Guard Clauses and Assertions

Two simple techniques to increase code quality, resilience, and ease debugging scenarios is to use guard clauses effectively and ensure that assertions are used liberally.

Guard Clauses

  • Any public method should perform guard clauses to ensure pre conditions are met.
  • Ensures the code's invariants are not broken.
  • Throw exceptions, because these are exceptional issues.
  • Developer and user assistance as it is possible for these clauses to fail at runtime.

Here we enforce that any PersonalDetails instance has a forename and surname. A forename must also be at least one character long. As long as these conditions are met, we finally assign the values internally. Guard clauses should also be used on dependencies that are services, checking that a service is not a null instance for example.


  • Used within private methods/functions where required.
  • Should be used for situations that should never happen, e.g the presence of a bug or invalid scenario.
  • Developer only assistance, the user should never see these ideally because automated/manual testing should have detected them.
  • Usually removed for release builds, though open to debate, best to judge on context. Is it better for the program to crash and inform the user, or carry on in an invalid state?
  • Great for documenting assumptions, e.g. code a level above ensures object is in a certain state.

While this method is private, we have essentially stated that we take no responsibility for validating that a name has been provided. This is the concern of another part of the code (the constructor in this case). However this simple assert statement means that if the method is used in a different manner, it will fail spectacularly at runtime. This will point at the incorrect use of the method and allow the developer to make the required changes.


Code quality will improve because less invalid scenarios should be allowed to happen. Due to clauses and assertions always being present they go hand in hand with automated tests, often catching scenarios that automated tests may miss. Debugging is easier because the stack trace points you at the source of the problem, rather than an initial problem hidden in layers of exceptions caused by invalid state. While applying clauses and assertions increases lines of code, they are easy to implement, and the return on investment is high. There are no excuses not to use them.