What't best way to implement logging system on a large project?

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C#

AOP

logging

Now our project has done half of year, we need to add logging framework on our code.For example:Enter/Exit funtion and occur exception is common requirement.I'd like to use AOP to finish.
But,spring.net AOP is face to interface, proxy only to use on interface,a class need inherit an interface can be proxy.Microsoft Enterprise library is also. The peoblem is we have lots of class that don't inherit interface. So if we change code structure will take a lot of offer, it is not a good way.PostSharp implement AOP through Attribute, it may be suitable for us, but PostSharp2.0 is not free.NAspect has little information can be found.
Above solutions are popular AOP solution, I hope use maturing framework to solve rather than coding by myself.Performance and extension are important to us.
I want to use AOP on our code, directly use on a class (don't inherit interface), do you have any solution, please help me.Thank you.