Simple modifications to a C# program developed in VS 2015

I have a C# program developed in VS 2015 that we use during online training courses in order to ensure that our students are attending the lesson. This program displays random characters (numbers and letters) on the screen at a random interval and stores these characters into a TXT file. Now I need to apply the following modifications to this program: 1) Bug to remove: if the same character is displayed twice consecutively, the second occurrence is not stored in the TXT file. 2) The program only write the TXT file when the user press the End button. Instead I want that it writes the file every time it displays a character, thus the End button simple close the program. 3) In order to remove ambiguity between characters, I want to remove from the set of numbers and letters the characters: O, 0 (zero), I, 1. I will provide the source code for VS 2012-2015 and I require the amended source code in return. I also require a test version of the program in order to test the modifications.