Magic string


In computer programming, a magic string is an input that a programmer believes will never come externally and which activates otherwise hidden functionality. A user of this program would likely provide input that gives an expected response in most situations. However, if the user does in fact innocently provide the pre-defined input, invoking the internal functionality, the program response is often quite unexpected to the user.

Background

Typically, the implementation of magic strings is due to time constraints. A developer must find a fast solution instead of delving more deeply into a problem and finding a better solution.
For example, when testing a program that takes a user's personal details and verifies their credit card number, a developer may decide to add a magic string shortcut whereby entering the unlikely input of "***" as a credit card number would cause the program to automatically proceed as if the card were valid, without spending time verifying it. If the developer forgets to remove the magic string, and a user of the final program happens to enter "***" as a placeholder credit card number while filling in the form, the user would inadvertently trigger the hidden functionality.

Resolution

Situations/issues of cause

Often there are significant time constraints out of the developer's control right from the beginning of their involvement in a project. Common issues that might lead to this anti-pattern as a result:
Restricting the format of the input is a possible maintenance solution. Essentially, this means validating input information to check that it is in the correct format, in order to reduce the possibility of the magic string being discovered by the user. Examples include validating a telephone number to ensure that it contains only digits or checking that a person's name has a forename and a surname. An exception is made for the magic string in the validation code so that it will not be rejected by validation. It is expected that, since a user would likely quickly notice the strict enforcement of formatting, it would likely not occur to the user to try inputting a string not conforming to the format. Therefore, it is very unlikely for the user to try the magic string.
As with any input validation process, it is important to ensure that the format is not restrictive in a way that unintentionally restricts the use of the application by some users. An example of this is restricting telephone number or postal code input based on one country's system, causing problems for legitimate users who are based in other countries.

Purposeful implementation

As is often the case with anti-patterns, there exists specific scenarios where magic strings are a correct solution for an implementation. Examples include cheat codes and Easter eggs. Furthermore, there are cases when users invent magic strings, and systems that have not coded to accept them can produce unexpected results such as missing license plates.