System.Text.Json: Add JsonReaderState.LineNumber and BytePositionInLine Property

[weifenluo]

Background and Motivation

In order to write a descent custom JSON deserializer, the LineNumber and BytePositionInLine, along with Path information should be provided in JsonException. The only way to get LineNumber and BytePositionInLine is through JsonReaderState struct. Unfortunately they're internal, which can only be consumed by the standard built-in deserializer.

JsonException has constructor to accept LineNumber and BytePositionInLine parameter, but this information is not available unless using reflection.

Proposed API

namespace System.Text.Json
{
    public struct JsonReaderState
    {
        public long LineNumber => _lineNumber;
        public long BytePositionInLine => _bytePositionInLine;
    }
}

Usage Examples

private static JsonException GetJsonExceptionToRethrow(string message, string path, JsonReaderState readerState, Exception ex)
{
    return new JsonException(message, path, readerState.LineNumber, readerState.BytePositionInLine, ex);
}

Alternative Designs

JsonReaderState is the only source of the information. Naturally this is the only way.

Risks

Don't see any risk.

[ghost]

Tagging subscribers to this area: @eiriktsarpalis, @layomia
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Issue Details

---

Background and Motivation

In order to write a descent custom JSON (de)serializer, the LineNumber and BytePositionInLine, along with Path information should be provided in JsonException. The only way to get LineNumber and BytePositionInLine is through JsonReaderState struct. Unfortunately they're internal, which can only be consumed by the standard built-in deserializer.

JsonException has constructor to accept LineNumber and BytePositionInLine parameter, but this information is not available unless using reflection.

Proposed API

namespace System.Text.Json
{
    public struct JsonReaderState
    {
        public long LineNumber => _lineNumber;
        public long BytePositionInLine => _bytePositionInLine;
    }
}

Usage Examples

private static JsonException GetJsonExceptionToRethrow(string message, string path, JsonReaderState readerState, Exception ex)
{
    return new JsonException(message, path, readerState.LineNumber, readerState.BytePositionInLine, ex);
}

Alternative Designs

JsonReaderState is the only source of the information. Naturally this is the only way.

Risks

Don't see any risk.

Author:	weifenluo
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json, untriaged
Milestone:	-

[layomia]

I agree this could be good information for custom serializers to provide, but would like to see more users ask for this before exposing API here (either on JsonReaderState or Utf8JsonReader itself). I'll move to future for now.

[sjschukin]

I have to write custom deserializer that skips some JSON fields with invalid data. Unfortunately I've to use reflection to show the position where the error occured. Voted for this feature.

[eiriktsarpalis]

Duplicate of #28482