System.Text.Json: Deserialize JSON with automatic casting

Question

Using .Net Core 3's new System.Text.Json JsonSerializer, how do you automatically cast types (e.g. int to string and string to int)? For example, this throws an exception because id in JSON is numeric while C#'s Product.Id is expecting a string:

public class HomeController : Controller
{
    public IActionResult Index()
    {
        var json = @"{""id"":1,""name"":""Foo""}";
        var o = JsonSerializer.Deserialize<Product>(json, new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true,
        });

        return View();
    }
}

public class Product
{
    public string Id { get; set; }
    public string Name { get; set; }
}

Newtonsoft's Json.Net handled this beautifully. It didn't matter if you were passing in a numeric value while C# was expecting a string (or vice versa), everything got deserialized as expected. How do you handle this using System.Text.Json if you have no control over the type format being passed in as JSON?

Answer 1

1. The new System.Text.Json api exposes a JsonConverter api which allows us to convert the type as we like.

   For example, we can create a generic number to string converter:

   public class AutoNumberToStringConverter : JsonConverter<object>
   {
       public override bool CanConvert(Type typeToConvert)
       {
           return typeof(string) == typeToConvert;
       }
       public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
       {
           if(reader.TokenType == JsonTokenType.Number) {
               return reader.TryGetInt64(out long l) ?
                   l.ToString():
                   reader.GetDouble().ToString();
           }
           if(reader.TokenType == JsonTokenType.String) {
               return reader.GetString();
           }
           using(JsonDocument document = JsonDocument.ParseValue(ref reader)){
               return document.RootElement.Clone().ToString();
           }
       }
   
       public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
       {
           writer.WriteStringValue( value.ToString());
       }
   }
   

2. When working with MVC/Razor Page, we can register this converter in startup:

   services.AddControllersWithViews().AddJsonOptions(opts => {
       opts.JsonSerializerOptions.PropertyNameCaseInsensitive= true;
       opts.JsonSerializerOptions.Converters.Insert(0, new AutoNumberToStringConverter());
   });
   

   and then the MVC/Razor will handle the type conversion automatically.

3. Or if you like to control the serialization/deserialization manually:

   var opts = new JsonSerializerOptions {
       PropertyNameCaseInsensitive = true,
   };
   opts.Converters.Add(new AutoNumberToStringConverter());
   var o = JsonSerializer.Deserialize<Product>(json,opts) ;
   

4. In a similar way you can enable string to number type conversion as below :

   public class AutoStringToNumberConverter : JsonConverter<object>
   {
       public override bool CanConvert(Type typeToConvert)
       {
           // see https://stackoverflow.com/questions/1749966/c-sharp-how-to-determine-whether-a-type-is-a-number
           switch (Type.GetTypeCode(typeToConvert))
           {
               case TypeCode.Byte:
               case TypeCode.SByte:
               case TypeCode.UInt16:
               case TypeCode.UInt32:
               case TypeCode.UInt64:
               case TypeCode.Int16:
               case TypeCode.Int32:
               case TypeCode.Int64:
               case TypeCode.Decimal:
               case TypeCode.Double:
               case TypeCode.Single:
               return true;
               default:
               return false;
           }
       }
       public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
       {
           if(reader.TokenType == JsonTokenType.String) {
               var s = reader.GetString() ;
               return int.TryParse(s,out var i) ? 
                   i :
                   (double.TryParse(s, out var d) ?
                       d :
                       throw new Exception($"unable to parse {s} to number")
                   );
           }
           if(reader.TokenType == JsonTokenType.Number) {
               return reader.TryGetInt64(out long l) ?
                   l:
                   reader.GetDouble();
           }
           using(JsonDocument document = JsonDocument.ParseValue(ref reader)){
               throw new Exception($"unable to parse {document.RootElement.ToString()} to number");
           }
       }
   
   
       public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
       {
           var str = value.ToString();             // I don't want to write int/decimal/double/...  for each case, so I just convert it to string . You might want to replace it with strong type version.
           if(int.TryParse(str, out var i)){
               writer.WriteNumberValue(i);
           }
           else if(double.TryParse(str, out var d)){
               writer.WriteNumberValue(d);
           }
           else{
               throw new Exception($"unable to parse {str} to number");
           }
       }
   }