Question

Please consider the following snippet from an implementation of the Interpreter pattern:

public override object Execute(Interpreter interpreter, object ctx) {
    var list = ctx as IEnumerable<string>;
    return (list != null) ? list.FirstOrDefault() : null;
}

What about if I want to use the same function for integers?

public override object Execute(Interpreter interpreter, object ctx) {
    var list = ctx as IEnumerable<string>;
    if (list != null) return list.FirstOrDefault();

    var list = ctx as IEnumerable<int>;
    return (list != null) ? list.FirstOrDefault() : null;
}

What I wanted was something like:

public override object Execute(Interpreter interpreter, object ctx) {
    var list = ctx as IEnumerable;
    return (list != null) ? list.FirstOrDefault() : null;
}

But Linq doesn’t act on IEnumerables. Instead, to get to this solution, I would be forced to write something like:

public override object Execute(Interpreter interpreter, object ctx) {
    var list = ctx as IEnumerable;
    if (list != null) foreach(var i in list) { yield return i; return; }
    return null;
}

Or use a generic method:

public override T Execute<T>(Interpreter interpreter, object ctx) {
    var list = ctx as IEnumerable<T>;
    return (list != null) ? list.FirstOrDefault() : null;
}

Which would break the Interpreter pattern (as it was implemented in this system). Covariance would also fail (at least in C#3), though would it work, it would be the exact behavior I wanted:

public override object Execute(Interpreter interpreter, object ctx) {
    var list = ctx as IEnumerable<object>;
    return (list != null) ? list.FirstOrDefault() : null;
}

So, my question is: what’s the best way to achieve the intended behavior?

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Addendum: digEmAll suggested something like the following code:

var list = ctx as IEnumerable;
return (list != null) ? list.Cast<object>().FirstOrDefault() : null;

This is a good answer for stuff like FirstOrDefault(). The problem comes with things like Reverse:

var list = ctx as IEnumerable;
return (list != null) ? list.Cast<object>().Reverse() : null;

I could fed this method a List<int> but I would get back a List<object>. No way to get around covariance, I know. So I guess digEmAll code is the best answer so far.

---

Thanks 🙂

Answer 1

Something like this, could fit your needs ?

public override object Execute(Interpreter interpreter, object ctx) {
    var nonGenericlist = ctx as IEnumerable;
    if (nonGenericlist == null)
       return null;
    var list = nonGenericlist.Cast<object>();
    return (list != null) ? list.FirstOrDefault() : null;
}