Find invalid values from JsonReaderException in Json.Net - asp.net-core

Question

I'm running the code below to purposely throw JsonReaderException. It correctly gives the exception message of "Could not convert string to boolean: aaa. Path 'Active', line 3, position 17."
Is there any way to get the value that has failed the validation directly from the JsonReaderException so I don't have to parse the exception message?
string json = #"{
'Email': 'james#example.com',
'Active': 'aaa',
'CreatedDate': '2013-01-20T00:00:00Z',
'Roles': [
'User',
'Admin'
]
}";
try
{
Account account = JsonConvert.DeserializeObject<Account>(json);
Console.WriteLine(account.Email);
}
catch (JsonReaderException exc)
{
// Do Something
}

Answer 1

It appears that the offending value is not saved as a property in JsonReaderException. The only possible location for this value would be the Exception.Data dictionary, however Json.NET does not add anything here.
However, with some work you can leverage Json.NET's serialization error event handling functionality to directly access the bad value at the time the exception is thrown. First, define the following helper method and ErrorEventArgs subtype:
public class ErrorAndValueEventArgs : Newtonsoft.Json.Serialization.ErrorEventArgs
{
public object ReaderValue { get; } = null;
public ErrorAndValueEventArgs(object readerValue, object currentObject, ErrorContext errorContext) : base(currentObject, errorContext)
{
this.ReaderValue = readerValue;
}
}
public static partial class JsonExtensions
{
public static TRootObject Deserialize<TRootObject>(string json, EventHandler<ErrorAndValueEventArgs> error, JsonSerializerSettings settings = null)
{
using (var sr = new StringReader(json))
using (var jsonReader = new JsonTextReader(sr))
{
var serializer = JsonSerializer.CreateDefault(settings);
serializer.Error += (o, e) => error(o, new ErrorAndValueEventArgs(jsonReader.Value, e.CurrentObject, e.ErrorContext));
return serializer.Deserialize<TRootObject>(jsonReader);
}
}
}
Now you will be able to access the value of JsonReader.Value at the time the exception was thrown:
object errorValue = null;
try
{
Account account = JsonExtensions.Deserialize<Account>(json, (o, e) => errorValue = e.ReaderValue);
Console.WriteLine(account.Email);
}
catch (JsonException exc)
{
// Do Something
Console.WriteLine("Value at time of {0} = {1}, Data.Count = {2}.", exc.GetType().Name, errorValue, exc.Data.Count);
// Prints Value at time of JsonReaderException = aaa, Data.Count = 0.
}
Notes:
Since you must manually create your own JsonTextReader, you will need to have access to the JSON string (or Stream) for this approach to work. (This is true in the example shown in your question.)
A similar technique for capturing additional error information is shown in JsonSerializationException Parsing.
You might want to enhance ErrorAndValueEventArgs to also record JsonReader.TokenType. In cases where the reader is positioned at the beginning of a container (object or array) at the time an exception is thrown, JsonReader.Value will be null.
Demo fiddle here.