Question

I have a dll in c++, it returns list, I want to use it in my c# app as List

[DllImport("TaskLib.dll", SetLastError = true, CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern List<int> GetProcessesID();

public static List<int> GetID()
{
    List<int> processes = GetProcessesID();//It is impossible to pack a "return value": The basic types can not be packed
    //...
}

Answer 1

Per Jared Par:

Generics as a rule are not supported in any interop scenario. Both PInvoke and COM Interop will fail if you attempt to Marshal a generic type or value. Hence I would expect Marshal.SizeOf to be untested or unsupported for this scenario as it is a Marshal specific function.

See:
Marshalling .NET generic types