When you first encounter the readonly keyword in C#, it seems simple: it prevents a field from being reassigned outside the constructor. However, when applied to reference types, important nuances arise that every developer should know to write safer and cleaner code. At Q2BSTUDIO, a custom software and application development company specialized in artificial intelligence, cybersecurity, and AWS and Azure cloud services, we work with these best practices to deliver robust and scalable custom software.
What does readonly really do in C#? In C#, readonly applies to fields and guarantees that the field can only be assigned in the declaration or within the constructor of the class or struct. Once assigned, the field cannot point to a different object. This protects the reference but does not prevent the referenced object from changing its internal state.
Conceptual example span public class User { public string Name { get; set; } } public class Account { private readonly User _user = new User(); public void ChangeUser() { // It is possible to modify the internal state _user.Name = NewName; // But this produces a compile-time error _user = new User(); } }
Frequent misunderstanding A readonly field of a reference type does not make the object immutable; it only blocks the reassignment of the reference. The object itself can mutate if its members allow mutation.
Value types vs. reference types with readonly With value types such as int, bool, or struct, readonly prevents changing the value after assignment. With reference types, readonly prevents reassigning the variable to another object but allows modifying the content of the object. Conceptual example span readonly int numero = 5; numero = 10; // compile error span readonly List<string> items = new List<string>(); items.Add(Test); // possible items = new List<string>(); // compile error
How to make a reference type truly immutable For an object's state to be immutable, it must be intentionally designed that way: avoid setters in properties; declare fields as readonly; use init for initialization only; do not expose internal collections directly; use immutable collections from System.Collections.Immutable or expose read-only interfaces like IReadOnlyList<T> and return defensive copies when necessary. Additionally, consider using record types and init properties introduced in C# 9 to facilitate immutable objects.
Example of an immutable approach conceptual public class User { public string Name { get; init; } } Another option is records record UserRecord(string Name); both approaches prevent modifications after initialization.
Readonly and properties Although readonly applies to fields, in properties a similar behavior can be emulated using init so that the property is immutable after initialization. Additionally, it is good practice not to expose mutable collections; instead, expose IReadOnlyCollection<T> or use System.Collections.Immutable to guarantee immutability.
Recommended best practices for development teams Avoid exposing public fields; prefer properties with get and init or getters only; use readonly on fields that must remain referenced to the same object; use immutable collections or defensive copies; apply deep immutability when the domain requires it by also making nested objects immutable; and clearly document design decisions to facilitate maintenance and security, especially in concurrent environments.
Impact on security and software quality At Q2BSTUDIO, we combine these practices with quality controls, automated testing, and secure design principles to offer custom software that meets cybersecurity and scalability requirements. Our experience in artificial intelligence (AI) for businesses, AI agents, business intelligence services, and Power BI allows us to design solutions that not only work but are maintainable and secure in cloud environments such as AWS and Azure cloud services.
Conclusion readonly prevents reference reassignment but does not guarantee the immutability of the referenced object. To obtain truly immutable objects, a set of design techniques must be applied: init properties or records, readonly fields, immutable collections, and defensive copies. If you are looking to develop custom applications, custom software, or integrate artificial intelligence and cloud services with a focus on cybersecurity and business intelligence, contact Q2BSTUDIO to take your project to the next level with solutions in Power BI, AI agents, and business intelligence services.
