mapperly

Mapperly writes your .NET object mapping code at compile time. Moving data from a database entity to an API response object means a pile of tedious property assignments, and AutoMapper handled that for years using runtime reflection. Mapperly does the same job as a Roslyn source generator: declare a partial class with a Mapper attribute and the method signatures, and the implementation lands in your build. Apache 2.0, free, on NuGet.

The wins are speed and debuggability. No reflection at runtime means mapping costs the same as hand-written assignments, and the generated code is plain C# you can step through and read in a diff. Mistakes become compile errors instead of production exceptions. AutoMapper's newer versions moved to commercial licensing, which sent a lot of .NET teams looking for exactly this.

One package reference. Nothing to host, nothing to configure at runtime, no ops burden at any team size.

The catch is that compile-time generation costs you runtime flexibility. Mapping shapes you only learn from data at request time is not what this is for, and complex custom conversions take more attribute wiring than the equivalent AutoMapper profile. You trade flexibility for a build that fails loudly instead of a request that fails quietly.