Hi all!
I have a problem of casting generics to their base type. In the code
below I first define a BaseList class that can hold items of any type
that inherits from BaseItem. I then define a DerivedList that can hold
items of the type DerivedItem, which inherits from BaseList.
So far so good... But if I somehow get an object that I know is a
subclass of BaseList (and therefor holds subclasses of BaseItem) and
try to cast it to BaseList<BaseItemi get an InvalidCastException
(also below).
Because both the list class and the item class(es) it contains can be
cast to their base classes I think that the whole should also be
castable? Or am I wrong? Is this an syntactic error or a thought fault?
=)
Regards,
Richard
Code (can also be found at http://pastebin.com/866977 )
class BaseItem {
}
class DerivedItem : BaseItem {
}
class BaseList<Twhere T : BaseItem {
}
class DerivedList : BaseList<DerivedItem{
}
class Program {
static void Main(string[] args) {
// Suppose i get this value (dl) from outside as an object.
// I only know that it is a subclass of BaseList and it is
// parameterized with some subclass of BaseItem
DerivedList dl = new DerivedList();
object o = dl;
// This cast fails!
BaseList<BaseItemdl2 = (BaseList<BaseItem>) o;
}
}
Exception:
System.InvalidCastException was unhandled
Message="Unable to cast object of type 'TestConsoleApp.DerivedList'
to type 'TestConsoleApp.BaseList`1[TestConsoleApp.BaseItem]'."
Source="TestConsoleApp"
StackTrace:
at TestConsoleApp.Program.Main(String[] args) in Program.cs:line
28
at System.AppDomain.nExecuteAssembly(Assembly assembly, String[]
args)
at System.AppDomain.ExecuteAssembly(String assemblyFile,
Evidence assemblySecurity, String[] args)
at
Microsoft.VisualStudio.HostingProcess.HostProc.Run UsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context( Object
state)
at System.Threading.ExecutionContext.Run(ExecutionCon text
executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
