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: 
I once had to hide a license verifier in code that the customer could modify. Conceivably, they could have removed it if they knew where to look. Here are some tricks that I used at the time.

1. Give your verifier classes, assembly names, and variable names that look like they actually do something else.
2. Call the verifier from multiple parts of the code.
3. Add a randomizer to the call for verification so that sometimes it runs, and sometimes it doesn't. This will make it harder to know where the verification code is actually coming from.

I should add that all of this is defeatable and could cause serious maintenance headaches, but in my particular scenario it worked.
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