Convert System.String generically to any complex type using “Convert.ChangeType()”

Question

I try to generically convert user input into either simple or complex types:

class Program
{
  static void Main(string[] args)
  {
    Console.WriteLine("Welcome, please provide the following info... Confirm with <RETURN>!");
    Console.WriteLine();    

    Console.Write("Name (e.g. 'Peggy Sue'): ");
    var user = GetUserInput<User>(Console.ReadLine());

    Console.WriteLine();
    Console.WriteLine();
    Console.WriteLine("Hi {0}, nice to meet you!", user.Forename);
    Console.WriteLine();

    Console.Write("Age: ");
    user.Age = GetUserInput<ushort>(Console.ReadLine());

    Console.WriteLine();
    Console.WriteLine("Thanks and goodbye!");
    Console.WriteLine("Press <RETURN> to quit...");
    Console.ReadLine();
  }

  static T GetUserInput<T>(string data)
  {
    return (T) Convert.ChangeType(data, typeof (T));
  }
}

class User
{
  public User(string name)
  {
    var splitted = name.Split(' ');
    Forename = splitted[0];
    Lastname = splitted[1];
  }

  public static implicit operator User (string value)
  {
    return new User(value);
  }

  public static explicit operator string (User value)
  {
    return string.Concat(value.Forename, " ", value.Lastname);
  }

  public string Forename { get; private set; }
  public string Lastname { get; private set; }

  public ushort Age { get; set; }
}

For the conversion to my "User" class, I always get the exception "Invalid cast from 'System.String' to 'ConsoleApplication1.User'.". Does anyone know how to fix this?

If I try something like this (not generically), it works just perfect:

Console.WriteLine((string) ((User) "Peggy Sue"));

Answer 1

No, Convert.ChangeType only works with a fixed set of types, I believe... or if the original object implements IConvertible , it can call IConvertible.ToType . That means you could implement IConvertible in your User class and have

Convert.ChangeType(user, typeof(string))

working, but that won't work the other way round.

Do you have a fixed set of types you need to convert? If so, you could have a Dictionary<Type, Func<string, object>> which you'd populate with conversion delegates. Then you just need to call the appropriate conversion and cast the return value. It's ugly, but probably your best bet.

Answer 2

One option here might be to associate a TypeConverter with the types you care about (you can do this at compile-time via [TypeConverter(...)] , or there is a trick for doing this at runtime if you don't control the types).

Then it is:

TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));
T obj = (T)conv.ConvertFromString(text); // or ConvertFromInvariantString

Answer 3

I fixed it. Check this:

class Program
{
  static void Main(string[] args)
  {
    Console.WriteLine("Welcome, please provide the following info... Confirm with <RETURN>!");
    Console.WriteLine();

    Console.Write("Name (e.g. 'Peggy Sue'): ");
    var user = GetUserInput<User>(Console.ReadLine());

    Console.WriteLine();
    Console.WriteLine();
    Console.WriteLine("Hi {0}, nice to meet you!", user.Forename);
    Console.WriteLine();

    Console.Write("Age: ");
    user.Age = GetUserInput<ushort>(Console.ReadLine());

    Console.WriteLine();
    Console.WriteLine("Thanks and goodbye!");
    Console.WriteLine("Press <RETURN> to quit...");
    Console.ReadLine();
  }

  static T GetUserInput<T>(string data)
  {
    TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));
    return (T) conv.ConvertFromInvariantString(data);
  }
}

[TypeConverter(typeof(UserConverter))]
class User
{
  public User(string name)
  {
    var splitted = name.Split(' ');
    Forename = splitted[0];
    Lastname = splitted[1];
  }

  public static explicit operator User (string value)
  {
    return new User(value);
  }

  public static explicit operator string (User value)
  {
    return string.Concat(value.Forename, " ", value.Lastname);
  }

  public string Forename { get; private set; }
  public string Lastname { get; private set; }

  public ushort Age { get; set; }
}

class UserConverter : TypeConverter
{
  public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
  {
    return (typeof(string) == sourceType);
  }

  public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
  {
    if (value is string)
    {
      return (User)(value as string);
    }

    return null;
  }
}

Answer 4

If you're looking to convert to numeric types, I prefer the much briefer and intent-exposing:

decimal.Parse(someString)

Or, in your example:

new User(userName)

No reason to create an entire method (or class, if you decide in the future to "make this reusable") just to wrap a cast. This is especially true when the language already has a less opaque way to express the intent of your code.