Confusing error "needs to have a constructor with 0 args or only optional args"

[thomaslevesque]

Source/destination types

// Source
record Foo(string x, int y);
// Destination
record FooDto(string x, int z); // Note the z instead of y

Mapping configuration

cfg.CreateMap<Foo, FooDto>();

Version: 10.1.1

Expected behavior

Should give an error saying that it can't resolve a value for constructor argument z, or something similar.

Actual behavior

Gives the following error:

ArgumentException: FooDto needs to have a constructor with 0 args or only optional args. (Parameter 'type')

This is confusing, because FooDto doesn't actually need to have a constructor with 0 args or only optional args (it works fine with record FooDto(string x, int y)). The problem is really that it can't resolve the value for z, so the error should say that.

I started encountering the error quite frequently since I started using records. If there's a mismatch between properties of the source and destination types, I get this error. Now I know what to look for, but the first time I saw it I was really confused and it took me a while to understand what the problem was.

Steps to reproduce

mapper.Map<FooDto>(foo);

[lbargaoanu]

That seems difficult in the general case. But you're welcome to try a PR.

[thomaslevesque]

That seems difficult in the general case.

Yes, I was looking at the code right now, it doesn't seem straightforward. Still, I think it would be a big improvement, given the number of issues that mention this error. If it's too complex, maybe just change the message to something like this?

FooDto needs to have a constructor with 0 args, only optional args, or only args whose value can be resolved from the source type.

It's not perfect, but it would be less misleading.

(Nitpicking: it should probably say "parameters" rather than "args". Parameters are what you declare in the constructor, arguments are what you pass when you call it)

[lbargaoanu]

A lot of people have no idea that constructor mapping exists :) So I'm not convinced this is an improvement. I see your point though.

[lbargaoanu]

Anyway, you should be calling AssertConfigurationIsValid. That one says `No available constructor'. Not that helpful perhaps, but at least closer to the truth.

[thomaslevesque]

A lot of people have no idea that constructor mapping exists :)

Sorry, but that's not a very compelling argument... A lot of people do know about it, and with records it's going to be more and more common to use constructor mapping.

So I'm not convinced this is an improvement.

Given that the current message is actually wrong (since the type doesn't actually need a parameterless constructor), I think it can't hurt to improve it.

Anyway, you should be calling AssertConfigurationIsValid. That one says `No available constructor'. Not that helpful perhaps, but at least closer to the truth.

It also mentions the unmapped property, which is helpful.

Unmapped members were found. Review the types and members below.
Add a custom mapping expression, ignore, add a custom resolver, or modify the source/destination type
For no matching constructor, add a no-arg ctor, add optional arguments, or map all of the constructor parameters
==================================
Foo -> FooDto (Destination member list)
UserQuery+Foo -> UserQuery+FooDto (Destination member list)

Unmapped properties:
z
No available constructor.

I guess that's a good thing to do anyway.

BTW, what is the recommended way to validate configuration when using AutoMapper.Extensions.Microsoft.DependencyInjection? There are a few issues about it, but they're either closed without an answer, or suggest using the static Mapper class, which is no longer recommended.

[bartdebever]

Would like to add to this as I've just ran into this error trying to map a class to a record. Doing this makes the error even more unclear than from record to record. Only due to the existance of this thread I understand and can fix the issue.
Think this should have a better error detailing which property is not mapped.

A lot of people have no idea that constructor mapping exists :)

This is true yet shouldn't be true, it's a powerful feature especially with records on the rise and can only see it needing to be used more and more in the future.