Runtime: [System.Text.Json] DictionaryKeyPolicy for nested Dictionary does not work as expected when been set to JsonNamingPolicy.CamelCase

[Balder1840]

When I use System.Text.Json.JsonSerializer to serialize nested dictionary,
I found the key of the first level of the dictionary not set to camel case when I set DictionaryKeyPolicy to JsonNamingPolicy.CamelCase.

Any workaround?

System.Text.Json v4.7.0
.Net Core v3.1.1

Sample Code

```c#
var firstLevelDic = new Dictionary>();
var secondLevelDic = new Dictionary {
{ "FOO bar", "23" },
{ "FOObar", "23" },
{ "FooBar", "23" }
};

firstLevelDic.Add("FOO bar", secondLevelDic);
firstLevelDic.Add("FOObar", secondLevelDic);
firstLevelDic.Add("FooBar", secondLevelDic);

var options = new JsonSerializerOptions();
options.DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
options.WriteIndented = true;
var json = JsonSerializer.Serialize(firstLevelDic, options);

generated json
```json
{
  "FOO bar": {
    "foo bar": "23",
    "foObar": "23",
    "fooBar": "23"
  },
  "FOObar": {
    "foo bar": "23",
    "foObar": "23",
    "fooBar": "23"
  },
  "FooBar": {
    "foo bar": "23",
    "foObar": "23",
    "fooBar": "23"
  }
}

I tried Newtonsoft.Json, which works as expected
Sample Code
```C#
var firstLevelDic = new Dictionary>();
var secondLevelDic = new Dictionary {
{ "FOO bar", "23" },
{ "FOObar", "23" },
{ "FooBar", "23" }
};

firstLevelDic.Add("FOO bar", secondLevelDic);
firstLevelDic.Add("FOObar", secondLevelDic);
firstLevelDic.Add("FooBar", secondLevelDic);

DefaultContractResolver contractResolver = new DefaultContractResolver
{
NamingStrategy = new CamelCaseNamingStrategy()
{
ProcessDictionaryKeys = true
}
};

var settings = new JsonSerializerSettings
{
ContractResolver = contractResolver,
Formatting = Formatting.Indented
};

var json = Newtonsoft.Json.JsonConvert.SerializeObject(firstLevelDic, settings);

Result
```json
{
  "foo bar": {
    "foo bar": "23",
    "foObar": "23",
    "fooBar": "23"
  },
  "foObar": {
    "foo bar": "23",
    "foObar": "23",
    "fooBar": "23"
  },
  "fooBar": {
    "foo bar": "23",
    "foObar": "23",
    "fooBar": "23"
  }
}

[eltiare]

I am experiencing this bug also and would like a work-around. Perhaps writing my own converter that overrides an existing one?

[IvanOliinychenko]

A workaround could be using set options.DictionaryKeyPolicy to JsonNamingPolicy.CamelCase
and when creating dictionary use options.DictionaryKeyPolicy.ConvertName(featureName). That should convert key according to selected policy. If DictionaryKeyPolicy isn't set calling convert name will throw an exception.

[layomia]

This bug has been fixed for 5.0. The workaround is to implement custom dictionary converters as required. The policy can be obtained from options.DicitonaryKeyPolicy.

[mariusGundersen]

This also applies if the dictionary values are an array of strings, Dictionary<string, string[]>, which is the case for the ValidationProblemDetails.Errors dictionary in ASP.NET core.

Here is an example that reproduces the problem.

var result = JsonSerializer.Serialize(new
{
    StringString = new Dictionary<string, string>
    {
        ["Key"] = "Value"
    },
    StringArray = new Dictionary<string, string[]>
    {
        ["Key"] = new []{ "Value" }
    }
}, options: new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DictionaryKeyPolicy = JsonNamingPolicy.CamelCase,
    WriteIndented = true
});

Console.WriteLine(result);
/*
{
  "stringString": {
    "key": "Value"
  },
  "stringArray": {
    "Key": [
      "Value"
    ]
  }
}
*/

What is the code for the workaround?

[tlentine]

This also applies if the dictionary values are an array of strings, Dictionary<string, string[]>, which is the case for the ValidationProblemDetails.Errors dictionary in ASP.NET core.

Here is an example that reproduces the problem.

var result = JsonSerializer.Serialize(new
{
    StringString = new Dictionary<string, string>
    {
        ["Key"] = "Value"
    },
    StringArray = new Dictionary<string, string[]>
    {
        ["Key"] = new []{ "Value" }
    }
}, options: new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DictionaryKeyPolicy = JsonNamingPolicy.CamelCase,
    WriteIndented = true
});

Console.WriteLine(result);
/*
{
  "stringString": {
    "key": "Value"
  },
  "stringArray": {
    "Key": [
      "Value"
    ]
  }
}
*/

What is the code for the workaround?

We have encountered this exact issue as well. Fortunately it was only a few areas of our code base that were impacted, but it would be nice if there was a solution to this.