A number of astute developers have noted that the C# code style enforced by Microsoft StyleCop differs in some ways from the style typically seen in sample code coming from the Microsoft Developer Division. For example, the very fine book Framework Design Guidelines by Krzysztof Cwalina and Brad Abrams includes many code samples which use a different style, and this style is briefly described in an appendix in the back of the book.
In fact, the differences between the "StyleCop style" and the "Framework Design Guidelines style" are relatively minor. One of the biggest differences is that the framework style prefixes private and internal fields with an underscore, while StyleCop style omits the underscore and instead prefixes all class members with this.
The reason for the differences between the two styles is largely historical. The team that developed the first version of the .Net CLR consisted almost entirely of C++ developers. Remember, at this time no-one had ever heard of C# and in fact the language only existed on paper. When the original CLR was stable enough for internal use, people from the CLR team began writing test and sample code in the new C# language, mainly to test the CLR itself. Being C++ developers, these people naturally wrote their first C# code in a style that very much resembled C++, with liberal use of the m_ prefix, very little whitespace or comments, short, cryptic variable names, lots of Hungarian notation, etc.
Soon after this, an offshoot of the CLR team began writing the first version of the .Net Framework. Most of this code was written in C#. This was the first production C# code written anywhere in the world! Again, the team chose a coding style that mostly resembled C++. Over time this style has changed slightly, but the Framework team still uses this style for its internal C# code, as well as sample C# code provided to customers and partners.
After the first version of the .Net Framework was released, many other teams within Microsoft began writing new code in C#. Some of the biggest early adopters were, naturally, the Office team and the Windows team. Within a couple of years, almost every product group at Microsoft was writing at least some percentage of its code in C#. The architects and managers in these teams (being architects and managers) each took the time to come up with their own, individual style guidelines for the new C# language. In some places these style guidelines differed wildly according to the whims and fancies of the leaders of these teams. However, over time some C# style trends began to form, and these trends did not always follow the original style used by the Framework team. C# style had begun to evolve.
A short time after this, a brilliant young developer at Microsoft (ok, it was me) decided to take it upon himself to write a little tool which could detect variances from the C# style used within his team. StyleCop was born. Over the next few years, we gathered up all of the C# style guidelines we could find from the various teams within Microsoft, and picked out all of best practices which were common to these styles. These formed the first set of StyleCop rules. One of the earliest rules that came out of this effort was the use of the this prefix to call out class members, and the removal of any underscore prefixes from field names. C# style had officially grown apart from its old C++ tribe.
Development on StyleCop continued on and off for the next five or six years, and over this time many new rules were introduced. As you might expect, many wars were fought over the nature of these rules, and much blood was shed. In the end, decisions about StyleCop rules always came down to the following:
StyleCop rules tend to encourage whitespace and openness in the code, as well as lots of comments and documentation. The rules also encourage developers to be explicit about what they are doing, and to minimize the need for assumption or guesswork on the part of the reader.
Today, StyleCop is very widely used within Microsoft, although it is still not an official, mandated tool internally. The .Net Framework team still writes code and samples using the original style derived from C++, and there are still some teams that have chosen to go their own way (and still some architects and managers with big egos). However, that is happening less and less internally, as more and more teams adopt the tool.
Now we have continued the evolution by releasing StyleCop to the world!