Why singlton pattern use ? while if we can do the same by Static class?

Let's see. Probably you think that singleton pattern is something good. I would not agree with that — it is at best only tolerable, and should not be overused. The singleton objects gives you draconian hassles, notably in multithreading. Yes, I use this pattern myself, but only in special, exclusive situations.

Now, one can implement singletons in different ways, good and bad. Static class is the class which would prohibit the good ways, will allow only bad ones. Why? Non-static class allows to have any member in class, static and non-static (instance) once. A static class limits you to static members only. You would not be able to use any instance members. But why would you need to use an instance member? Well, to make the singleton implementation a bit less disgusting that it could be. How? Here is the example of good implementation: http://csharpindepth.com/Articles/General/Singleton.aspx[^]. (Referenced from http://en.wikipedia.org/wiki/Singleton_pattern[^].)

Please read the both articles. After reading then, you should come to understanding why your idea of using of the static class as a singleton was so bad, compared to better implementation.

—SA