It probably isn't even possible to do this, but I will ask anyway. Is it possible to create a function that receives a string and then uses it as a right side argument for the goes to operator (=>) used in lambda?
Actually, what I want to do is to be able to redefine an specific method of a specific class during runtime. I want to be write down a function with the program running and attaching it to a delegate. Is it possible?
The easiest way to do it is probably DLINQ as TcKs suggested.
The fastest (I believe, in 3.5) is to create a DynamicMethod. Its also the scariest method as well. You're essentially building a method using IL, which has about the same feel as writing code in machine language.
I needed to do this to dynamically attach event handlers in some thing or another (well, I didn't need to do it, I just wanted to make unit testing events easier). It seemed a bit daunting at the time because I don't know crap about IL, but I figured out a simple way to accomplish this.
What you do is create a method that does exactly what you want. The more compact the better. I'd provide an example if I could figure out exactly what you're trying to do. You write this method in a class within a DLL project and compile it in release mode. Then you open the DLL in Reflector and disassemble your method. Reflector gives you the option of what language you wish to disassemble to--select IL. You now have the exact calls you need to add to your dynamic method. Just follow the example on MSDN, switching out the example's IL for your reflected methods' code.
Dynamic methods, once constructed, invoke at about the same speed as compiled methods (saw a test where dynamic methods could be called in ~20ms where reflection took over 200ms).