Custom JsonConverter with parameters in .NET Core

Question

I wrote my own JsonConverter (using ASP.Net Core 3.1), which I intended to use on my DTOs.

In previous versions (Newtonsoft.Json), you could add a constructor on the custom JsonConverter class, and specify parameters using JsonConverterAttribute:

[JsonConverter(typeof(MyDecimalConverter), 3)]
public decimal MyProp { get; set; }

However, after the migration to System.Text.Json.Serialization, this option is not there anymore. The new JsonConverterAttribute simply does not have a constructor for it.

What is the new way of achieving this ?

Answer 1

After comparing the JsonConverterAttribute definition in Newtonsoft.Json and System.Text.Json.Serialization, we can find that: when using the System.Text.Json.Serialization, it doesn't allow to enter the converter parameters.

So, as dbc said, you could create a Custom JsonConverter to convert the decimal with 3 digits, like this:

public class MyDecimalConverter3Digit : JsonConverter<decimal>
{
    public override decimal Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
    public override void Write(Utf8JsonWriter writer, decimal value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(Decimal.Round(value, 3).ToString());
    }
}

Then, register the JsonConverter in Startup.ConfigureServices method:

        services.AddControllersWithViews().AddJsonOptions(options =>
        {
            options.JsonSerializerOptions.Converters.Add(new MyDecimalConverter3Digit());
        }); 

After that you could use it in the model, like this:

public class Calculate
{
    public decimal Price { get; set; }
    [JsonConverter(typeof(MyDecimalConverter3Digit))]
    public decimal Rate { get; set; }
}

Besides, you could also configure your application to use the Newtonsoft.Json serialize and deserialize json. Please refer the following steps:

1. Install the Microsoft.AspNetCore.Mvc.NewtonsoftJson package via NuGet or use the following command:

    Install-Package Microsoft.AspNetCore.Mvc.NewtonsoftJson
   

2. Add .AddNewtonsoftJson() method at the end of the AddControllersWithViews(), like this:

    services.AddControllers().AddNewtonsoftJson();
    services.AddControllersWithViews().AddNewtonsoftJson();
    services.AddRazorPages().AddNewtonsoftJson(); 
   

   When you create custom converter, remember to use the Newtonsoft reference, instead of System.Text.Json.Serialization. Like this:

   After that, you could use the custom converter with parameters.