Strategy for splitting a large JSON file

You can use JsonWriter.WriteToken(JsonReader reader, true) to stream individual array entries and their descendants from a JsonReader to a JsonWriter. You can also use JProperty.Load(JsonReader reader) and JProperty.WriteTo(JsonWriter writer) to read and write entire properties and their descendants.

Using these methods, you can create a state machine that parses the JSON file, iterates through the root object, loads "prefix" and "postfix" properties, splits the array property, and writes the prefix, array slice, and postfix properties out to new file(s).

Here's a prototype implementation that takes a TextReader and a callback function to create sequential output TextWriter objects for the split file:

    enum SplitState
    {
        InPrefix,
        InSplitProperty,
        InSplitArray,
        InPostfix,
    }

    public static void SplitJson(TextReader textReader, string tokenName, long maxItems, Func<int, TextWriter> createStream, Formatting formatting)
    {
        List<JProperty> prefixProperties = new List<JProperty>();
        List<JProperty> postFixProperties = new List<JProperty>();
        List<JsonWriter> writers = new List<JsonWriter>();

        SplitState state = SplitState.InPrefix;
        long count = 0;

        try
        {
            using (var reader = new JsonTextReader(textReader))
            {
                bool doRead = true;
                while (doRead ? reader.Read() : true)
                {
                    doRead = true;
                    if (reader.TokenType == JsonToken.Comment || reader.TokenType == JsonToken.None)
                        continue;
                    if (reader.Depth == 0)
                    {
                        if (reader.TokenType != JsonToken.StartObject && reader.TokenType != JsonToken.EndObject)
                            throw new JsonException("JSON root container is not an Object");
                    }
                    else if (reader.Depth == 1 && reader.TokenType == JsonToken.PropertyName)
                    {
                        if ((string)reader.Value == tokenName)
                        {
                            state = SplitState.InSplitProperty;
                        }
                        else
                        {
                            if (state == SplitState.InSplitProperty)
                                state = SplitState.InPostfix;
                            var property = JProperty.Load(reader);
                            doRead = false; // JProperty.Load() will have already advanced the reader.
                            if (state == SplitState.InPrefix)
                            {
                                prefixProperties.Add(property);
                            }
                            else
                            {
                                postFixProperties.Add(property);
                            }
                        }
                    }
                    else if (reader.Depth == 1 && reader.TokenType == JsonToken.StartArray && state == SplitState.InSplitProperty)
                    {
                        state = SplitState.InSplitArray;
                    }
                    else if (reader.Depth == 1 && reader.TokenType == JsonToken.EndArray && state == SplitState.InSplitArray)
                    {
                        state = SplitState.InSplitProperty;
                    }
                    else if (state == SplitState.InSplitArray && reader.Depth == 2)
                    {
                        if (count % maxItems == 0)
                        {
                            var writer = new JsonTextWriter(createStream(writers.Count)) { Formatting = formatting };
                            writers.Add(writer);
                            writer.WriteStartObject();
                            foreach (var property in prefixProperties)
                                property.WriteTo(writer);
                            writer.WritePropertyName(tokenName);
                            writer.WriteStartArray();
                        }
                        count++;
                        writers.Last().WriteToken(reader, true);
                    }
                    else
                    {
                        throw new JsonException("Internal error");
                    }
                }
            }
            foreach (var writer in writers)
                using (writer)
                {
                    writer.WriteEndArray();
                    foreach (var property in postFixProperties)
                        property.WriteTo(writer);
                    writer.WriteEndObject();
                }
        }
        finally
        {
            // Make sure files are closed in the event of an exception.
            foreach (var writer in writers)
                using (writer)
                {
                }

        }
    }

This method leaves all the files open until the end in case "postfix" properties, appearing after the array property, need to be appended. Be aware that there is a limit of 16384 open files at one time, so if you need to create more split files, this won't work. If postfix properties are never encountered in practice, you can just close each file before opening the next and throw an exception in case any postfix properties are found. Otherwise you may need to parse the large file in two passes or close and reopen the split files to append them.

Here is an example of how to use the method with an in-memory JSON string:

    private static void TestSplitJson(string json, string tokenName)
    {
        var builders = new List<StringBuilder>();
        using (var reader = new StringReader(json))
        {
            SplitJson(reader, tokenName, 2, i => { builders.Add(new StringBuilder()); return new StringWriter(builders.Last()); }, Formatting.Indented);
        }
        foreach (var s in builders.Select(b => b.ToString()))
        {
            Console.WriteLine(s);
        }
    }

Prototype fiddle.