The REPL in .NET
Didn't you guys heard about Test Driven Development? You can get it in most languages now aday. In .Net, the common framework is NUnit (but I like MbUnit as well). In C++, you have CppUnitLite. The issues he is describing sounds like normal legacy code and can be solved using the techniques listed in Working Effectively With Legacy Code.
REPL is nice, but it is good for playing with the langauge; to see what will happen if you do XYZ, not to verify that XYZ indeed happens. I know that Joel's Test doesn't include Unit Testing, but still...
This is sad. Benjamin, get yourself a copy of the following books and read them, you'll likely learn a lot.