Using ConfigurationBuilder with a json file to add null to list

Question

I am trying to put a list of enums into appsettings.json that will be iterated through at one point in the code. The problem is that if I put a null value into the list in the json file, it does not populate a null value into the list when the Settings are loaded from the file.

minimum code example:

using Microsoft.Extensions.Configuration;
using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        var builder = new ConfigurationBuilder().
            AddJsonFile($"appsettings.json");

        var config = builder.Build();
        
        var test = config.GetRequiredSection("Settings").Get<Settings>();
        test.EnumList.Add(null);
    }
}

class Settings
{
    public enum EnumValues
    {
        Value1,
        Value2
    }
    public List<EnumValues?> EnumList { get; set; }
}

json file:

{
    "Settings": {
        "EnumList" :  [null, "Value1"]
    }
}

The test.EnumList.Add(null) will predictably and correctly add a null to the list, however, there will be no null in the list before that line, despite being in the json file. I would like to actually be able to put a null in the json file and have it show up in list.

How can I do this? I've hunted for a bit and I've come up with nothing.

Is there a way that you are using JsonOptions in your startup class? Are you using Newtonsoft? — Pablo Claus
– Pablo Claus, Commented Jul 8, 2022 at 21:40
Unfortunately, putting quotes around it does not work. I'm able to set other properties, not in lists, to null using just null. — EatATaco
– EatATaco, Commented Jul 12, 2022 at 14:51
Should have been more clear, Pablo, that I am using Microsoft.Extensions.Configuration. — EatATaco
– EatATaco, Commented Jul 12, 2022 at 14:53

pfx · Accepted Answer · 2022-07-12 21:33:16Z

2

+50

Retrieving that EnumList as a list of strings, shows that that null value is present, but has been stored as an empty string.

var strings = config.GetRequiredSection("Settings:EnumList").Get<List<string>>();

It looks like that null value gets lost when making that Get<Settings>() call.

The source code of the ConfigurationBinder at GitHub shows that an empty string or null value gets skipped for a generic nullable type - here List<Settings.EnumValues?>.

if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
{
    if (string.IsNullOrEmpty(value))
    {
        return true;
    }
    return TryConvertValue(Nullable.GetUnderlyingType(type)!, value, path, out result, out error);
}

To get arround that, declare that EnumList property as an EnumValues?[] array.

public class Settings
{
    public enum EnumValues
    {
        Value1,
        Value2
    }

    public EnumValues?[] EnumList { get; set; }
}

Now the parsed Settings will contain that null value - see image.

var test = config.GetRequiredSection("Settings").Get<Settings>();

With this array in place, you'll have to find an other way to add an item to it, e.g. something like below.

test.EnumList = test.EnumList
    .Concat(new Settings.EnumValues?[] 
    { 
        null // Or other value
    }).ToArray();

edited Jul 12, 2022 at 21:33

answered Jul 12, 2022 at 20:53

pfx

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1 Comment

EatATaco Over a year ago

Excellent. Thank you so much. This is a static value that is set only at start up, I actually encapsulate it so it can't be changed, so adding things to it is not necessary. I only did that to test that I could actually add a null to the list.

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