Question 109166 in Json Question posted in
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System.Text.Json serializes/deserialize nullable reference types to "{}" as opposed to "null"

Trying to convert an old ASP.NET Core project from Newtonsoft.Json to System.Text.Json but it changes the behavior of nullable reference type conversion. System.Text.Json serializes/deserialize nullable reference types to "{}" as opposed to "null". How do I fix it? I want to get rid of Newtonsoft.Json.

services.AddMvc().AddNewtonsoftJson();

Here is the type for that List public IList<IDictionary<string, object>> List { get; set; }. The actual value type is DBNull.

System.Text.Json

{
    "data": {
        "list": [
            {
                "headers": [],
                "list": [
                    {
                        "id": "3faab29e-982d-ccb9-197b-08db24ef09e7",
                        "contractType": "Basis",
                        "tEvent": "Edit",
                        "Basis": 0.0000,
                        "fees": {},
                        "FlatPrice": {},
                        "theirContract": {}
                    },

Newtonsoft.Json

{
    "data": {
        "list": [
            {
                "headers": [],
                "list": [
                    {
                        "id": "3faab29e-982d-ccb9-197b-08db24ef09e7",
                        "contractType": "Basis",
                        "tEvent": "Edit",
                        "Basis": 0.0000,
                        "fees": null,
                        "FlatPrice": null,
                        "theirContract": null
                    },
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2

Answers


  1. 0

    As stated here there is not built-in support for DBNull.

    You have to use a custom converter:

    class DBNullConverter : System.Text.Json.Serialization.JsonConverter<DBNull>
{
    public override DBNull Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        throw new NotSupportedException();           
    }

    public override void Write(Utf8JsonWriter writer, DBNull value, JsonSerializerOptions options)
    {
        writer.WriteNullValue();
    }
}

    Then add it to the options:

    var serializeOptions = new JsonSerializerOptions
{
    Converters =
    {
        new DBNullConverter()
    }
}
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  2. 0

    As at the time of writing, System.Text.Json does not have built-in support for DBNull.

    Instead, you can write a custom JsonConverter to handle DBNull values, by overriding the converter’s Write() method to print out the JSON literal null instead:

    public class DBNullJsonConverter : JsonConverter<DBNull> 
{ 
    public override bool CanConvert(Type typeToConvert) 
    { 
        return typeToConvert == typeof(DBNull); 
    } 
 
    public override DBNull Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) 
    { 
        throw new NotImplementedException(); 
    } 
 
    public override void Write(Utf8JsonWriter writer, DBNull value, JsonSerializerOptions options) 
    { 
        // write the JSON literal null
        writer.WriteNullValue(); 
    } 
}

    Then, pass this converter to the JsonSerializerOptions of your JsonSerializer:

    var options = new JsonSerializerOptions();
options.Converters.Add(new DBNullJsonConverter());

// you should get null for fields of type DBNull here
var json = JsonSerializer.Serialize(someObject, options);
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