As extension method

As

public static class ObjectExtensions
{
     public static T As<T>(this object instance)
     {
         if (instance == null) return default(T);

         var casted = instance as T;
         if (casted == null)
             throw new ArgumentException("Cannot cast  '" + instance.GetType().FullName + "' to '" + typeof(T).FullName + "'.");
         return casted;
     }
}

The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint. Fix: public static T As(this object instance) where T : class