Weird Behavior with Class + Primary Constructor vs Record Type

[ardalis]

Had a weird behavior today. ChatGPT solved it but I don't know exactly why the behavior was how it was.

Situation

I had a working set of code with basically 2 projects (web and usecases). I refactored it into 4 projects so I could share some of the base types involved and apply a modular monolith approach. After the refactor, I made sure to have MediatR scan all of the assemblies in play, but I was getting an error on a .Send() call.

"No service for type 'MediatR.IRequestHandler`2[myQuery,myResult]' has been registered." in System.Private.CoreLib.dll

The Query (which worked fine before!)

public class TasksByAssignedIdQuery(int employeeId) : IQuery<Result<List<TaskDto>>>
{
	public int EmployeeId { get; set; } = employeeId;
}

The Diagnosis

ChatGPT said:

By writing it as:

public class TasksByAssignedIdQuery(int employeeId) : IQuery<...>

you've created a class with a primary constructor, which was only added in C# 12 and does not behave like a record — it does not auto-generate value-based equality, and more importantly:

It does not automatically make the query implement IRequest unless everything aligns perfectly — and in practice, it often does not work well with frameworks expecting traditional constructor injection or reflection.

Use a record instead.

The Fix

I changed the message type to:

public record TasksByAssignedIdQuery(int EmployeeId) : IQuery<Result<List<TaskDto>>>
{
	//public int EmployeeId { get; set; } = employeeId;
}

and indeed that fixed it. My question is, why? And is this a bug in MediatR that it doesn't handle message types with primary constructors in some scenarios?

[jbogard]

This is a bit odd, there must be something wonky here. I'll take a look!