方法1

JObject json = (JObject)JsonConvert.DeserializeObject(ptString);

Dictionary<string, object> d = new Dictionary<string, object>(json.ToObject<IDictionary<string, object>>(), StringComparer.CurrentCultureIgnoreCase);

String f = d["FROM"].ToString();

方法2

var json = @"{UPPER: 'value'}";

var jObj = JObject.Parse(json);

var upper = jObj.GetValue("upper", StringComparison.OrdinalIgnoreCase)?.Value<string>();

Console.WriteLine(upper); // value