Can Swift convert a class / struct data into dictionary?

You can just add a computed property to your struct to return a Dictionary with your values. Note that Swift native dictionary type doesn't have any method called value(forKey:). You would need to cast your Dictionary to NSDictionary:

struct Test {
    let name: String
    let age: Int
    let height: Double
    var dictionary: [String: Any] {
        return ["name": name,
                "age": age,
                "height": height]
    }
    var nsDictionary: NSDictionary {
        return dictionary as NSDictionary
    }
}

You can also extend Encodable protocol as suggested at the linked answer posted by @ColGraff to make it universal to all Encodable structs:

struct JSON {
    static let encoder = JSONEncoder()
}
extension Encodable {
    subscript(key: String) -> Any? {
        return dictionary[key]
    }
    var dictionary: [String: Any] {
        return (try? JSONSerialization.jsonObject(with: JSON.encoder.encode(self))) as? [String: Any] ?? [:]
    }
}

struct Test: Codable {
    let name: String
    let age: Int
    let height: Double
}

let test = Test(name: "Alex", age: 30, height: 170)
test["name"]    // Alex
test["age"]     // 30
test["height"]  // 170

You could use Reflection and Mirror like this to make it more dynamic and ensure you do not forget a property.

struct Person {
  var name:String
  var position:Int
  var good : Bool
  var car : String

  var asDictionary : [String:Any] {
    let mirror = Mirror(reflecting: self)
    let dict = Dictionary(uniqueKeysWithValues: mirror.children.lazy.map({ (label:String?, value:Any) -> (String, Any)? in
      guard let label = label else { return nil }
      return (label, value)
    }).compactMap { $0 })
    return dict
  }
}


let p1 = Person(name: "Ryan", position: 2, good : true, car:"Ford")
print(p1.asDictionary)

["name": "Ryan", "position": 2, "good": true, "car": "Ford"]

A bit late to the party, but I think this is great opportunity for JSONEncoder and JSONSerialization. The accepted answer does touch on this, this solution saves us calling JSONSerialization every time we access a key, but same idea!

extension Encodable {

    /// Encode into JSON and return `Data`
    func jsonData() throws -> Data {
        let encoder = JSONEncoder()
        encoder.outputFormatting = .prettyPrinted
        encoder.dateEncodingStrategy = .iso8601
        return try encoder.encode(self)
    }
}

You can then use JSONSerialization to create a Dictionary if the Encodable should be represented as an object in JSON (e.g. Swift Array would be a JSON array)

Here's an example:

struct Car: Encodable {
    var name: String
    var numberOfDoors: Int
    var cost: Double
    var isCompanyCar: Bool
    var datePurchased: Date
    var ownerName: String? // Optional
}

let car = Car(
    name: "Mazda 2",
    numberOfDoors: 5,
    cost: 1234.56,
    isCompanyCar: true,
    datePurchased: Date(),
    ownerName: nil
)

let jsonData = try car.jsonData()

// To get dictionary from `Data`
let json = try JSONSerialization.jsonObject(with: jsonData, options: [])
guard let dictionary = json as? [String : Any] else {
    return
}

// Use dictionary

guard let jsonString = String(data: jsonData, encoding: .utf8) else {
    return
}

// Print jsonString
print(jsonString)

Output:

{
  "numberOfDoors" : 5,
  "datePurchased" : "2020-03-04T16:04:13Z",
  "name" : "Mazda 2",
  "cost" : 1234.5599999999999,
  "isCompanyCar" : true
}

Use protocol, it is an elegant solution.
1. encode struct or class to data
2. decode data and transfer to dictionary.

/// define protocol convert Struct or Class to Dictionary
protocol Convertable: Codable {

}

extension Convertable {

    /// implement convert Struct or Class to Dictionary
    func convertToDict() -> Dictionary<String, Any>? {

        var dict: Dictionary<String, Any>? = nil

        do {
            print("init student")
            let encoder = JSONEncoder()

            let data = try encoder.encode(self)
            print("struct convert to data")

            dict = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? Dictionary<String, Any>

        } catch {
            print(error)
        }

        return dict
    }
}

struct Student: Convertable {

    var name: String
    var age: Int
    var classRoom: String

    init(_ name: String, age: Int, classRoom: String) {
        self.name = name
        self.age = age
        self.classRoom = classRoom
    }
}

let student = Student("zgpeace", age: 18, classRoom: "class one")

print(student.convertToDict() ?? "nil")

ref: https://a1049145827.github.io/2018/03/02/Swift-%E4%BB%8E%E9%9B%B6%E5%AE%9E%E7%8E%B0%E4%B8%80%E4%B8%AAStruct%E6%88%96Class%E8%BD%ACDictionary%E7%9A%84%E9%9C%80%E6%B1%82/

This answer is like the above which uses Mirror. But consider the nested class/struct case.

extension Encodable {
    func dictionary() -> [String:Any] {
        var dict = [String:Any]()
        let mirror = Mirror(reflecting: self)
        for child in mirror.children {
            guard let key = child.label else { continue }
            let childMirror = Mirror(reflecting: child.value)
            
            switch childMirror.displayStyle {
            case .struct, .class:
                let childDict = (child.value as! Encodable).dictionary()
                dict[key] = childDict
            case .collection:
                let childArray = (child.value as! [Encodable]).map({ $0.dictionary() })
                dict[key] = childArray
            case .set:
                let childArray = (child.value as! Set<AnyHashable>).map({ ($0 as! Encodable).dictionary() })
                dict[key] = childArray
            default:
                dict[key] = child.value
            }
        }
        
        return dict
    }
}