[fabian2k]

For null checking I'd use the relatively new ArgumentNullException.ThrowIfNull, that automatically adds the parameter name in the exception and is a very compact and obvious way to check for null in your method parameters.

For enums I use ArgumentExceptions, something called this code with an invalid enum argument. I think there are some cases where Unreachable is a better error, but I'd probably be more inclined to use that for logic errors, where the code itself should not be reachable according to your understanding and reaching it would mean the code is wrong. The enums and null are cases where the arguments are unexpected, which to me is different.