ArrayOrObjectJsonConverter

App.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Spectre.Console" Version="0.43.0" />
  </ItemGroup>

</Project>

Program.cs

using System.Runtime.CompilerServices;
using System.Text.Json;
using System.Text.Json.Serialization;
using Spectre.Console;

const string Null = "null";
const string EmptyArray = "[]";
const string WrapperWithEmptyArray = @"{
  ""items"": []
}";
const string Array = @"[
  {
    ""id"": 123,
    ""text"": ""some text""
  },
  {
    ""id"": 456,
    ""text"": ""some more text""
  }
]";
const string Wrapper = @"{
    ""items"": [
        {
           ""id"": 123,
            ""text"": ""some text""
        },
        {
            ""id"": 456,
            ""text"": ""some more text""
        }
    ]
}";

var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    WriteIndented = true
};
options.Converters.Add(new ArrayOrObjectJsonConverter<SomeDto>());

DeserializeAndPrint(Null);
DeserializeAndPrint(EmptyArray);
DeserializeAndPrint(WrapperWithEmptyArray);
DeserializeAndPrint(Array);
DeserializeAndPrint(Wrapper);

void DeserializeAndPrint(string json, [CallerArgumentExpression("json")] string title = "")
{
    var result = JsonSerializer.Serialize(JsonSerializer.Deserialize<IReadOnlyCollection<SomeDto>>(json, options), options);

    AnsiConsole.Write(new Table()
        .AddColumn(new TableColumn(title))
        .AddRow(new Table {Alignment = Justify.Center}
            .AddColumn("original")
            .AddRow(new Text(json)))
        .AddRow(new Text("↓↓↓↓↓") {Alignment = Justify.Center})
        .AddRow(new Table { Alignment = Justify.Center }
            .AddColumn("deserialized")
            .AddRow(new Text(result))));
}

public record SomeDto(int Id, string Text);

public class ArrayOrObjectJsonConverter<T> : JsonConverter<IReadOnlyCollection<T>>
{
    public override IReadOnlyCollection<T>? Read(
        ref Utf8JsonReader reader,
        Type typeToConvert,
        JsonSerializerOptions options)
        => reader.TokenType switch
        {
            JsonTokenType.StartArray => JsonSerializer.Deserialize<T[]>(ref reader, options),
            JsonTokenType.StartObject => JsonSerializer.Deserialize<Wrapper>(ref reader, options)?.Items,
            _ => throw new JsonException()
        };

    public override void Write(Utf8JsonWriter writer, IReadOnlyCollection<T> value, JsonSerializerOptions options)
        => JsonSerializer.Serialize(writer, (object?) value, options);

    private record Wrapper(T[] Items);
}