C# creating my own class that acts like a string

hpjchobbes

I am trying to create some classes that act like a string, but allow me to test the type at a later date. Because string and String are sealed, I can't seem to inherit from them. I also found out you can't override the assignment operator. I found something close to what I want here on stackoverflow, but it uses structs which can't be inherited. Here's a example of what I want to do, but I can't figure out how:

public class ORSerialLotNumber : string {}
public class SerialNumber : ORSerialLotNumber {}
public class LotNumber : ORSerialLotNumber {}

// ......

List mySLNList = new List();
SerialNumber sNum1 = "Sd-23s-3sf";
LotNumber nNum1 = "2342434";

mySLNList.Add(sNum1);
mySLNList.Add(nNum1);

// ......

foreach(var v in mySLNList)
if(v.GetType() == typeof(SerialNumber))
string serial = v;
if(v.GetType() == typeof(LotNumber))
string lot = v;

Lost User

That example doesn't really depend on it being a struct, it's about the implicit conversion operator. Here's a minimal implementation of a class to implicitly converts to and from string:

class CustomString
{
public static implicit operator CustomString(string s)
{
return new CustomString();
}

public static implicit operator string(CustomString s)
{
    return "";
}

}

Do that for both subclasses. By the way, as far as I can tell you should be able to use is[^] instead of the GetType() == typeof() "not-quite-anti-pattern-but-close".

hpjchobbes

Thanks, this seems to be just what I need, though I imagine that I need to save the actual string value and have the constructor set that value and the operator string(CustomString s) return the string instead of an empty string?

class CustomString
{
private string _value;

private CustomString(string s)
{
    \_value = s;
}

public static implicit operator CustomString(string s)
{
    return new CustomString(s);
}

public static implicit operator string(CustomString s)
{
    return s.\_value;
}

}

Would I also be able to implement INotifyPropertyChanged with this class? I don't see how as there's no public property. Would I just use "_value" even though it's private?

Lost User

You could just give it a public property though, right?

OriginalGriff

There wouldn't be a whole lot of point in implementing the interface if he doesn't! :laugh:

Those who fail to learn history are doomed to repeat it. --- George Santayana (December 16, 1863 – September 26, 1952) Those who fail to clear history are doomed to explain it. --- OriginalGriff (February 24, 1959 – ∞)

hpjchobbes

So the idea would be to include a public property, but still have the implicit operator act on that property? Would that then allow someone to either use the implicit operator or the property directly?

Lost User

Yes, is it giving you any trouble?

hpjchobbes

No, I just didn't even think about this, or even know that it was possible. I'm reading more about `implicit`, which seems very useful. It doesn't seem to be touched on in most 'beginner' reading, though.

BobJanova

I'm not quite sure what you're trying to achieve here. I think it is that you want a data object that has a 'number' (a string identifier) and also a type. Using GetType or is/as functions is generally a sign that you haven't quite got the solution you want; switching based on type usually implies you need more polymorphic behaviour. I think the better solution is

enum ItemType { Serial, Lot };
struct ObjectIdentifier {
string Identity { get; set; }
ItemType Type { get; set; }
}

List<ObjectIdentifier> mySLNList = ...;

foreach(ObjectIdentifier oi in mySLNList)
switch(oi.Type)
...