The type or namespace name ' Server ' was not found (is a using directive or an assembly reference missing?) ) _ Self-Study Process

Today will webserice inside of a code, copy to a C # class, the result of running a compilation error.
DataBase = Server.MapPath ("Data.mdb");
Error: Cannot find type or namespace name ' Server ' (using directive or assembly reference is missing?) )

But I did cite the name space using System.Web; why?
Because the full path to the Server is System.Web.HttpContext.Current.Server. Using system.web is clearly not sufficient, but it is possible to write Server.MapPath "Data.mdb" directly in classes that inherit from the built-in page class, because the page class already contains these classpath.

The solution to this type of error is summarized as follows
1, ensure the correct assembly reference:
Make sure that the assembly that contains the namespace is referenced. If you are developing in Visual Studio. NET, you can go to the Project menu and click Add Reference. On the. NET tab, select the assembly that contains the namespace that you want to import, or browse to the folder that contains the assembly. If you are using the command-line compiler, add the corresponding switch (/reference) to the compilation statement.
2, the right namespace:
(1)* If you execute this statement from a class that inherits from the page class, you can simply use the
DataBase = Server.MapPath ("Data.mdb");
(2) If in the ordinary class
A the using statement is placed at the top of the code file containing the type reference, ensuring that the namespace is imported into the project:
Using System.Web;
DataBase = HttpContext.Current.Server.MapPath ("Data.mdb");
B If there is no using statement, the reference to the type is fully qualified:
DataBase = System.Web.HttpContext.Current.Server.MapPath ("Data.mdb");