It is common for UML tools -- not just Visio -- to reverse engineer classes from your code into a UML model but not to create diagrams of those classes automatically.
The reason is that class diagrams are merely "views" onto the coded classes and it's up to you to decide which classes to show on which diagrams. If your project has 100 classes you would not want a single diagram showing all of those classes. More likely you would want to draw a diagram for each use case showing the classes that participate in that use case. Or a diagram for each subsystem showing only the classes in that subsystem. Less likely but possible would be a diagram showing all classes begining with letter A, and a separate diagram showing all classes beginning with letter B etc.
[note also that a single class may appear on many diagrams to show its use in different contexts]
Visio cannot possibly know which classes you would wish to place on which diagrams.
I hope that helps,
Co-author, Professional UML with Visual Studio .NET
quote:Originally posted by amitjoshipune
I am working on C# project. In mid of it I
now need to draw UML diagrams of the code (in C#)
I am using MS Visual Studio 2003 and Visio.
But it does not draw a single diagram on its own.
How to achieve this ?