如何将对象强制转换为IList <T> T型未知

Question

I have a object[] containig some values. I want to extract infomation out of it, but I fail at casting the second object in the array (a WeakReference ) to a IList with T as the third value in the array.

Take a look at my code:

object[] vals = GetValues(); //vals[2] = WeakReference, vals[3] = Type of T, vals[4] = index in the list
IList<((Type)vals[3])> list = (IList<((Type)vals[3])>)(((WeakReference)vals[2]).Target); //This line does not even compile, seems like I'm doing something wrong..
object oo = list.ElementAt((int)vals[4]);
//Do something with oo...

Any suggestions how I can cast the Target of the WeakReference to a IList interface with T = vals[3] ?

Answer 1

It's really strange that you are packing so much heterogeneous information into an array . Arrays are normally used to store elements of the same type. Why not encapsulate the data in a proper type?

But to answer the question as asked - in C# 4, you can use dynamic :

var target = ((dynamic)vals[2]).Target;

if(target != null)
{
    object oo = Enumerable.ElementAt(target, vals[4]);
    //Do something with oo...
}

(EDIT: If you want to minimize the use of dynamic here, cast to a WeakReference and leave the dynamic call to the end. This way, type-safety is 'maximized'.)

Otherwise, you can use reflection:

object target = ((WeakReference)vals[2]).Target;

if (target != null)
{
    object oo = target.GetType()
                      .GetProperty("Item")
                      .GetValue(target, new[] { vals[4] });
    //Do something with oo...
}

(EDIT: If the indexer could be explicitly implemented, you'll probably need to use the interface mappings.)

Answer 2

It was already suggested that you could use dynamic , but it sounds like you are also supposed to check that the object has the specified type. I've also kept use of dynamic to a minimum:

object target = ((WeakReference) vals[2]).Target;

if (target == null)
    throw new InvalidOperationException("Target cannot be null.");

object result = Enumerable.ElementAt((dynamic) target, (int) vals[4]);

if (result != null && !((Type) vals[3]).IsAssignableFrom(result.GetType()))
    throw new InvalidOperationException(string.Format("The retrieved object is a {0}, but the expected type was {1}.", result.GetType(), (Type) vals[3]));

return result;

Answer 3

If you just need the object from an enumerable, at a given index, here is a simple function that does it:

    public static object GetObjectAt(IEnumerable enumerable, int index)
    {
        int i = 0;
        foreach (object obj in enumerable)
        {
            if (i == index)
                return obj;

            i++;
        }
        throw new IndexOutOfRangeException();
    }

And in your case, you would do this:

        object oo = GetObjectAt((IEnumerable)(WeakReference)vals[2], (int)vals[4]);

Of course there are alternatives that look more sexy (see other answers), with fancy linq queries and cool C# 4 dynamic new trendy stuff :-) but, in general, if you don't "need" the T type (in your example code, you don't need it), you don't need generics. This solution is actually supported with any .NET Framework and C# versions, from 1 to 4.

Answer 4

I think you're working too hard to be able to use the extension methods which don't seem to be necessary.

object[] vals = GetValues();
var list = ((WeakReference)vals[2]).Target as IList;
object oo = null;
if (list != null)
{
    oo = list[(int)vals[4]];
}