If you are developing with Visual Studio, or using the .NET Framework with some other dev tool (including my personal weakness, a text editor), and you encounter what you believe is a bug - for example the behavior of a method does not agree with the documentation - you can supply your feedback directly to the development teams within Microsoft via this link:

https://connect.microsoft.com/visualstudio/feedback

Any bugs entered there go directly to the defect databases monitored by the engineering teams for the .NET Framework and Visual Studio. 

BUT! If you are using Windows Communication Foundation (WCF) or Workflow Foundation (WF), and you encounter what you believe is a bug, you can use these dedicated links:

WCF: http://connect.microsoft.com/wcf/Feedback
WF: http://connect.microsoft.com/wf/Feedback

These ones go directly to the sub-teams that handle WCF and WF. 

All of these places are completely open to the public and all feedback items go straight into our the product studio DBs for servicing.