I'm looking for a tool like SonarJ but for .NET instead of Java.
SonarJ helps you to find deviations between he architecture and the code within minutes. It can be integrated into your IDE to help you avoid the introduction of new architetural violations to your code base. You can also use it to maintain metric based software quality rules which will keep complexity under control.
I googled and searched SO without satisfying results.
Here are two others:
Lattix (http://www.lattix.com) NDepend (http://www.ndepend.com)
Structure101 is now also available for .Net.
VS2010's Ultimate Edition also has good support for dependency management and architectural governance.
Concerning .NET code, I think NDepend is the tool you are looking for. NDepend comes with a dependency graph, a dependency matrix, a treemap/metric view, and a Code Query Language dedicated to explore the code base, see a few screenshots below:
