HttpRuntime.Cache only provides severals methods and I think I’m able
to implement these methods with Dictionary by myself.
You think wrong. HttpRuntime.Cache is much more than a simple dictionary. It offers thread-safety and cache expiration policies. It provides possibilities of using custom implementation and benefit from distributed caching which is helpful in web farms. Implementing this with dictionaries could be lots of work that you probably don’t want to venture into as you will be basically reinventing the wheels and even if reinventing wheels doesn’t bother you chances for getting it right are slim.
2)If HttpRuntime.Cache is much better than Dictionary, why some people
would like to implement their own cache framework.
People wouldn’t want to do that.
3) How about MS Enterprise Cache Block?
That’s heavy artillery which is not always necessary when you need simple caching which could be achieved with what the framework already provides you out of the box.
Remark: In .NET 4.0 you should use the new System.Runtime.Caching namespace instead of HttpRuntime.Cache.
So to answer your question: Should I use HttpRuntime.Cache
Yes, unless you are using .NET 4.0 in which case you should use classes from the new System.Runtime.Caching namespace.