Algorithm visualization for C#

I don't know about inside a function, but VS2010 has sequence diagram generation from code - see here or here

I think you may be looking for Code Rocket.

It provides flowchart and pseudocode visualizations of code methods and algorithms, embedded directly inside Visual Studio and Eclipse - and there is a separate Designer application for working outwith IDEs too.

AiVosto has a set of tools to visualize source code from many languages: Visustin. It is on the market since a long time. I tried it a very long time ago, was not 100% convinved. Maybe you want to give it a try and evaluate if it's worth the money for you.

For me it was so that in order to understand some complex algorithm, I have anyway to experiment with it, having a graph helps a bit but as a coder you probably can visualize loops and decision trees well without software. I don't want to discourage you from using that, just try it out before investing money, having graphs is nice but shall also be useful.