reference to the base type.

Upvotes: 0

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