Source files in Visual Studio allow you to collapse blocks of code. The idea is that in most cases you can reduce the amount of onscreen code, which seems to separate other modules within a given class, by collapsing the code so it isn't visible; this feature is known as outlining. For example, if you are comparing the load and save methods and in between you have several other blocks of code, then you can effectively "hide" this code, which isn't part of your current focus.
By default, there is a minus sign next to every method (sub or function). This makes it easy to hide or show code on a per-method basis. If the code for a method is hidden, the method declaration is still shown and has a plus sign next to it indicating that the body code is hidden. This feature is very useful when you are working on a few key methods in a module and you want to avoid scrolling through many screens of code that are not relevant to the current task.
It is also possible to create custom regions of code so you can hide and show portions of your source files. For example, it is common to see code where all of the properties are placed in one region, and all of the public methods are placed in another. The #Region directive is used for this within the IDE, though it has no effect on the actual application. A region of code is demarcated by the #Region directive at the top and the #End Region directive at the end. The #Region directive that is used to begin a region should include a description, which appears next to the plus sign shown when the code is minimized.
The outlining enhancement was in part inspired by the fact that the original Visual Studio designers generated a lot of code and placed all of this code in the main vb file for that form. It wasn't until Visual Studio 2005 and partial classes that this generated code was placed in a separate file. Thus the region allowed the generated code section to be hidden when a source file was opened. Being able to see the underpinnings of your generated UI does make it is easier to understand what is happening, and possibly to manipulate the process in special cases. However, as you can imagine, it can become problematic; hence the #Region directive, which can be used to organize groups of common code and then visually minimize them.
Visual Studio 2010 developers, but not Express Edition developers, can also control outlining throughout a source file. Outlining can be turned off by selecting Edit O Outlining O Stop Outlining from the Visual Studio menu. This menu also contains some other useful functions. A section of code can be temporarily hidden by highlighting it and selecting Edit O Outlining O Hide Selection. The selected code will be replaced by ellipses with a plus sign next to it, as if you had dynamically identified a region within the source code. Clicking the plus sign displays the code again.
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