23) Diffecernce between static class and sealed class

Sealed class..
The main purpose of a sealed class to take away the inheritance feature from the user so they cannot derive a class from a sealed class. One of the best usage of sealed classes is when you have a class with static members
Static class..
Static classes are used when a class provides functionality that is not specific to any unique instance. Here are the features of static classes in C# 2.0.

Static classes can not be instantiated.
Static classes are sealed so they can not be inherited.
Only static members are allowed.
Static classes can only have static constructor to initialize static members.