jSonDocument property: sometimes null, sometimes not!

[vaz83]

Hi,
i have the following code:
`void read()
{
if (SPIFFS.begin())
{
if(SPIFFS.exists("/dev.json"))
{
File infoFile = SPIFFS.open("/dev.json", "r");
if(infoFile)
{
size_t size = infoFile.size();
std::unique_ptr<char[]> buf(new char[size]);
infoFile.readBytes(buf.get(), size);

    DynamicJsonDocument jDoc(1500);
    DeserializationError error = deserializeJson(jDoc,buf.get());
    if(!error)
    {
      if(jDoc["GUID"]!=NULL)
      {
          String tempstr = jDoc["GUID"];
          if(tempstr.startsWith("SECURED."))
          {
            tempstr.replace("SECURED.","");
            jDoc["GUID"] = tempstr;
            tempstr.toCharArray(GUID,80);
            XORcrypt(GUID);
          }
          else
            strcpy(GUID, jDoc["GUID"]);
      }
    }
  }
}

}`

What i am facing now is that the condition if(jDoc["GUID"]!=NULL) is returning false, and when i pass jDoc["GUID"] to a string variable it has value! the property exists and has data. Why is it returning null? in another part of the code when i open other json files this condition works just fine, please help

[bblanchon]

Hi @vaz83,

The problem is that NULL is not a pointer, it is the integer 0, so ArduinoJson applies the conversion to integer. Since the value in doc["GUID"] is not an integer, the conversion returns 0, so the if clause runs.

I'll fix the comparison operator so that it returns false when the value has the wrong type. I'll also to add support for nullptr_t, so you can write if (doc["GUID"] != nullptr) instead.

Best Regards,
Benoit

[bblanchon]

As a temporary workaround, please use JsonObject::containsKey()

[vaz83]

As a temporary workaround, please use JsonObject::containsKey()

Will do. I also found out other problem, i will make a new thread. Thanks!

[UltimatPronin]

I have similar problem (on V6):
1)in my sketch I use an DynamicJsonDocument _item(256);
2)periodically I change data stored in this object and send json to my service:
_item["last_time"] = timeClient.getFormattedTime(); //string data
_item["rnd"] = random(2); //int data
and after about 100 operations of changing _item["last_time"] it continiously return null and I can not assign any value to it.
3)For now a workaround for me is pass [long type] variable [string] place to _item["last_time"]. With it all is ok.