JSON: Invalid Class Typecast in Delphi

Question

This works, when it is an object:

{"data":{"url":"stackoverflow.com"}}

This does not work, when it is an array:

{"data":[{"url":"stackoverflow.com"}]}

Error: Invalid class typecast

procedure TForm1.Button1Click(Sender: TObject);
var
  json : string;
  obj, data : TJSONObject;
  url : string;

begin
  json := '{"data":{"url":"stackoverflow.com"}}';
  obj := TJSonObject.ParseJSONValue(json) as TJSONObject;

  try
    data := obj.Values['data'] as TJSONObject;
    url := data.Values['url'].value;
    showMessage(url);
  finally
    obj.Free;
  end;
end;

I know I have to use TJSONArray, but I don't know how to implement it.

Remy Lebeau · Accepted Answer · 2022-03-12 05:16:42Z

2

This is not hard to understand.

In the first case, the value of data is an object, so obj.Values['data'] as TJSONObject is correct.

In the second case, the value of data is an array, so obj.Values['data'] as TJSONObject is wrong. it needs to be obj.Values['data'] as TJSONArray instead, and then you access the TJSONObject from the elements of the array, eg:

procedure TForm1.Button1Click(Sender: TObject);
var
  json : string;
  obj, data : TJSONObject;
  arr: TJSONArray;
  url : string;

begin
  json := '{"data":[{"url":"stackoverflow.com"}]}';
  obj := TJSonObject.ParseJSONValue(json) as TJSONObject;

  try
    arr := obj.Values['data'] as TJSONArray;
    data := arr[0] as TJSONObject;
    url := data.Values['url'].Value';
    ShowMessage(url);
  finally
    obj.Free;
  end;
end;

answered Mar 12, 2022 at 5:16

Remy Lebeau

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1 Comment

ErikROCHA Over a year ago

Now I see it, pass it to an array and then access that position. Thanks!

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