Hmm. I'm not sure why this is a non-existing property error.

The DTO has the property $extra, with a default of null (or something else).

The source object is a simple StdClass that doesn't have the property. It's not setting a non-existing property of the target object, but rather set a default on the target object when the source object is missing the field.

I think it's a pretty common case -- the source and target have some common fields, create the target object and map with the property name is the same or the property has a #[Map()] attribute.

the "isset" was just something I tried because it seemed like logic that would work, but didn't. Could be a different issue.

The source object is a simple StdClass that doesn't have the property. It's not setting a non-existing property of the target object, but rather set a default on the target object when the source object is missing the field.

In your example:

$mapper = new ObjectMapper();
$target = 'App\Dto\MyClass';
$obj = (object) ['id'=>'abc'];
$entity = $mapper->map($obj, $target);

The stdClass does not have any information about how it should be mapped, therefore we use the data from the $target (here App\Dto\MyClass. App\Dto\MyClass has a default but when we read that property on the stdClass it does not exists. So it's not "setting a non-existing property" but "reading a non-existing property" which leads to a NoSuchPropertyException. Looks fine to me.
The default value works fine when using the PropertyAccessor with the proper option see my assertions in the tests:

        $this->assertEquals('abc', $b->id);
        $this->assertEquals(null, $b->optional);

I think it's a pretty common case -- the source and target have some common fields, create the target object and map with the property name is the same or the property has a #[Map()] attribute.

stdClass is quite specific, use DTOs on both ends of the mapping and you probably won't have any issues.

the "isset" was just something I tried because it seemed like logic that would work, but didn't. Could be a different issue.

Isset would be a weird use case but NotNull would be quite useful.

Thanks. My use case is that I'm getting data from csv and JSON, and I want to map the data to a DTO. But the field names and often type are wrong.

So

First Name,Last Name,Country,Language Spoken At Home
Bob,Roberts,USA,English

And I want a DTO of a class with properties $firstName, $lastName,$countryCode,$langCode

So I want to import from $row['First Name'] to $dto->firstName and transform the country and language names to codes them with a service. ObjectMapper seems like the perfect tool for that. So this step is to go from the stdClass to a DTO. What approach do you suggest? I've been using league/csv-reader for some parts, (which will map to a DTO), but the problem is that I don't have a DTO yet.