Чтение строки с типом Object

source_domain_address = (object)i["source_domain_address"]

"source_domain_address": null

var Metas = par["dataset"]["metas"]["default"].Select(i => new
            {
                records_count = (int)i["records_count"],
                modified = (DateTime)i["modified"],
                source_domain_address = (object)i["source_domain_address"],
                geographic_reference = (string)i["geographic_reference"],
                timezone = (object)i["timezone"],
                title = (string)i["title"],
                parent_domain = (object)i["parent_domain"],
                theme = (string)i["theme"],
                modified_updates_on_data_change = (bool)i["modified_updates_on_data_change"],
                metadata_processed = (DateTime)i["metadata_processed"],
                data_processed = (DateTime)i["data_processed"],
                territory = (string)i["territory"],
                description = (string)i["description"],
                modified_updates_on_metadata_change = (bool)i["modified_updates_on_metadata_change"],
                shared_catalog = (object)i["shared_catalog"],
                source_domain = (object)i["source_domain"],
                attributions = (string)i["attributions"],
                geographic_area_mode = (object)i["geographic_area_mode"],
                geographic_reference_auto = (bool)i["geographic_reference_auto"],
                geographic_area = (object)i["geographic_area"],
                publisher = (string)i["publisher"],
                language = (string)i["language"],
                license = (string)i["license"],
                source_dataset = (object)i["source_dataset"],
                metadata_languages = (string)i["metadata_languages"],
                oauth_scope = (object)i["oauth_scope"],
                c = (bool)i["c"],
                license_url = (string)i["license_url"],
 
            }).ToList();

"metas": {
      "default": {
        "records_count": 4377,
        "modified": "2016-02-14T02:54:45+00:00",
        "source_domain_address": null,
        "references": null,
        "keyword": [ "Weather", "Rainfall", "Temperature" ],
        "source_domain_title": null,
        "geographic_reference": [ "world_us" ],
        "timezone": null,
        "title": "Local Weather Archive",
        "parent_domain": null,
        "theme": [ "Environment" ],
        "modified_updates_on_data_change": false,
        "metadata_processed": "2021-05-25T08:00:22.583000+00:00",
        "data_processed": "2021-05-24T21:00:08+00:00",
        "territory": [ "United States of America" ],
        "description": "<p>Pull weather data as collected at Raleigh-Durham International Airport by NOAA.</p><p>This dataset contains Raleigh Durham International Airport\u00a0weather data pulled from the NOAA\u00a0web service described at:</p><p><a href=\"http://www.ncdc.noaa.gov/cdo-web/webservices/v2\">http://www.ncdc.noaa.gov/cdo-web/webservices/v2</a></p><p>We have pulled this data and converted the data to commonly used units.\u00a0</p>",
        "modified_updates_on_metadata_change": false,
        "shared_catalog": null,
        "source_domain": null,
        "attributions": [ "NOAA - NCEI" ],
        "geographic_area_mode": null,
        "geographic_reference_auto": true,
        "geographic_area": null,
        "publisher": "National Oceanic and Atmospheric Administration - National Centers for Environmental Information",
        "language": "en",
        "license": "Open Database License (ODbL)",
        "source_dataset": null,
        "metadata_languages": [ "en" ],
        "oauth_scope": null,
        "c": false,
        "license_url": "http://opendatacommons.org/licenses/odbl/"
      }

var metas =  JsonConvert.DeserializeObject<Metas>(json);
object s = metas.source_domain_address;
 
//или так, смотря какой тип у поля будет.
string s = metas.source_domain_address;

records_count = (int)ipar["dataset"]["metas"]["default"]["records_count"]