VS 2005 uses a code-behind model conceptually the same as VS 2003. Specifically, each .aspx page continues to inherit from a code-behind class that contains protected control field references for each control in the .aspx page:
What is different between VS 2003 and VS 2005 is that Visual Studio no longer injects its tool-specific wire-up code in the developer’s code-behind file. Instead, it takes advantage of a new language feature in C# and VB called "partial types" (or partial classes) to split the code-behind implementation across two files. One of these partial class files is the developer-owned code-behind file that contains developer-written event-handlers and code for the page. The other partial class file is then a tool-generated/maintained file that contains the protected control field declarations and the other design-time code that Visual Studio requires. The benefit of splitting them out into two separate files at design-time is that it ensures that the code that VS creates and maintains never interferes (or deletes) code that a developer writes. At compile-time, these files are compiled together and generate a single code-behind class.
With the VS 2005 Web Application project model, the design-time partial class is generated and persisted on disk by VS 2005. This new design-time partial-class file has the filename naming pattern: PageName.aspx.designer.cs. If you expand any new page created within your VS 2005 Web Application project, you can see this file listed under the associated Page.aspx file along with the developer-owned code-behind file:
If you open up the code-behind file of the page (Default.aspx.cs), you’ll then see the code-behind logic of the page — which contains all of the code and event handlers that a developer writes (and no tool-generated "code-spit" content — which means it stays very clean):
If you open the Default.aspx.designer.cs file, you’ll then see the design-time code of the page — which contains the field declarations for controls within the .aspx page:
Because the MyWebProject._Default class is marked as "partial" in both of the above two files, the compiler will merge them into a single generated class at compile-time. This means that any variable, method or field generated in the default.aspx.designer.cs file can be used from the default.aspx.cs code-behind file (just as if it was declared in the code-behind file itself). For example, within the Page_Load event handler we could easily add the below code that uses the "Label1" and "Calendar1" control:
This will compile clean and run just fine — because the "Label1" and "Calendar1" field references have been defined within the default.aspx.designer.cs file.
When you do a build inside a VS 2005 Web Application project, all pages, user-controls, master pages (and their associated code-behind files+design-time generated files), along with all other standalone classes within the project are compiled into a single assembly. This is the same behavior as with VS 2003.