How to validate JSON using System.Text.Json before deserialization

Question

In .NET Core 5.0, using System.Text.Json.JsonSerializer Deserialize(someJsonFile) i get:

System.Text.Json.JsonException: 'The JSON value could not be converted to System.Guid. Path: $.someGuid | ..

which is expected, since the the someGuid property is of type System.Guid and the value of someGuid in the JSON file/string is:

{
  "someGuid": ""
}

which can not be deserialized properly .. (since it's not Guid.Empty)..

To my question.

What's a good and generic implementation to validate the Json before deserializing it (in general)? like TryParse or JsonDocument.Parse? sure, try-catch but that's dirty (imho).

btw: i don't want to use Newtonsoft

thanks for your suggestions (and critics of course).

Seems it's on the road-map. There are 3rd party solutions available. — Andy
– Andy, Commented Apr 21, 2021 at 15:16
I think you're stuck with try/catch or implementing your own converters for each type and some post-deserialisation validation code. — DavidG
– DavidG, Commented Apr 21, 2021 at 15:26

Sergio A. · Accepted Answer · 2021-10-27 20:31:26Z

I created a custom converter using the example is this answer: The JSON value could not be converted to System.Int32

public class StringToGuidConverter : JsonConverter<Guid>
    {
        public override Guid Read(ref Utf8JsonReader reader, Type type, JsonSerializerOptions options)
        {
            if (reader.TokenType == JsonTokenType.String)
            {
                ReadOnlySpan<byte> span = reader.ValueSpan;
                if (Utf8Parser.TryParse(span, out Guid guid, out int bytesConsumed) && span.Length == bytesConsumed)
                {
                    return guid;
                }

                if (Guid.TryParse(reader.GetString(), out guid))
                {
                    return guid;
                }
            }

            return Guid.Empty;
        }

        public override void Write(Utf8JsonWriter writer, Guid value, JsonSerializerOptions options)
        {
            writer.WriteStringValue(value.ToString());
        }
    }

In my case my model to deserialize to can't take Nullable Guid so I return an empty GUID and then validate this in my logic.

Because I'm create a web api using .Net standard, I can't register this in the services at the startup class. But you can register the custom converter using the JsonSerializerOptions property when calling the Deserialize method like this:

var options = new JsonSerializerOptions
{
    PropertyNameCaseInsensitive = true,
    Converters = { new NERDS.API.Helpers.StringToGuidConverter() }
};

StreamReader reader = new StreamReader(HttpContext.Current.Request.InputStream);
string json = reader.ReadToEnd();
return JsonSerializer.Deserialize<T>(json, options);

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