The JSON value could not be converted to System.Nullable[System.Int32]

Question

I updated an ASP.NET Core 2.2 API to ASP.NET Core 3.0 and I am using System.Json:

services
  .AddMvc()
  .SetCompatibilityVersion(CompatibilityVersion.Version_3_0)
  .AddJsonOptions(x => {}) 

I then tried to post JSON data using Angular 8, which was working before:

{
  "name": "John"
  "userId": "1"
}

The model in the ASP.NET Core 3.0 API is:

public class UserModel {
  public String Name { get; set; }
  public Int32? UserId { get; set; } 
}

And the API Controller action is as follows:

[HttpPost("users")]
public async Task<IActionResult> Create([FromBody]PostModel) { 
}

When I submit the model I get the following error:

The JSON value could not be converted to System.Nullable[System.Int32]. 

Do I need to do something else when using System.Json instead of Newtonsoft?

Answer 1

Microsoft has removed Json.NET dependency from ASP.NET Core 3.0 and using System.Text.Json namespace now for serialization, deserialization and more.

You can still configure your application to use Newtonsoft.Json. For this -

1. Install Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package

2. In ConfigureServices() add a call to AddNewtonsoftJson()-

   services.AddControllers().AddNewtonsoftJson();

Read more on https://devblogs.microsoft.com/dotnet/try-the-new-system-text-json-apis/

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to

Answer 2

Here Via the json you pass a string value for UserId but your model refer a int32? value for UserId. Then how your value convert from string to int32?

Answer 3

I faced this issue: and all those solutions did not work for me! so I did the following:-

• First of all the data returned to me as the following:-
  

  I need to make year as an int and also want to make value as double!
  You should make custom JsonConverter, it worked for me after a lot of search, and here is sample of:-

StringToDoubleConverter

public sealed class StringToDoubleConverter : JsonConverter<double>
{
    public override double Read(
        ref Utf8JsonReader reader,
        Type typeToConvert, 
        JsonSerializerOptions options)
    {
        double.TryParse(reader.GetString(),out double value);
        return value;
    }

    public override void Write(
        Utf8JsonWriter writer,
        double value, 
        JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

Then you can save it to db! play around as you like