c# - Unexpected Duplicate Configuration Values in .NET 8 WorkerService - Stack Overflow

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I'm developing a .NET 8 WorkerService project where I load configuration from multiple sources (e.g., appsettings.json and appsettings.Development.json). My configuration file defines a section Units that holds an array of objects, each containing a name and an array of paths.

Here's a simplified version of my code:

Program.cs:

var builder = Host.CreateApplicationBuilder(args);

var units = builder.Configuration.GetSection("Units").Get<Unit[]>();

foreach (var unit in units)
{
    foreach (var item in unit.Paths)
    {
        Console.WriteLine(item);
    }
}

public record Unit(string Name, string[] Paths);

appsettings.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "Units": [
    {
      "name": "test1",
      "paths": [
        "p1",
        "p2",
        "p3"
      ]
    },
    {
      "name": "test2",
      "paths": [
        "p4",
        "p5",
        "p6"
      ]
    }
  ]
}

appsettings.Development.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  }
}

When I run the project, I expect the output to be:

p1
p2
p3
p4
p5
p6

However, the actual output is:

p1
p2
p3
p1
p2
p3
p4
p5
p6
p4
p5
p6

Question: Why am I getting duplicate values? Has anyone encountered this behavior before and can shed some light on what's causing it?

Any insights would be greatly appreciated!

I'm developing a .NET 8 WorkerService project where I load configuration from multiple sources (e.g., appsettings.json and appsettings.Development.json). My configuration file defines a section Units that holds an array of objects, each containing a name and an array of paths.

Here's a simplified version of my code:

Program.cs:

var builder = Host.CreateApplicationBuilder(args);

var units = builder.Configuration.GetSection("Units").Get<Unit[]>();

foreach (var unit in units)
{
    foreach (var item in unit.Paths)
    {
        Console.WriteLine(item);
    }
}

public record Unit(string Name, string[] Paths);

appsettings.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "Units": [
    {
      "name": "test1",
      "paths": [
        "p1",
        "p2",
        "p3"
      ]
    },
    {
      "name": "test2",
      "paths": [
        "p4",
        "p5",
        "p6"
      ]
    }
  ]
}

appsettings.Development.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  }
}

When I run the project, I expect the output to be:

p1
p2
p3
p4
p5
p6

However, the actual output is:

p1
p2
p3
p1
p2
p3
p4
p5
p6
p4
p5
p6

Question: Why am I getting duplicate values? Has anyone encountered this behavior before and can shed some light on what's causing it?

Any insights would be greatly appreciated!

• c#
• asp-core
• -core
• configuration
• record

Share Improve this question Follow edited Mar 14 at 16:37 Ivan Petrov 5,46222 gold badges1111 silver badges2424 bronze badges asked Mar 14 at 11:36 Christopher FontaineChristopher Fontaine 3566 bronze badges 3

• I tried to recreate your problem in a .NET 8 console app with the code you provided and I got the expected output. You're probably accidently importing the appsettings twice or something. – Shoejep Commented Mar 14 at 12:08
• @Shoejep The provided code exactly produces the behavior I described. In the accepted answer, Ivan Petrov explains the underlying cause of the duplicate binding, clarifying that the issue is due to how the configuration binder processes the record with a primary constructor. Please refer to his answer for the detailed explanation. – Christopher Fontaine Commented Mar 16 at 8:09
• 1 I realised I installed Microsoft.Extensions.Hosting 9.0.3 when trying to reproduce your issue and the issue doesn't exist there. Apologies. – Shoejep Commented Mar 17 at 9:17

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1 Answer 1

Sorted by: Reset to default 1

The issue is with you using a record which by default

1. doesn't have a parameterless constructor
2. also has properties that even if init only have a setter that configuration binding apparently uses to set values a SECOND time

Workaround if you want to continue using a record is to declare a parameterless constructor:

public record Unit(string Name, string[] Paths)
{
    public Unit() : this(string.Empty, Array.Empty<string>()) { }
}

If you want to reproduce it, you can declare a normal class that resembles a record:

public class Unit
{
    public Unit(string name, string[] paths)
    {
        Name = name;
        Paths = paths;
    }
    public string Name { get; init }
    public string[] Paths { get; init }
}

If you remove the init - you won't get duplicate values inside the Paths array.

So in .NET7+ where the feature of binding to classes with parameterized constructor was made available the logic seems to be:

1. Execute the parameterized constructor and pick the configuration elements for the parameters based on type or some other heuristic
2. If a property has a setter -> Find what is there by calling the getter and then instead of overwriting it, append to it in case of a colleciton

I couldn't find a documentation for this behavior, but the following issues might shed some light if it's a bug or a feature:

1. Support immutable types with configuration binding
2. [Feature Request] Allow Options service to bind config to class with parameterised ctor
3. Support immutable types with configuration binding

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