Generic Class Members in C#?

Question

Hey, I think I have the wrong idea here, but I'm not sure what is best. I want a class with a member variable that can be of any type, depending on what is needed at the time. So far, I have something like this:

    public class ConfigSetting<T> {
    private T value;

    public T GetValue() {
        return value;
    }
    public void ChangeValue() {

    }

    public ConfigSetting(string heading, string key) {
        this.value = DerivedMethods.configsettings.SettingGroups[heading].Settings[key].RawValue;
    }
}

The type returned by the right side of the 'this.value' line is a string, currently. I know here it seems like I have no need to use anything other than the string type, but eventually I will expand the constructor, such that 'this.value' could be a string, int, float, or bool.

Anyway, my compiler says "Cannot convert 'string' to 'T'", so I assume I'm doing something very backwards.

Thank you.

Answer 1

You're running into problems because this is not a good use of generics. If the generic type parameter can only be constructed in four different ways -- string, float, bool and int -- then this isn't very generic. I expect that a generic thing can be any type at all.

If I had a thing that could only be one of four types then I would model it like this:

abstract class ConfigSetting
{ /* shared code here */  }

class TextSetting : ConfigSetting
{ /* Code here to handle string settings */ }

class BooleanSetting : ConfigSetting
{ /* ... 

and so on. I would probably then give each of them an internal constructor, and make the base class into a factory for the derived classes, using the factory pattern.

Only use generics if your solution is truly generic. Like List<T>, for example, can be a list of anything: ints, strings, arrays, dictionaries, functions, whatever. If the thing you are modeling has a small number of possible types, just make one for each type.

Answer 2

Well, what conversion did you expect it to apply? If you expect the value to already be of the right type, you could do:

object tmp = DerivedMethods.configsettings.SettingGroups[heading].Settings[key].RawValue;
this.value = (T) tmp;

Note that you have to go through object either implicitly (as here) or with an explicit cast:

this.value = (T)(object) DerivedMethods.configsettings... (etc);

The set of conversions provided for generic types is somewhat limited. But it should work if the original value is genuinely correct.