Casting generic type "as T" whilst enforcing the type of T

Question

I'm missing a trick here I think and can't believe I've never done this before. However, how can I cast a generic type using the as keyword?

[Serializable] public abstract class SessionManager<T> where T : ISessionManager {      protected SessionManager() { }      public static T GetInstance(HttpSessionState session)     {          // Ensure there is a session Id         if (UniqueId == null)         {             UniqueId = Guid.NewGuid().ToString();         }          // Get the object from session         T manager = session[UniqueId] as T;         if (manager == null)         {             manager = Activator.CreateInstance<T>();             session[UniqueId] = manager;         }          return manager;      }      protected static string UniqueId = null;  } 

The line T manager = session[UniqueId] as T; throws the following error:

The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint

Now, I think understand the reason for this; I've not physically told the compiler that T is a class. If I replace:

public abstract class SessionManager<T> where T : ISessionManager 

with

public abstract class SessionManager<T> where T : class 

... then the code builds successfully.

But my question is thus; how can I have both the class and ISessionManager enforcements on the generic type? I'm hoping there's a very simple answer for this.

EDIT: Just to add I had tried: where T : ISessionManager, class, turns out I hadn't read my compiler error properly. Simply enough, just putting class before ISessionManager fixes the issue. The error I hadn't read was:

"The 'class' or 'struct' constraint must come before any other constraints".

Dumb moment over.

Answer 1

... where T : class, ISessionManager 

In case you want to use the where keyword on methods here is an example that also uses generics

    public void store<T>(T value, String key)     {         Session[key] = value;     }      public T retrieve<T>(String key) where T:class     {         return  Session[key] as T ;     }