Behavior changed during binding to collections in Net 8

[bbenameur]

Description

Duplicated properties when deserialize array from appsetting.json only with .Net 8

Configuration

on the appsetting.json i have an object with this structure:

 "ArrayObject": [
   {  
     "Elements": 
       {
         "Type": "xxxx",
         "BaseUrl": "xxxxxxxx",
         "one": "",
         "xxxx": ""

       }
     
   }
 ]

There are a stranger behavior only with .Net 8
using

var builder = WebApplication.CreateBuilder(args);

var result = builder.Configuration.GetSection("ArrayObject").Get<List<ArrayObject>>();
 the result this case will be 

Regression?

As you see Elements should be null and empty but with Net 8 no it's take the number of properties on the elements objects.

Other information

It's ok with Net 6 see image

[ghost]

Tagging subscribers to this area: @dotnet/area-extensions-configuration
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

Duplicated properties when deserialize array from appsetting.json only with .Net 8

Configuration

on the appsetting.json i have an object with this structure:

 "ArrayObject": [
   {  
     "Elements": 
       {
         "Type": "xxxx",
         "BaseUrl": "xxxxxxxx",
         "one": "",
         "xxxx": ""

       }
     
   }
 ]

There are a stranger behavior only with .Net 8
using

var builder = WebApplication.CreateBuilder(args);

var result = builder.Configuration.GetSection("ArrayObject").Get<List<ArrayObject>>();
 the result this case will be 

Regression?

As you see Elements should be null and empty but with Net 8 no it's take the number of properties on the elements objects.

Other information

It's ok with Net 6 see image

Author:	bbenameur
Assignees:	-
Labels:	untriaged, area-Extensions-Configuration
Milestone:	-

[bbenameur]

@ericstj

[bbenameur]

@eerhardt

[bbenameur]

@davidfowl

[eerhardt]

@bbenameur - can you post a complete repro? The code above doesn't compile since it doesn't define ArrayObject.

[bbenameur]

Hello @eerhardt
this is the project
https://github.com/bbenameur/demo-project
debug please and see the result on program.cs
var result = builder.Configuration.GetSection("ArrayObject").Get<List>();

[ericstj]

Confirmed the change in behavior, here's a simplified repro in console app. Having a look for the cause to determine if it's intentional or not.
binding-repro.zip

[bbenameur]

Yes i confirm that the behavior is changed

[bbenameur]

@ericstj Am I in the right group dotnet/runtime ?

[ericstj]

Yes this is the right group and area. I'm updating the issue as I understand more. Thank you for the report.

This is the cause: e7e9dbd#diff-2e6f75e1d4f577e1c8c1b6a32d158ce7812a38ad8fcd3b4d762157b92c03e890R420

I read through that PR and didn't see why we were adding another case here. Moreover - we'll only create the instance and never bind any of it's properties which seems wrong. If I change the type to have a constructor, then it will fail:

public class Element
{
	public Element(string type) { Type = type; }
	public string Type { get; set; }
}

var result = config.GetSection("ArrayObject").Get<List<ArrayObject>>(o => o.ErrorOnUnknownConfiguration = true);

System.InvalidOperationException: 'Cannot create instance of type 'Element' because parameter 'type' has no matching config. Each parameter in the constructor that does not have a default value must have a corresponding config entry.'

This is definitely a regression that we'll look into fixing. @bbenameur - is this blocking you? Seems like your JSON might have been set up in a way that didn't match your objects and you weren't noticing that until now. Can you fix either the JSON or your type shape to make things work correctly?

[bbenameur]

Thanks @ericstj for your quick response,
We were able to bypass this regression with a piece of code (not clean), but no blocking for the moment,
please I would like to receive a notification when the problem is resolved

[tarekgh]

Looks we fixed this in 9.0 #96737. We were watching to see if more reports on the issue before porting it to servicing. I'll confirm first the fix address this issue and then will port it to servicing.

[tarekgh]

Update:

#96737 doesn't address this issue. I'll look more here at the right fix.

[ericstj]

I originally misidentified the regressing change, it was actually #86485 which was fixing Configuration Binding a dictionary with a key that has empty value skips the key as well - that makes more sense.

It still seems wrong to create an instance when operating on a config section with a value, that value cannot be directly assigned, and there are no child sections. Probably the fix for the above didn't scope down the config input enough - it handled no-child values, but didn't check for no value. Maybe we only do this if the config.value is null as well.

[bbenameur]

The behavior is ok only if the element class has a constructor and without o => o.ErrorOnUnknownConfiguration = true

public class Element
{
	public Element(string type) { Type = type; }
	public string Type { get; set; }
}

[tarekgh]

@bbenameur

The behavior is ok only if the element class has a constructor and without o => o.ErrorOnUnknownConfiguration = true

This is expected because the binder will not be able to create instance of the Element class. When public Element(string type) is there, the binder can create the instance using the default constructor provided by the compiler.

[bbenameur]

Hello @tarekgh
So you confirm that it will be fixed on .Net 9?

[tarekgh]

@bbenameur For sure we are going to fix it in 9.0. We'll consider porting the fix back 8.0 if the changes are not risky and scoped. The fix for this is going to span the source generator too which I am currently looking at.

[tarekgh]

This is now fixed in 9.0. We'll watch if we'll get more reports and then consider porting the fix to 8.0.