Help me understand Resharper background compilation

Question

So Jeff Atwood rightly complained about Visual Studio not performing background compilation see: http://www.codinghorror.com/blog/2007/05/c-and-the-compilation-tax.html

The solution from most sources seems to be Reshaper which will incrementally perform background compilation as you write. This leads to their great realtime re-factoring tips and error detection.

But what I don't understand is with R# continually compiling my code, why does it take so long when executing a compilation via VS (i.e. Ctrl + Shift + B or similar). What I mean by this is, if R# has already compiled my code then why would I need a recompilation?

My assumption is of course that R# is not overriding the assemblies in my bin directories but instead holding the compilation results in memory. In which case, is it possible to tell R# to simply override my assemblies when compilation is successful?

Answer 1

I don't know about "rightly complained" - that's an opinion I happen to disagree with:)

However, the VB.NET (and probably Resharper c#) background compilers do not actually compile full assemblies - they cannot! If you think about it, the natural state of your code while you are working is not compilable! Almost every keystroke puts your code in an invalid state. Think of this line:

var x = new Something();

As you type this, from the key "v" to the key ")", your code is "wrong". Or what if you are referencing a method you haven't defined yet? And if this code is in an assembly that another assembly requires, how would you compile that second assembly at all, background or not?

The background compilers get around this by compiling small chunks of your code into multiple transient "assemblies" that are actually just metadata holders - really, they don't care about the actual effects of the code as much as the symbols defined, used, etc. When you finally hit build, the actual full assemblies still need to be built.

So no, I don't believe it's possible because they're not built to do actual full compilation - they are built to check your code and interpret symbols on the fly.