My favorite super-pattern in developing and supporting software.  Always assume the user is wrong.  Remember, you are User Number Zero, so… blame yourself.

Defensive Programming is a technique similar to Defensive Driving. Always assume the user will break your program. Defensive programming will save you time in the long run by reducing support and increasing user satisfaction.

Its aim is to reduce the risk of collision by anticipating dangerous situations, despite adverse conditions or the mistakes of others – Defensive Driving – Wikipedia 2018

We must have a balance of making programs flexible enough to allow the user to use it for intentions unforeseen by the tool-makers themselves, and to ensure they don’t fall off the rails and lose data because they entered a word instead of a number.

Input Validation

• Never trust Input Text Boxes
  • Always convert numerical values using ParseFloat or equivalent
  • Never  Eval() or Shell() with input string
  • Never use string to build SQL Query without proper escaping. see SQL Injection
  • Always trim whitespace at beginning and end of string
• Give clear and actionable error messages when providing feedback to user on their incorrect inputs
• … This post could go on for a while, TBF

 

Further Reading

• Apple responds to iPhone 4 reception issues: you’re holding the phone the wrong way
• The First Rule of Programming: It’s Always Your Fault