public class UserInfo
{
    [JsonProperty("id")]
    public int Id{ set; get; }
    [JsonProperty("userName")]
    public string UserName{ set; get; }
}

 var user = new { Name = "john", Age = 19 }; 
 var serializerSettings = new JsonSerializerSettings
            {
                // 设置为驼峰命名
                ContractResolver = new CamelCasePropertyNamesContractResolver()
            };
var userStr = JsonConvert.SerializeObject(user, Formatting.None, serializerSettings);