Deserialize JSON string in D365FO using X++ code

[DataContract]
internal final class TestJSONContract
{
    String50 parmNationId, parmNation,parmYearId,parmYear,parmPopulation,parmSlugNation;
    
    [DataMemberAttribute('ID Nation')]
    public String50 parmNationId(String50 _parmNationId = parmNationId)
    {
        parmNationId = _parmNationId;
        return parmNationId;
    }

    [DataMemberAttribute('Nation')]
    public String50 parmNation(String50 _parmNation = parmNation)
    {
        parmNation = _parmNation;
        return parmNation;
    }

    [DataMemberAttribute('ID Year')]
    public String50 parmYearId(String50 _parmYearId = parmYearId)
    {

        parmYearId = _parmYearId;
        return parmYearId;
    }

    [DataMemberAttribute('Year')]
    public String50 parmYear(String50 _parmYear = parmYear)
    {
        parmYear = _parmYear;
        return parmYear;
    }

    [DataMemberAttribute('Population')]
    public String50 parmPopulation(String50 _parmPopulation = parmPopulation)
    {
        parmPopulation = _parmPopulation;
        return parmPopulation;
    }

    [DataMemberAttribute('Slug Nation')]
    public String50 parmSlugNation(String50 _parmSlugNation = parmSlugNation)
    {
        parmSlugNation = _parmSlugNation;
        return parmSlugNation;
    }

}

Step 2 : Create contract class for outer node.

In your case if there is more than one outer node then you need to create your class according to your JOSN. ex- if you have status and data node in your JSON but "status" is string type and "data" node contains multiple sub node then you need to create one more attribute in below class with string type and data with list type.

[DataContract]
internal final class TestJSONDataContract
{
    List parmData;
    
    [DataMemberAttribute('data'),DataCollectionAttribute(Types::class, classStr(TestJSONContract))]
    public List parmData(List _parmData = parmData)
    {
        parmData = _parmData;
        return parmData;
    }
}

Step 3: Create runnable class to call your API and get the result.

internal final class TestConsumeAPITest
{
    public static void main(Args _args)
    {
        System.Net.HttpWebRequest   webRequest;
        System.Net.HttpWebResponse  webresponse;
        System.Exception            ex;
        System.IO.Stream            responseStream;
        System.IO.StreamReader      reader;
        str                         output;
        ListEnumerator              listEnumerator;
    
        try
        {
          new InteropPermission(InteropKind::ClrInterop).assert();
    
          webRequest = System.Net.WebRequest::Create("##########"); //your API URL goes here.   
    
          webRequest.Method = "GET";
          webRequest.ContentType = "application/json";
          webresponse = webRequest.GetResponse();
          responseStream = webresponse.GetResponseStream();
          reader = new System.IO.StreamReader(responseStream);
    
          output = reader.ReadToEnd();
    
          TestJSONDataContract TestJSONDataContract = FormJsonSerializer::deserializeObject(classNum(TestJSONDataContract),output);
    
          List listData = new List(Types::Class);
    
          listData          = TestJSONDataContract.parmData();
          listEnumerator    = listData.getEnumerator();
    
          while (listEnumerator.moveNext())
          {
            TestJSONContract jsonDataContract = listEnumerator.current();
    
            info(strFmt("Nation id %1 : %2 = %3",jsonDataCOntract.parmNationId(), jsonDataContract.parmNation(), jsonDataContract.parmPopulation()));
          }
    
        }
        catch
        {
          ex = CLRInterop::getLastException().GetBaseException();
          error(ex.get_Message());
        }
    
      }

}

Step Final : Execute your runnable class to test the API and JSON format.