ASP WebAPI generic List optional parameter

I'm really struggling with this one. I need a generic list parameter for my Get method, but it needs to be optional. I just did this:

public dynamic Get(List <long> ManufacturerIDs = null)

Unfortunately on runtime i get the error:

Optional parameter 'ManufacturerIDs' is not supported by 'FormatterParameterBinding'.

How to get a generic list as an optional parameter here?

Solution 1:

What's the point of using an optional parameter? List<T> is a reference type and if the client doesn't supply a value it will simply be null:

public HttpResponseMessage Get(List<long> manufacturerIDs)
{
    ...
}