Accessing JSON data after Indy POST

Question

I am struggling to know how to access the response to an Indy POST request. I post the data either as JSON or paramstring. My code when using JSON is as follows.

params := TStringList.Create;
try
  params.Text :=
    '{'
    + format ('"client_secret":"%s",', [FilesFrm.ClientSecret])
    + format ('"client_id":"%s",', [FilesFrm.ClientId])
    + '"grant_type":"authorization_code",'
    + '"redirect_uri":"http://localhost:8080",'
    + format ('"code":"%s"', [fCode])
    + '}';
  idLogFile1.Active := true;
  // Make sure it uses HTTP 1.1, not 1.0
  IdHTTP1.HTTPOptions := IdHTTP1.HTTPOptions + [hoKeepOrigProtocol];
  IdHTTP1.Request.ContentType := 'application/json';
  IdHttp1.Request.Accept := 'application/vnd.hmrc.1.0+json';

  try
    result := IdHTTP1.Post (
      'https://test-api.service.hmrc.gov.uk/oauth/token',
      params);
  except
    on E: Exception do
    memo1.lines.add (E.ClassName + ': ' + E.message);
    end;

  memo1.Lines.add (result);
  memo1.Lines.add (idHTTP1.ResponseText);
finally
  params.free;
  end;

The result of printing out result and RepsonseText is just

EIdHTTPProtocolException: HTTP/1.1 400 Bad Request

HTTP/1.1 400 Bad Request

However, because I have a TidLogFile component attached to the TidHTTP, I can see what actually arrives, which is the following.

Recv 2/1/2019 7:56:07 AM: HTTP/1.1 400 Bad Request<EOL>
Content-Type: application/json<EOL>
Content-Length: 72<EOL>
Cache-Control: no-cache,no-store,etc, etc...
; Secure; HTTPOnly<EOL><EOL>
{"error":"invalid_request","error_description":"grant_type is required"}

Aside from the fact that grant_type appears to be in the original request data, I would like to be able to access the JSON response at the end, since "grant_type_is_required" is much more helpful than "Bad request", but I cannot find where it is.

I have subsequently found Response.ContentLength, which contains the value 72, and Response.ContentStream, which should in theory contain the 72 bytes of data, but produces access violations when I try to extract the data.

len := idHTTP1.Response.ContentLength;
memo1.Lines.Add(format ('Length = %d', [len]));
if assigned (idHTTP1.Response.ContentStream) then
  begin
  //idHTTP1.Response.ContentStream.Position := 0;
  result := ReadStringFromStream (idHTTP1.Response.ContentStream, len);
  end;
memo1.lines.add (result);

Answer 1

Here is an example which shows how the HTTP body can be accessed.

The body can be accessed if you catch the EIdHTTPProtocolException exception.

  on E: EIdHTTPProtocolException do
  begin
    WriteLn(E.Message);
    WriteLn(E.ErrorMessage);
  end;

Full example code:

program JSONPostExample;

{$APPTYPE CONSOLE}

uses
  IdHTTP, IdGlobal, SysUtils, Classes;

var
  HTTP: TIdHTTP;
  RequestBody: TStream;
  ResponseBody: string;
begin
  HTTP := TIdHTTP.Create;
  try
    try
      RequestBody := TStringStream.Create('{"日本語":42}',
        TEncoding.UTF8);
      try
        HTTP.Request.Accept := 'application/json';
        HTTP.Request.ContentType := 'application/json';
        ResponseBody := HTTP.Post('https://httpbin.org/post',
          RequestBody);
        WriteLn(ResponseBody);
        WriteLn(HTTP.ResponseText);
      finally
        RequestBody.Free;
      end;
    except
      on E: EIdHTTPProtocolException do
      begin
        WriteLn(E.Message);
        WriteLn(E.ErrorMessage);
      end;
      on E: Exception do
      begin
        WriteLn(E.Message);
      end;
    end;
  finally
    HTTP.Free;
  end;
  ReadLn;
  ReportMemoryLeaksOnShutdown := True;
end.

Note that you must not use a TStringList for the POST body. That version of TIdHTTP.Post() formats the data according to the application/x-www-form-urlencoded media type, which is not appropriate for JSON and will corrupt it.