Parsing large JSON file in .NET

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As you’ve correctly diagnosed in your update, the issue is that the JSON has a closing ] followed immediately by an opening [ to start the next set. This format makes the JSON invalid when taken as a whole, and that is why Json.NET throws an error.

Fortunately, this problem seems to come up often enough that Json.NET actually has a special setting to deal with it. If you use a JsonTextReader directly to read the JSON, you can set the SupportMultipleContent flag to true, and then use a loop to deserialize each item individually.

This should allow you to process the non-standard JSON successfully and in a memory efficient manner, regardless of how many arrays there are or how many items in each array.

    using (WebClient client = new WebClient())
    using (Stream stream = client.OpenRead(stringUrl))
    using (StreamReader streamReader = new StreamReader(stream))
    using (JsonTextReader reader = new JsonTextReader(streamReader))
    {
        reader.SupportMultipleContent = true;

        var serializer = new JsonSerializer();
        while (reader.Read())
        {
            if (reader.TokenType == JsonToken.StartObject)
            {
                Contact c = serializer.Deserialize<Contact>(reader);
                Console.WriteLine(c.FirstName + " " + c.LastName);
            }
        }
    }

Full demo here: https://dotnetfiddle.net/2TQa8p

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