Parsing Json response with not defined structure in C#

To define a type that will deserialize the given json into a type that mimicks the known structure + some unknown structure, use Json.Net and use this type:

public class Response
{
    [JsonProperty("COMPLETED_COUNT")]
    public int CompletedCount { get; set; }

    [JsonProperty("INPROGRESS_COUNT")]
    public int InProgressCount { get; set; }

    [JsonProperty("FAILED_COUNT")]
    public int FailedCount { get; set; }

    [JsonExtensionData]
    public Dictionary<string, object> ExtraData { get; } = new Dictionary<string, object>();
}

Please know that the "Results" sub-property in the first json will be difficult to handle in this concept, so you should still strive to make a "detection" algorithm that looks at a subset of the properties, and if some specific key properties exists, pick a different and more correct type to deserialize the json into.

For instance, here is a simple such heuristics:

public static ResponseBase Classify(string json)
{
    var response = JsonConvert.DeserializeObject<ResponseBase>(json);

    if (response.ExtraData.ContainsKey("Results"))
        return JsonConvert.DeserializeObject<ResponseWithResults>(json);

    return response;
}

public class ResponseBase
{
    [JsonProperty("COMPLETED_COUNT")]
    public int CompletedCount { get; set; }

    [JsonProperty("INPROGRESS_COUNT")]
    public int InProgressCount { get; set; }

    [JsonProperty("FAILED_COUNT")]
    public int FailedCount { get; set; }

    [JsonExtensionData]
    public Dictionary<string, object> ExtraData { get; } = new Dictionary<string, object>();
}

public class ResponseWithResults : ResponseBase
{
    public int BatchId { get; set; }
    public List<ResponseResult> Results { get; } = new List<ResponseResult>();
}

public class ResponseResult
{
    public int ItemId { get; set; }
    public string ResultMessage { get; set; }
}

Do note that this deserializes the json twice. There are different ways to handle/avoid that but you will have to decide if this is a valid way forward.