Get the Count of a List of unknown type

You could do this

var property = typeof(ICollection).GetProperty("Count");
int count = (int)property.GetValue(list, null);

assuming you want to do this via reflection that is.

Cast it to ICollection and use that .Count

using System.Collections;

List<int> list = new List<int>(Enumerable.Range(0, 100));

ICollection collection = list as ICollection;
if(collection != null)
{
  Console.WriteLine(collection.Count);
}