Empty objects loaded via ConfigurationBuilder.AddJsonFile() are ignored

[halter73]

Description

Empty objects loaded via ConfigurationBuilder.AddJsonFile() are ignored.

---

Repro

config.json

{
    "a": "b",
    "c": {
        "d": "e"
    },
    "f": "",
    "g": null,
    "h": {},
    "i": {
        "k": {}
    } 
}

Program.cs

using System;
using Microsoft.Extensions.Configuration;

namespace EmptyConfigSection
{
    class Program
    {
        static void Main(string[] args)
        {
            var config = new ConfigurationBuilder()
                .AddJsonFile("config.json")
                .Build();

            Console.WriteLine("{");

            foreach (var child in config.GetChildren())
            {
                PrintConfigSection(child, padding: "    ");
            }

            Console.WriteLine("}");
        }

        static void PrintConfigSection(IConfigurationSection configSection, string padding)
        {
            if (configSection.Value is null)
            {
                Console.WriteLine($@"{padding}""{configSection.Key}"": {{");

                foreach (var child in configSection.GetChildren())
                {
                    PrintConfigSection(child, $"    {padding}");
                }

                Console.WriteLine($"{padding}}},");
            }
            else
            {
                Console.WriteLine($@"{padding}""{configSection.Key}"": ""{configSection.Value}"",");
            }
        }
    }
}

EmptyConfigSection.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="5.0.0-preview.7.20364.11" />
  </ItemGroup>

  <ItemGroup>
    <Content Include="config.json">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
  </ItemGroup>

</Project>

---

Expected output

{
    "a": "b",
    "c": {
        "d": "e",
    },
    "f": "",
    "g": "",
    "h": {
    },
    "i": {
        "k": {
        },
    },
}

Actual output

{
    "a": "b",
    "c": {
        "d": "e",
    },
    "f": "",
    "g": "",
}

"h", "i" and "k" are not present in the actual output even though they should be.

The top-level call to IConfiguration.GetChildren() in Program.Main() should return an IEnumerable containing 6 top-level IConfigurationSections: "a", "c", "f", "g", "h" and "k". Instead it returns 4 top-level IConfiguration sections omitting both "h" and "i".

The repro program never even makes a call to GetChildren() that should return the "k" subsection, because it never sees "k"'s parent "i" in the first place.

Configuration

F:\dev> dotnet --info
.NET SDK (reflecting any global.json):
 Version:   5.0.100-preview.7.20330.3
 Commit:    eeb77e1a55

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.20161
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   F:\dev\aspnet\AspNetCore\.dotnet\sdk\5.0.100-preview.7.20330.3\

Host (useful for support):
  Version: 5.0.0-rc.1.20370.4
  Commit:  0e0e648770

I do not think this issue is specific to my configuration however.

Regression?

• I don't think this is a regression, but I cannot say for sure.

Tagging subscribers to this area: @maryamariyan
See info in area-owners.md if you want to be subscribed.

[davidfowl]

@HaoK do you remember if this was intentional?

[HaoK]

I don't think this is intentional, I'd imagine this is going to depend on the behavior of the json parser code: https://github.com/dotnet/runtime/blob/master/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs

One thing to try @halter73 , compare against the old Newtonsoft provider:

https://github.com/dotnet/extensions/blob/master/src/Configuration/Config.NewtonsoftJson/src/NewtonsoftJsonConfigurationExtensions.cs#L37

[HaoK]

Looking at the code: https://github.com/dotnet/runtime/blob/master/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs#L47

Doesn't look like empty objects are handled, so might just be a bug?

[HaoK]

Basically maybe there needs to be a special case in there that inserts an empty child section, right now looks like empty objects are just dropped with no entry into config (maybe the old json provider did the same thing though...) as the code is similar: https://github.com/dotnet/extensions/blob/master/src/Configuration/Config.NewtonsoftJson/src/NewtonsoftJsonConfigurationFileParser.cs#L41

[HaoK]

I could see how maybe this is intentional, as the only real functional difference between omitting empty sections would be for code that is looking at the hierarchy of config, as you would never get anything out of empty sections...

[HaoK]

I wasn't involved in the initial designs of config, so the actual design/intent for this code was before my time

[halter73]

I could see how maybe this is intentional, as the only real functional difference between omitting empty sections would be for code that is looking at the hierarchy of config, as you would never get anything out of empty sections...

And that's exactly why I noticed this now. See dotnet/aspnetcore#24286.

[HaoK]

Yeah, seems reasonable to fix the behavior so the config hierarchy gets empty sections so you don't have to do wacky stuff like add random junk to your sections for them to exist

[alefranz]

If this is up from grabs, I could try to look into it.

What is the expected behaviour?
Should it only add an empty child section when it is not already there?
If it already exists, it will not be clearing out the section from all the properties, right?

[halter73]

What's the normal behavior when multiple sources define the same config section?

I assume we'd want to do what we normally do when multiple sources define the same section. If sections with the same path from multiple sources normally get merged, then clearing out sections because the JSON source defined those sections as empty objects seems wrong. If it's the last source wins, and a JSON file defines sections as empty objects, then clearing out those sections seems right.

I don't know if this will be up for grabs. It hasn't been labeled that way yet.