Indexes in C# run from 0 to N - 1 inclusive, where N is the number of items in the collection.
So if your have 5 elements in an array
arr
, say, then valid indexes would be
arr[0]
arr[1]
arr[2]
arr[3]
arr[4]
Any other value as an index (i.e. negative, or greater than or equal to N) will cause an index out of range exception because there is no value at that location.
We can't tell you what the problem is in your code precisely: it needs your code running with your data to work that out, and we don't have access to that at all.
So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. If you don't know how to use it then a quick Google for "Visual Studio debugger" should give you the info you need.
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!