What Is a Property List?

A property list in computer programming is a list of key-value pairs that describe the specific attributes of a program or object.‭ ‬A key-value pair consists of a key,‭ ‬or the name of the attribute‭ ‬that‭ ‬is being referred to,‭ ‬and a value for that attribute.‭ ‬Property lists can be used to save objects to a file during the process of serialization or can be used to store information such as user preferences.‭ ‬They usually are human-readable and therefore can be platform-independent.

In modern computing,‭ ‬a property list that is going to be written to a file usually employs the extensible markup language‭ (‬XML‭) ‬to define the keys and values.‭ ‬XML is human-readable text that also can be read by any program,‭ ‬allowing the property list file to be moved between operating systems.‭ ‬Using XML has the added benefit of being supported by several‭ ‬programming libraries, which makes reading the list from a stream easier.

During data serialization,‭ ‬a property‭ ‬list is generated that contains key-value pairs that represent all of the state information‭ ‬in a given data object.‭ ‬The key refers to the name of the field being serialized, and the value is the actual state information being saved.‭ ‬Using mechanisms such as reflection,‭ ‬the property list can be read and the fields appropriately assigned to a new instance of the object class.

Some operating systems and computer languages have made property lists an integral part of their operating structure.‭ ‬These programs have taken the standard XML format of a property list and created proprietary implementations that allow the system to optimize the reading and writing of the lists.‭ Although these still are property lists,‭ ‬they are not portable to other systems or programs‭ ‬unless they are specifically designed to read the modified format.

A property list is valuable in certain applications because it is human-readable.‭ ‬This has led to the development of specific programs that are used to edit property lists in a way that maintains the structure and makes it easier for the computer user.‭ ‬Certain operating systems even have built-in support for the editing and creation of property lists.

The structure and arguments of a property list that is created with XML can be defined with a block of code known as a document type definition‭ (‬DTD‭)‬.‭ ‬A DTD can help a program that is writing or reading a property list by defining the valid keys that can be used and the allowable ranges of the values assigned.‭ ‬This means that key-value‭ ‬pairs can be checked for correct syntax, and errors can be caught at the time of writing or reading before any invalid data has a chance to cause problems in a running program.