Specification VS Result

If so, then look into this

Also, to minimize duplication of code and optimize your architecture i suggest using something like https://github.com/ardalis/Specification

This is a great candidate project for the Ardalis.Specifications library.

This is honestly such an amazing way to make sure you have a clear view of what your Linq does, see https://github.com/ardalis/Specification - note that the examples on the front git page uses a repository, but he later allowed the use directly on DbSet which finally made it possible for me to use the library.

Check out the specification pattern - it is designed to encapsulate query logic similar to your example. Steve Smith has a nice library for it on GitHub: https://github.com/ardalis/Specification

I'm thinking that the only way to abstract both of them would be to have the repositories return IEnumerables and have well defined aggregates for the entire application to avoid an exploding number of methods on the interface, but this is not possible in my project*. I've looked at Steve Smith's Specification library, but since it works with IQueryable, it would not work with Dapper.

Using the Nuget Package Ardalis.Result for mapping result objects to HTTP Response Codes and associated objects; FastEndpoint has its own way of approaching this problem, which seems to be to hang everything off an "Endpoint" and optionally using Swagger to redundantly describe the route's behavior; this seems like a step backwards from more advanced Web frameworks in other languages. The advantage of Ardalis.Result should be clear - each closed form of Ardalis.Result can have its own knowledge of what it does, and therefore automatically describe the abstract transformations it does on a request. In functional programming, this "abstract transformation" is typically called a monad transformer or Arrow. Once you have a type that describes the abstract transformation, plugging that into Swagger should be straight-forward.

You could also use the Ardalis Guard Clause, Ardalis Result and Fluent Validation

I use this generic package. https://github.com/ardalis/Result

For you're Result.cs , look at [Result pattern]https://github.com/ardalis/Result) as an example.