Dynamic return type that depends on subclass in C#?

Question

I would like to declare a base class Command with abstract getter GetValue . Then I subclass this to TalkCommand and SpendCommand for example.

Both class must implement GetValue , however, I want to TalkCommand to return string type for GetValue (what to talk?) and I want the SpendCommand to return int type. (how much to spend?)

I don't know how to write this in the Command base class. Is this design possible in C#?

Answer 1

In addition to Manfred's answer, I want to give you a hint to go further.

While generics provide you a valuable tool to define GetValue return type, you wouln't be able to perform the following upcast:

abstract class Command<T>
{
    public abstract T GetValue();
}

class TalkCommand : Command<string>
{
    public override string GetValue()
    {
        return "Test";
    }
}

// TalkCommand isn't Command<object>
Command<object> cmd = new TalkCommand();

Instead of defining your command as an abstract class, I would start defining it as an interface with covariance :

public interface ICommand<out T>
{
    T GetValue();
}

And the whole abstract class could implement ICommand<out T> :

abstract class Command<T> : ICommand<T>
{
    public abstract T GetValue();
}

Now you can upcast an implementation of ICommand<string> to ICommand<object> :

ICommand<string> cmd = new TalkCommand();
ICommand<object> cmd2 = cmd;

This is good, because you get the best of two worlds:

• Your commands have strong return types.
• Your commands can be generically treated as implementations of ICommand<object> and this let you call GetValue and return object in some situations where you don't know the generic argument to provide or you don't need it at all.

Answer 2

If return type must be different depending on subclass, you would be better off not having this method in the base class.

The idea behind having a base type in the first place is to let you deal with all subclasses in a uniform way. One should be able to write

Command cmd = ... // One of subclasses
<some-type> val = cmd.GetValue();

and replace <some-type> with the actual type. In your case, however, there is no common base type other than object for int and string , so having a different return type becomes pointless.

You can implement GetValue as generic, like this:

abstract class Command {
    abstract T GetValue<T>();
    ...
}

but that would require the caller to pass the desired type as type parameter of GetValue , which would defeat the purpose of having GetValue in the base class.

Possible fix: One approach that lets callers stay away from knowing the type is visitor - make an interface that lets Command "push" the value into the caller, like this:

IValueGetter {
    void SetInt(int v);
    void SetString(string v);
}

abstract class Command {
    abstract void GetValue(IValueGetter g);
}
class TalkCommand {
    override void GetValue(IValueGetter g) {
        g.SetString("hello");
    }
}
class BuyCommand {
    override void GetValue(IValueGetter g) {
        g.SetInt(123);
    }
}

Now the caller can interact with Command without knowing the type of its GetValue . Commands that produce an int would call back SetInt , while commands that produce string would call SetString .

Answer 3

Only if your base class Command is generic or you don't mind working with object .

Generic Example

If you always work with a known instance type, this would be the best approach since you can with the data type you need without casting.

abstract class Command<T>
{
    public abstract T GetValue();
}

class TalkCommand : Command<string>
{
    public override string GetValue()
    {
        return "Test";
    }
}

class SpendCommand : Command<int>
{
    public override int GetValue()
    {
        return 1234;
    }
}

Object Example

This is a very universal approach, pretty much what WPF does with DependencyProperties and non-generic collections like IList or IEnumerable .

abstract class Command
{
    public abstract object GetValue();
}

class TalkCommand : Command
{
    public override object GetValue()
    {
        return "Test";
    }
}

class SpendCommand : Command
{
    public override object GetValue()
    {
        return 1234;
    }
}

Hybrid Example

If you want to be able to use both, you'll need a non-generic base class and inherit another abstract generic class from it.

abstract class Command
{
    public abstract object GetValue();
}

abstract class Command<T> : Command
{
    public T GetTypedValue()
    {
        return (T)GetValue();
    }
}   

class TalkCommand : Command<string>
{
    public override object GetValue()
    {
        return "Test";
    }
}

class SpendCommand : Command<int>
{
    public override object GetValue()
    {
        return 1234;
    }
}