C# generic list <T> how to get the type of T?

Type type = pi.PropertyType;
if(type.IsGenericType && type.GetGenericTypeDefinition()
        == typeof(List<>))
{
    Type itemType = type.GetGenericArguments()[0]; // use this...
}

---

More generally, to support any IList<T>, you need to check the interfaces:

foreach (Type interfaceType in type.GetInterfaces())
{
    if (interfaceType.IsGenericType &&
        interfaceType.GetGenericTypeDefinition()
        == typeof(IList<>))
    {
        Type itemType = type.GetGenericArguments()[0];
        // do something...
        break;
    }
}

Given an object which I suspect to be some kind of IList<>, how can I determine of what it's an IList<>?

Here's the gutsy solution. It assumes you have the actual object to test (rather than a Type).

public static Type ListOfWhat(Object list)
{
    return ListOfWhat2((dynamic)list);
}

private static Type ListOfWhat2<T>(IList<T> list)
{
    return typeof(T);
}

Example usage:

object value = new ObservableCollection<DateTime>();
ListOfWhat(value).Dump();

Prints

typeof(DateTime)

Marc's answer is the approach I use for this, but for simplicity (and a friendlier API?) you can define a property in the collection base class if you have one such as:

public abstract class CollectionBase<T> : IList<T>
{
   ...

   public Type ElementType
   {
      get
      {
         return typeof(T);
      }
   }
}

I have found this approach useful, and is easy to understand for any newcomers to generics.