<p>I would like to access a class everywhere in my application, how can I do this?</p>

<p>To make it more clear, I have a class somewhere that use some code. I have an other class that use the same code. I do not want to duplicate so I would like to call the same code in both place by using something. In php I would just include("abc.php") in both... I do not want to create the object everytime I want to use the code.</p>

The concept of global classes in C# is really just a simple matter of referencing the appropriate assembly containing the class. Once you have reference the needed assembly, you can refer to the class of choice either by it's fully qualified Type name, or by importing the namespace that contains the class. (
Concrete instance or Static access
to that class)
Or
You can have a
Singleton class
to use it everywhere but some people won't recommend you this way to proceed.