Json Array into ListBox/Memo in Delphi xe7 - json

Question

I'm trying to catch the following JSON array :
[{"name":"Bryan","email":"Bryan#hotmail.com"},
{"name":"Louis","email":"Louis#hotmail.com"},
{"name":"Maria","email":"Maria#hotmail.com"},
{"name":"Test","email":"test#hotmail.com"},
{"name":"Anthony","email":"anthony#hotmail.com"}]
and put it in Memo or ListBox in Delphi :
the code is the following :
procedure TForm1.Button1Click(Sender: TObject);
var jv: TJSONValue;
jo: TJSONObject;
jp: TJSONPair;
ja: TJSONArray;
i: integer;
j: integer;
begin
RESTRequest1.Execute;
jv:=RESTResponse1.JSONValue;
jo:= TJSONObject.ParseJSONValue(jv.ToString) as TJSONObject;
try
for i := 0 to jo.Size - 1 do
begin
jp := jo.Get(i);
if jp.JsonValue is TJSONArray then
begin
ja := jp.JsonValue as TJSONArray;
for j := 0 to ja.Size -1 do
Memo1.Lines.Add(ja.Get(i).ClassName + ': ' + ja.Get(j).ToString);
end
else
Memo1.Lines.Add(jp.ClassName + ': '+ jp.ToString);
end;
finally
jo.Free;
end;
end;
When I click in Button I got the following error message :
Invalid class typecast
during debugging the following line has a problem :
jo:= TJSONObject.ParseJSONValue(jv.ToString) as TJSONObject;
I don't know how to resolve this problem or this mistake ,
Could you please help me ?
Thanks.

Answer 1

This could perfectly well be worked out by reading the code and looking at the JSON. However, I'd like to show you how to go about debugging such a problem in case you cannot work it out by static analysis. When an as cast fails that is always because the object on the left hand side of the as does not derive from the type on the right hand side. The next step then is always to inquire as to what the type of the object on the left hand side is. I've included a short MCVE above as a means to demonstrate.
The output of this program:
{$APPTYPE CONSOLE}
uses
System.JSON;
const
JSON = '[{"name":"Bryan","email":"Bryan#hotmail.com"},' +
' {"name":"Louis","email":"Louis#hotmail.com"},' +
' {"name":"Maria","email":"Maria#hotmail.com"},' +
' {"name":"Test","email":"test#hotmail.com"},' +
' {"name":"Anthony","email":"anthony#hotmail.com"}]';
begin
Writeln(TJSONObject.ParseJSONValue(JSON).ClassName);
end.
is
TJSONArray
Now, TJSONArray does not derive from TJSONObject. Hence your as cast raise a runtime error. If you cast the value returned by ParseJSONValue to TJSONArray that will succeed.
This is to be expected since the root of your JSON is an array and is not an object.
You need to modify your code so that it does not assume that the root level is always an object. You need different behaviour for arrays and objects.

Answer 2

I'm not sure what the problem of TJSONObject is with the string you posted.
For some reason it will parse it if you changed it.
{"Persons":[{"name":"Bryan","email":"Bryan#hotmail.com"},{"name":"Louis","email":"Louis#hotmail.com"},{"name":"Maria","email":"Maria#hotmail.com"},{"name":"Test","email":"test#hotmail.com"},{"name":"Anthony","email":"anthony#hotmail.com"}]}
If I run the code as it is I get the following result
If you don't mind using something different than default Delphi units I would suggest superobject (Link here)
superobject will parse your JSON edited and as posted.
Your code would look like this:
Const
MyJSON = '[{"name":"Bryan","email":"Bryan#hotmail.com"},{"name":"Louis","email":"Louis#hotmail.com"},{"name":"Maria","email":"Maria#hotmail.com"},{"name":"Test","email":"test#hotmail.com"},{"name":"Anthony","email":"anthony#hotmail.com"}]';
procedure ParseJSON;
var
obj: ISuperObject;
Ar: TSuperArray;
I: integer;
begin
obj := SO(MyJSON);
if obj.IsType(stArray) then
begin
Ar := obj.AsArray;
try
for I := 0 to Ar.Length-1 do
L.Add(Ar.O[I].AsString);
finally
Ar.Free;
end;
end
else
L.Add(Obj.AsString);
end;
Result:
For Koul, to get the element names and values.
Like I said not very pretty code but ok.
Ar.O[0].AsObject.GetNames.AsArray.S[0]
To cut it up in pieces a bit.
Ar.O[0] //Get the first element in the array as ISuperObject
.AsObject //Get it as TSuperTableString
.GetNames //Gets all names in the array, in this case "name" and "email"
.AsArray[0]//Get the first name in the names array.
It will result in email (Names are sorted A-Z)
You can do the same for the values by calling GetValues instead of GetNames.
I think the prettiest way to get it will be defining 2x more TSuperArray
procedure PrintNamesAndValues;
Var
Ar, ArNames, ArValues:TSuperArray;
I: Integer;
begin
Ar := SO(<JSON string>).asArray;
ArNames := Ar.O[0].AsObject.GetNames.AsArray;
ArValues := Ar.O[0].AsObject.GetValues.AsArray;
For I := 0 to ArNames.Length-1 do
WriteLn(Format('%s: %s',[ArNames.S[I], ArValues.S[I]]));
end;
Hope it's all clear enough :)