Troubleshooting and Debugging C#

The AutoMapper Issue Took Me Hours

Lately I got an easy task: transform some existing data models to new models defined in a third party library.

As agreed in the team, AutoMapper will be used for this job. Quickly, I finished most of them, but got stuck into a very simple one.

The existing model is Period , which is defined as

And the corresponding data model in the 3rd party library is looking like

As we can see, the fields Startand Endare exactly same:

• same property names
• same property types

So, the mapping code should be easy and straightforward

Unfortunately, the test failed, with the error messages below:

Error mapping types.
Mapping types:
Period -> Period
MyWork.Model.Period -> Library.Model.Period
Type Map configuration:
Period -> Period
MyWork.Model.Period -> Library.Model.Period
Destination Member:
Library.Model.Period.Void .ctor(Library.Model.FhirDateTime, Library.Model.FhirDateTime).parameter start

WHAT?!

The error message says, it failed at the member Start . However, it’s such a simple one!

Wait, let’s try to ignore the member Start and expect it to fail at End.

CreateMap<MyWork.Model.Period, Library.Model.Period>()
  .ForMember(d => d.Start, opt => opt.Ignore());

Unfortunately, it fails again, at the member Start , with the exactly same error messages!!!

I tried to

• map Start explicitly … got same errors!
• map StartElement manually … got same errors!
• ignore all members! Sorry, same errors!

I’ve been driven mad, by such a task, which looks so easy.

Ok, calm down.

I have to read the error messages again. And, found this line:

Destination Member:
Library.Model.Period.Void .ctor(Library.Model.FhirDateTime, Library.Model.FhirDateTime).parameter start

Wait, ctor is … Constructor!

Saying, it tried to create the object with this constructor,

instead of the one without parameters

and then, it found that the given parameters are strings, but NOT FhirDateTime

SO, IT FAILED.

The solution is simple: just use ConstructUsing to specify which constructor should be used.

CreateMap<MyWork.Model.Period, Library.Model.Period>()
   .ConstructUsing(x => new Library.Model.Period());

Now it works, as expected.

After put up the Pull Request, I checked the official docs of AutoMapper and found the reason: Constructor Mapping

Saying, if parameters of a constructor match the members, the mapped constructor will be used by default.

The doc also mentions how to disable it (see details from the link below).

Construction - AutoMapper documentation