How would you unit test this method?

I'm looking to unit test this method but not sure how to go about doing that with a Stream reader and array created in the method. Any ideas?

public static List<Associations> CollectingAssociations(StreamReader sr, string MainUID, string AssocTrainUID, string AssocLoc) {


        string line;
        List<Associations> associationsArray = new List<Associations>();
        //skips the header
        sr.ReadLine();
        //reads top of the file, association records
        while((line = sr.ReadLine()).Substring(0, 2) == "AA") {


            if (line.Substring(3, 6).Contains(MainUID) && 
                line.Substring(9, 6).Contains(AssocTrainUID) && 
                line.Substring(37,7).Contains(AssocLoc)) {

                    associationsArray.Add(new Associations(line));
            }

        }
        return associationsArray;
    }