Figured it Out Hope This Helps import Foundation let json = """ {     "from": "Guille",     "text": "Look what I just found!",     "attachments": [         {             "name": "Test1",             "type": "image",             "payload": {                 "url": "url1",                 "width": 640,                 "height": 480             }         },         {             "name": "Test2",             "type": "audio",             "payload": {                 "title": "title1",                 "url": "url2",                 "shouldAutoplay": true,             }         },         {             "name": "Test3",             "type": "audio",             "payload": {                 "title": "title2",                 "url": "url3",                 "shouldAutoplay": true,             }         }     ] } """.data(using: .utf8)! struct ImageAttachment: Codable {     let url: URL     let width: Int     let height: Int } struct AudioAttachment: Codable {     let title: String     let url: URL     let shouldAutoplay: Bool } struct Test: Codable {     let name: String } enum Attachment {     case image(String, ImageAttachment)     case audio(String, AudioAttachment)     case unsupported } extension Attachment: Codable {     private enum CodingKeys: String, CodingKey {         case type         case name         case payload     }          init(from decoder: Decoder) throws {         let container = try decoder.container(keyedBy: CodingKeys.self)                  let name = try container.decode(String.self, forKey: .name)                  let type = try container.decode(String.self, forKey: .type)         switch type {         case "image":             let payload = try container.decode(ImageAttachment.self, forKey: .payload)             self = .image(name, payload)         case "audio":             let payload = try container.decode(AudioAttachment.self, forKey: .payload)             self = .audio(name, payload)         default:             self = .unsupported         }     }          func encode(to encoder: Encoder) throws {         var container = encoder.container(keyedBy: CodingKeys.self)                  switch self {         case .image(let name, let payload):             try container.encode("image", forKey: .type)             try container.encode(payload, forKey: .payload)             try container.encode(name, forKey: .name)         case .audio(let name, let payload):             try container.encode("audio", forKey: .type)             try container.encode(payload, forKey: .payload)             try container.encode(name, forKey: .name)         case .unsupported:             let context = EncodingError.Context(codingPath: [], debugDescription: "Invalid attachment.")             throw EncodingError.invalidValue(self, context)         }     } } struct Message: Codable {     let from: String     let text: String     let attachments: [Attachment] } let decoder = JSONDecoder() let message = try decoder.decode(Message.self, from: json) for attachment in message.attachments {     switch attachment {     case .image(let name, let payload):         print("image", name)         print("image", payload)     case .audio(let name, let payload):         print("audio",name)         print("audio", payload)     case .unsupported:         print("unsupported")     } }