expando

Expando is a term used in software development to describe an object or data structure whose members can be added, removed, or modified at runtime, effectively allowing its shape to expand beyond a fixed schema. The concept is common in dynamic programming languages and in systems that require flexible data models, such as deserialization of loosely structured data or runtime object composition.

In .NET, ExpandoObject embodies this concept. Provided by the System.Dynamic namespace, ExpandoObject enables properties to be added or removed on the fly while the object is accessed through dynamic binding. As a dynamic object, properties can be created at runtime, and the object also implements standard dictionary interfaces to enumerate and manipulate its members. This makes it convenient for scenarios such as constructing data transfer objects on demand or wrapping loosely typed data sources like JSON.

Other languages and environments treat the idea similarly without necessarily using the term expando. JavaScript objects, Python dictionaries with dynamic attributes, or OpenStruct-like types in some languages allow runtime extension of an object's attributes.

Advantages of expando-like structures include flexibility and rapid prototyping, while drawbacks include reduced compile-time type safety, potential performance costs, and possible difficulties with refactoring and tooling support.