C# type of empty list

Question

I can get a type of non empty list:

private Type GetListType(IEnumerable list)
{
  return list.GetType().GetGenericArguments()[0];
}

How i can get a type of empty list?

private Type GetListType(IEnumerable list)
{
...
}

Answer 1

Your code works for empty lists too. The [0] is an indexer over the array of type arguments returned by the GetGenericArguments call, not your list's contents.

Answer 2

The code you have works, eg in

GetListType(new List<string>()); // typeof(string)

It will not work if the runtime type doesn't have a generic parameter, eg

public class MyList : List<SomeObject> { }
GetListType(new MyList()); // there is no 0'th generic argument!

Or it might not return what you're expecting:

GetListType(new Dictionary<string, int>()); // typeof(string)
// even though it ought to be KeyValuePair<string, int>

Maybe you should use this instead:

private Type GetListType<T>(IEnumerable<T> list)
{
  return typeof(T);
}

The biggest downside of this is that your type must implement IEnumerable<T> , not just IEnumerable (but if you wanted to get a useful type from an empty list, it'd have to do this, or otherwise declare its type, anyway).

Answer 3

As others have pointed out the code you have will work fine with an empty collection. But it will only work when the collection type directly contains a generic variable. It will not work for concrete collections. For example

class IntList : List<int> { } 

GetListType(new List<int>()) // Works
GetListType(new IntList())   // Fails

The type IntList has no generic parameters hence the index operation [0] will fail.

To make a more general purpose implementation you should pick a particular interface or class type in which to query for the generic parameter. I would recommend IEnumerable<T> .

static Type GetListType(IEnumerable enumerable)
{
    var type = enumerable.GetType();
    var enumerableType = type
        .GetInterfaces()
        .Where(x => x.GetGenericTypeDefinition() == typeof(IEnumerable<>))
        .First();
    return enumerableType.GetGenericArguments()[0];
}