Delphi XE 6 using System.JSON methods and implementations failing

Question

I created a simple test app in Delphi XE6, I want to start using Delphi's JSON to handle requests from a custom coded web server (one I am busy with)

procedure TForm1.Button1Click(Sender: TObject);
var
  Servermethods : TServerMethods1Client;
  JsonArray : TJSONArray;
  JsonValue: TJSONValue;
  JSonObj: TJSONObject;
  JSPair: TJSONPair;
  s: String;
begin
  JSonObj := TJSONObject.Create;
  JSonObj.AddPair(TJSONPair.Create('a','abcde'));
  JsonArray := TJSONArray.Create;
  JsonArray.AddElement(JSonObj);

  JsonValue := JsonArray.Items[0];
  JSonObj := (JsonValue as TJSONObject);

  JSPair := TJSONPair(JSonObj);

  s := JSPair.JsonString.Value;
end;

When you Inspect/Evaluate any JSON object (CTRL+F7) there is simply no info, in the inspector I simply get "()" -[aka empty]-

But for arguments sake if I change the call to JSONObj.Tostring I get the full json string ({"a", "abcde"}) and thats cool thats cool but when it gets to the last line

s := JSPair.JsonString.Value;

Boom! Access Violation.

Any help would be appreciated

* Edit * What I'm actually asking i for someone to test this in XE6 to see if it's a bug? or am I missing something very obvious?

Hi, thanks for the reply - The Pair is now part of the object - you'll see that JSonObj.AddPair(TJSONPair.Create('a','abcde')) so the pairs parent is the JSonObj ... IE it exists, you should then be able to access the pair Via the JsonObj like so (s := TJSONPair(JSonObj).JsonString.Value;) — Ryno Coetzee, May 06 '14 at 19:34
But the variable JSPair is never set, so it contains mostly garbage. — Uwe Raabe, May 06 '14 at 19:35
Hi - Sorry, I saw that small oversight, thanks for pointing it out, fixed it and amended this post but still - Access Violation — Ryno Coetzee, May 06 '14 at 19:41

score 2 · Accepted Answer · answered May 06 '14 at 19:52

2

The JSONPair is part of the JSONObject, so you cannot cast the JSONObject to a JSONPair. Use JSONObject.Get to get the pair.

answered May 06 '14 at 19:52

Uwe Raabe

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Thank you! I maybe looked at that method 100 times without registering the return value of the TJSONOBJECT.Get method was it's TJSONPair – Ryno Coetzee May 06 '14 at 20:09

brezniczky · Answer 2 · 2014-05-06T19:44:28.313

It seems to me that you are also casting a TJSONValue to a TJSONPair.

The last few lines assign a standalone TJSONValue object to JSONObj. Then that is cast to a JSONPair. (in the unsafe way - there is no runtime type compatiblity check then). I think a value here is just a member of a pair and is not compatible with that otherwise.

You could try using "as" operator in that case too, might make discovering the features a lot easier for you.

I.e.

JSPair := JSonObj as TJSONPair;

should complain for the typecasting issue when run.

It would also tell if I am right to say these classes are really sitting on different "benches" of the class inheritance tree, and hence your access violation.

Hi, Thank you for your answer, you where right about the typecasting and the inheritance hierarchy however @Uwe Raabe's answer was the one i'm looking for — Ryno Coetzee, May 06 '14 at 20:07

Delphi XE 6 using System.JSON methods and implementations failing

2 Answers2