Task From A Code Test

Earlier this week I took a coding test and the first task in the test was interesting and alittle fun to complete. At the start you are given a text file with integers (one per line) that are used as an input into another process. This process has been failing because of problems with the input file. You are tasked with writing a program to validate the input files and output any errors to a separate file for use in troubleshooting. The task is to validate the file according to a set of rules.

The rules are as follows:
1.Verify that all integers in the file fall between 1 and N where N is the total number of lines in the text file.
2.Verify that all integers between 1 and N exist in the file (So if the file contains 3 lines, they should be 1, 2, and 3 in any order).
3.Output any errors to another file.

Rather than use a bunch of loops directly to iterate through the lines, I opted for a LINQ approach. Having done LINQ for so long, that approach usually looks better to me than writing the loops directly. The code is on GitHub and is available by clicking here.