Why C# Records Outperform Mutable Classes for DTOs in Modern .NET

A technical analysis argues that mutable classes with get/set properties are the wrong data structure for Data Transfer Objects (DTOs) in C# development, a practice that has persisted since .NET Framework 2.0. Since C# 9 introduced records, developers have had a purpose-built, immutable alternative better suited to representing data snapshots passed across system boundaries. The article draws on examples from fintech and banking systems, where silent mutations to shared objects can cause real-world failures such as duplicate payments, corrupted audit trails, and race conditions in async pipelines. DTOs, described as snapshots of data at a point in time, should be immutable by design — much like a printed bank statement that no one should be able to alter after the fact. The case is made for records not merely as a stylistic preference but as a structurally correct choice that reduces production risk in distributed systems.