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In object-oriented (OO) and functional programming, an immutable object (unchangeable [1] object) is an object whose state cannot be modified after it is created. [2] This is in contrast to a mutable object (changeable object), which can be modified after it is created. [3]
In C#, a class is a reference type while a struct (concept derived from the struct in C language) is a value type. [5] Hence an instance derived from a class definition is an object while an instance derived from a struct definition is said to be a value object (to be precise a struct can be made immutable to represent a value object declaring attributes as readonly [6]).
The relatively new System.Collections.Immutable package, available in .NET Framework versions 4.5 and above, and in all versions of .NET Core, also includes the System.Collections.Immutable.Dictionary<TKey, TValue> type, which is implemented using an AVL tree. The methods that would normally mutate the object in-place instead return a new ...
Immutable objects are easily shared, but require creating new extrinsic objects whenever a change in state occurs. In contrast, mutable objects can share state. Mutability allows better object reuse via the caching and re-initialization of old, unused objects. Sharing is usually nonviable when state is highly variable.
The immutable keyword denotes data that cannot be modified through any reference. The const keyword denotes a non-mutable view of mutable data. Unlike C++ const, D const and immutable are "deep" or transitive, and anything reachable through a const or immutable object is const or immutable respectively. Example of const vs. immutable in D
In computer science, having value semantics (also value-type semantics or copy-by-value semantics) means for an object that only its value counts, not its identity. [1] [2] Immutable objects have value semantics trivially, [3] and in the presence of mutation, an object with value semantics can only be uniquely-referenced at any point in a program.
Because fields in C# classes are always mutable, variantly parameterized classes in C# would not be very useful. But languages which emphasize immutable data can make good use of covariant data types. For example, in all of Scala, Kotlin and OCaml the immutable list type is covariant: List [Cat] is a subtype of List [Animal].
Many languages have explicit pointers or references. Reference types differ from these in that the entities they refer to are always accessed via references; for example, whereas in C++ it's possible to have either a std:: string and a std:: string *, where the former is a mutable string and the latter is an explicit pointer to a mutable string (unless it's a null pointer), in Java it is only ...