Invalid typecast error processing JsonResult in Delphi

Question

I have the JSON below:

{
    "value": "[{\"field1\":\"value1\",\"field2\":\"value2\",\"field3\":\"value3\"}]",
    "formatters": [],
    "contentTypes": [],
    "declaredType": null,
    "statusCode": 200
}

I need to get hold of value as an array using Delphi.

var
  obj2: TJSonObject;
  arr:  TJSONArray;

The overall JSON is an object, so I first do:

 obj2 := TJSONObject.ParseJSONValue(strJSon) as TJSONObject;

I then need to get the value part as an array, so I use:

arr := obj2.GetValue('value') as TJSONArray;

I get an Invalid Class Typecast error on the line above.

Does anyone know why?

In the JSON you have shown, value is NOT an array (TJSONArray), it is a string instead (TJSONString), hence the cast error. Note the quote chars surrounding the value's data. If it were a true array, those quotes would not be present. — Remy Lebeau
– Remy Lebeau, Commented Oct 9, 2018 at 0:08
Ok that makes sense is there a best practice way to decide this so that it can become an array? — Paul
– Paul, Commented Oct 9, 2018 at 0:09
value is a string in the JSON, period. The JSON has no concept of what the string represents. You will have to parse the string yourself to make your own array from it. I suppose the simplest approach would be to retrieve the string and pass it to another ParseJSONValue() call, and cast the resulting TJSONValue to TJSONArray. — Remy Lebeau
– Remy Lebeau, Commented Oct 9, 2018 at 0:10
Yuk this in reality could become a very big string with a lot of elements in it. The objects inside have a lot of properties — Paul
– Paul, Commented Oct 9, 2018 at 0:12
Why is JSON data being stringified at all? What is the value supposed to represent exactly? — Remy Lebeau
– Remy Lebeau, Commented Oct 9, 2018 at 0:18

GatoSoft · Accepted Answer · 2019-02-19 03:15:43Z

1

You can apply this transformation to your StrJson variable, before doing the typecast

Uses StrUtils;

Var
strJson: String;

strJson:= ReplaceStr(strJson,'"[','[');
strJson:= ReplaceStr(strJson,']"',']');
strJson:= ReplaceStr(strJson,'\"','"');
vJSonArray:= TJSONObject.ParseJSONValue(strJson) as TJSONArray; //OK

answered Feb 19, 2019 at 3:15

GatoSoft

1161 silver badge3 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

GatoSoft Over a year ago

The format in which you get the JSON is not valid, or at least it is not what you expect. The text: "value": "[{\"field1\":\"value1\",\"field2\":\"value2\",\"field3\":\"value3\"}]" means you has created the entry with something like: ObjJson.AddPair(String, String) but, instead you should use: ObjJson.AddPair(String, TJSOnArray) The following example is valid, but the value is not an array, but a string MyValue:= '[{"field1":"Value1"}]'; ObjJson.AddPair('value', MyValue); This is the one you are looking for: ObjJson.AddPair(Texto, TJSONObject.ParseJSONValue(MyValue) as TJSONArray)

Remy Lebeau · Accepted Answer · 2018-10-09 00:26:53Z

In the JSON you have shown, the value field is a string, not an array. That is why you are getting the cast error. Had it been an actual array, it would have looked like this instead:

"value": [{"field1":"value1","field2":"value2","field3":"value3"}],

Note the surrounding quotes are gone, and the inner quotes are not escaped as \".

If you know that the value string represents a JSON array, you can access the array like this:

var
  val1, val2: TJSONValue;
  obj: TJSONObject;
  arr: TJSONArray;
begin
  val1 := TJSONObject.ParseJSONValue(strJSon);
  try
    obj := val1 as TJSONObject;
    val2 := TJSONObject.ParseJSONValue(obj.GetValue('value').Value);
    try
      arr := val2 as TJSONArray;
      // use arr as needed ...
    finally
      val2.Free;
    end;
  finally
    val1.Free;
  end;
end;

Collectives™ on Stack Overflow

Invalid typecast error processing JsonResult in Delphi

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related