I can follow this now. It's interesting to come back to this 7 years later, it makes much more sense to me now.

One line. Excellent!

I used almost the same approach, passed tests under Attempt button and was happy it's done.
Yes, I thought about "what if first element will be default?"
The answer depends on environment we work in:

• If we're talking about real TDD production - I can say it counts as solution because tests passed. If tests were not designed to check this case - ask the guy who wrote these test. No need to fill code with garbage checks for everyting. If it will break somehow - we'll see an exception, fix tests, fix the code to pass tests. Happy end.
• If we talk about unpredictable startup thing where you really don't know what to expect and have no tests - yes you all are right, we have to think about it.

You may just return false withut var rez. And in if construction return true

watafuk bro

Just when I thought that one line is impossible.

Same

The rez variable is unnecessary here. Your solution can be presented in a simpler way:
int sum1 = 0;
int sum2 = 0;

for (int i = 0; i < arr1.Length; i++)
{
sum1 += arr1[i];
}

for (int i = 0; i < arr2.Length; i++)
{
sum2 += arr2[i];
}

return sum1 + sum2;