Question

I wrote this extension method:

public static DataTable ToDataTable<T>(this IList<T> list)
{...}

It works well if called with a type known at compile time:

DataTable tbl = new List<int>().ToDataTable();

But how to call it if the generic type isn’t known?

object list = new List<int>();
...
tbl = Extension.ToDataTable((List<object>)list); // won't work

Answer 1

This occurs because a List<int> is not a List<object> — the List type is not covariant in its element type parameter. Unfortunately you would need to get a typed version of the generic method and call it using reflection:

Type listItemType = typeof(int);   // cheating for simplicity - see below for real approach
MethodInfo openMethod = typeof(Extension).GetMethod("ToDataTable", ...);
MethodInfo typedMethod = openMethod.MakeGenericMethod(typeof(listItemType));
typedMethod.Invoke(null, new object[] { list });

An alternative may be to create a version of your extension method that accepts IList rather than IList<T>. The List<T> class implements this non-generic interface as well as the generic interface, so you will be able to call:

public static DataTable WeakToDataTable(this IList list) { ... }

((IList)list).WeakToDataTable();

(In reality you’d probably use an overload rather than a different name — just using a different name to call out the different types.)

---

More info: In the reflection solution, I skipped over the problem of how to determine the list element type. This can be a bit tricky depending on how sophisticated you want to get. If you’re assuming that the object will be a List<T> (for some T) then it’s easy:

Type listItemType = list.GetType().GetGenericArguments()[0];

If you’re only willing to assume IList<T> then it’s a bit harder, because you need to locate the appropriate interface and get the generic argument from that. And you can’t use GetInterface() because you’re looking for a closed constructed instance of a generic interface. So you have to grovel through all the interfaces looking for one which is an instance of IList<T>:

foreach (Type itf in list.GetType().GetInterfaces())
{
  if (itf.IsGenericType && itf.GetGenericTypeDefinition == typeof(IList<>))  // note generic type definition syntax
  {
    listItemType = itf.GetGenericArguments()[0];
  }
}

This will work for empty lists because it goes off the metadata, not the list content.