[esmbro73]

what does this actually mean or how do I go about doing this.

Write a C# program which will read in any file, compute its modular sum and output a new
version of the file (let's call it a “.xum” file) with the checksum appended to it

[modi123_1]

Typically I go to my instructor for clarification on his/her homework versus a random set of strangers on the internet, but so it goes.

It says you nee to do four things.
1. read in a file.
2. compute modular sum from the values in the file.
3. write the output to a file with a specific extension on the file.
4. Append the checksum of the file to it.

[tlhIn`toq]

You need to talk to your teacher about this. We can't be responsible for interpreting every assignment from every teacher from every school in the country. We're not going to get our heads bit off because you asked us and not your teacher only to get a poor grade because we couldn't guess correctly.