Duplicate List<T> items when binding class with default constructor containing params keyword to IConfiguration

[DawidBartniczak]

Description

Reading JSON configuration file (appsettings.json) and binding it to a class with default constructor containing params keyword using ConfigurationBinder.Get<T> results in a duplicate list items.

Reproduction Steps

1. Create a class with default constructor containing params keyword and a getter only List field that has the value from default constructor assigned
2. Create a section in appsettings.json
3. Bind that section to the class
4. The resulting list has a duplicated item

Example that triggers the bug

public sealed record ErpConfigurationOptions(
    NexoConfiguration? NexoConfiguration = null,
    BlackBaseConfiguration? BlackBaseConfiguration = null)
{
    public const string SectionName = "ErpConfiguration";
}

public class BlackBaseConfiguration(params List<BlackBaseInstance> instances)
{
    public List<BlackBaseInstance> Instances { get; } = instances;
}

public record BlackBaseInstance(int Id, string InstanceName, char Symbol, string DbfDirectory);

var erpConfig = configuration.GetSection(ErpConfigurationOptions.SectionName)
  .Get<ErpConfigurationOptions>() ?? throw new InvalidOperationException("ERP configuration is missing.");

{
  "ErpConfiguration": {
    "NexoConfiguration": {
      "SqlConnectionString": ""
    },
    "BlackBaseConfiguration": {
      "Instances": [
        {
          "Id": 1,
          "InstanceName": "Poznan",
          "Symbol": "P",
          "DbfDirectory": "/mnt/dbf"
        }
      ]
    }
  }
}

Expected behavior

No duplicate should occur.

Actual behavior

A duplicated list item is read from JSON config which only contains one list item.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

Microsoft.Extensions.Configuration v10.0.9

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[MihuBot]

I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):

• Attempting to Get a configuration section using a type defined as a record will duplicate collection values. #83803

[tarekgh]

Thanks for reporting this.

The root cause is in the reflection based configuration binder. When a type is created through a constructor and the constructor parameter name differs only by case from a collection property (for example a parameter instances and a property Instances), the binder bound the property a second time in addition to the constructor, which appended the items again and produced the duplicates. The rest of the binder matches constructor parameters and properties case-insensitively, so this was an inconsistency in one comparison.

Plan:

• PR Fix duplicated collection items when a constructor parameter name differs only by case from the property #129775 fixes the reflection based runtime binder for this case (class with a constructor, including a params collection constructor). This issue will be closed when that PR is merged.
• The configuration source generator has the same problem and is tracked by Attempting to Get a configuration section using a type defined as a record will duplicate collection values. #83803. The source generator fix should cover both the record case and the class case, and it will be handled with that tracking issue.