Flutter POST json request body is empty/null on server side

user_14416830 0
Viewed 164

I am sending a post request from my flutter to a localhost api. However, the body on the server side is always empty/null. Whereas, if I send the exact same request on postman everything works fine.

String json = jsonEncode(toCheck);
Map<String,String> headers = {
  "Accept": "application/json",
  "content-type": "application/json",
};

var response = await http.post(
    Uri.parse(userPath + 'validateUser/'),
    headers:  headers,
    body: "{\"email\":\"david@hotmail.com\",\"username\":\"\",\"mobile_phone_number\":\"\",\"password\":\"david\"}");
print(response.statusCode);
if(response.statusCode == HttpStatus.ok){
  return response.body as bool;
}
print(response.body.toString());
return false;

The status code is always 500 with the following error:

{"Message":"An error has occurred.","ExceptionMessage":"The value cannot be null"...

This error happens because the parameter "json" is null:

  [HttpPost]
  [Route("validateUser/", Name = "validadeUser")]
  public async Task<IHttpActionResult> ValidateUser([FromBody] string json)
  {
      //get the username and password to validate the user
      UserClient client = JsonConvert.DeserializeObject<UserClient>(json); //error happens here because the string json is null
      //...
  }

However, when making the same exact request on postman, everything is okay: postman request

The host on the android emulator app is the following: https://10.0.2.2:44304/api/user/validateUser/

PS: looks like a duplicate of Flutter POST request body is empty on server side and Flutter Dart HTTP POST request body is empty on server side but none of those solutions have worked for me, therefore I posting a new question.

dart flutter http json .net
2 Answers

It took me a while to spot the error... but in the end, I did!

Your API takes a String argument from the request body:

  ValidateUser([FromBody] string json)

That may seem alright at first, since indeed, you are sending a json String as your request body. But the thing is that since you're also using these headers:

{
  "Accept": "application/json",
  "content-type": "application/json",
}

This bit: "content-type": "application/json" will translate your correctly formatted json String into an actual json Object! So when it reaches your API, it will no longer be a String, and so your API won't be able to receive it properly.

The solution would be to either remove the "content-type": "application/json" from your headers and keep the request body and the API function as they are, or to change the API to:

ValidateUser([FromBody] object json)

The latter is probably the most convenient, since you were going to parse it from json String to json Object anyway!

(With this API function, you COULD also remove your "content-type": "application/json" bit from the headers, and then send the request body like this:

  body: jsonDecode("{\"email\":\"david@hotmail.com\",\"username\":\"\",\"mobile_phone_number\":\"\",\"password\":\"david\"}"),

Or just not jsonEncode it in the first place... Just send it as an object. This will have the same effect as sending it as a json String with "content-type": "application/json" in the header.)

Btw, as a general rule, make sure you add lots of print and log statements in your code as you develop it! It will help you spot anything that didn't go quite as you had expected, along the way. (That's how I spotted this.)

user_143356550

Before sending the request to the server please encode your object to complete the body property of the request. Try this:

body: json.encode({\"email\":\"david@hotmail.com\",\"username\":\"\",\"mobile_phone_number\":\"\",\"password\":\"david\"})

Cheers.

user_20954250
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