Question: I want to avoid my program being simple to have the license-verifier part removed from.

I don't want to use a commercial obfuscator because:

1. Of the cost. And though they can do a better job than I – they
too don't make it impossible to crack, just harder.
2. It seems that sometimes obfuscators cause bugs in the generated
code.

Obviously, I will be keeping an un-obfuscated copy for maintenance.

Answer: 
If your intent is to make it harder, but not impossible, one way is to have multiple code points that check your licence file is valid.

Lets say you have a licence file with some key like so

```
abc-def-fhi-asdf

```

So, four parts to the key. We would then create four different methods that check for the various parts of the key.

By doing this, and varying the methods used through the code (ideally, randomly choosing the verification method at runtime), you make it significantly more difficult to remove the validation.

on top of this, one method would be to have a publish process that inlined your verification method, subtly changing it each time it is called.

for example something like this:

```
*user clicks a common function
// [VALIDATION STUB]
*perform user action

```

The new publish process runs through the code, pulling out // [VALIDATION STUB] and replacing it with your validation code (before the code is compiled), which as I say should vary as much as possible each time.

The main thing to pull from my answer really is that obfuscation is hard, but not impossible. Especially if you resign yourself to the reality that the malevolent user will always break it eventually
I have some suggestions that you may find usefull. 

First of course you can use free obfuscators like the one that comes with VisualStudio. It's better than nothing. 

Second you can write your license verification code and once it's working fine, refactor it as much as you can, change class names, member variables, local variables and methods to something like c1, v1, l1, m1 and so on. That's basically what obfuscators do.

Third, do all of the above.

Fourth, write your licence verification in unmanaged code (C++, Delphi) and make it a DLL named something important like core.dll, net.dll etc. You can also put some decoy methods in there that would do nothing important. Make many calls to that DLL from multiple places of your code and pretend that you do something with the results of those calls.