I think you're probably confusing yourself by trying to develop the whole program at once. Take just one part, e.g. just the "if (choice == "JPY")" block, implement that and get it working.

Then add the rest bit by bit, testing as you go along.

Also when you post code with errors, state what the errors are. Often code can be fixed by looking at the errors, but not stating what they are means I have to try and guess, which is more work, or I have to muck about creating a VS project to run it in, which is more mucking about, so just post the errors and if I need to create a VS project to answer your questions then that's my choice.