readonly

Readonly is a designation used across software and systems to indicate that a value or resource should not be altered during normal operation. Depending on context, immutability may be enforced by a language’s type system, a runtime check, or by access permissions at the operating system or application level. Different domains use similar concepts under different terminology.

In programming languages, readonly or equivalent constructs protect data from modification after initialization. In C#, the readonly keyword applies to fields and allows assignment only in a constructor, after which the field is immutable for the object's lifetime; it contrasts with const, which is a compile-time constant. TypeScript supports readonly on properties and on array types to restrict reassignment and mutation through the type system, though runtime enforcement is not guaranteed. Some languages implement immutability via final, sealed, or immutable data structures, which can differ in scope and semantics.

In user interfaces and web forms, an element marked readonly cannot be edited by the user, while still submitting its value with the form. This differs from disabled controls, which are not submitted. The readonly attribute is commonly used for fields that should be shown and submitted but not edited.

In file systems, a read-only attribute or mount option prevents writing to a file or directory. Attempting to modify such resources typically results in a permission error, unless the attribute is removed or the access mode is changed.

Read-only modes occur in databases and APIs to guarantee data stability during certain operations, such as long-running queries or audits. Overall, readonly is a broad concept used to protect data integrity by preventing modifications in appropriate contexts.