Generic type struct in dictionary

Question

I want to create a C# struct that stores a class type property which later can be instantiated.

I created the struct like this:

internal struct Command<T>
{
    internal string trigger;
    internal T clazz;
    internal string category;
}

Then I want to store these Command structs in a dictionary:

private Dictionary<string, Command> _commandMap;

How do I define the dict so that its value type is set correctly for Command? Or is my whole approach wrong?

Are you trying to store multiple different kinds of command in the dictionary, e.g. a `Command` and a `Command`? If so, you're definitely going to run into problems. (I'd also strongly recommend keeping fields private, and using properties, as well as making the struct immutable.) — Jon Skeet, Feb 23 '19 at 14:54
@JonSkeet The command shouldn't be of types like int or string but of class types (e.g. MyCustomCommandClass). — BadmintonCat, Feb 23 '19 at 14:56
Well string is a class as well... it sounds like you might want a constraint on `T`. But back to the dictionary - would you want to be able to store a `Command` and a `Command` in the same dictionary? If you could give us more context, it will be easier to help you. — Jon Skeet, Feb 23 '19 at 14:57
@JonSkeet yes there can be different command classes but they all implement a certain interface, e.g. ICommand. Sorry for the lack of context! I'm still figuring the whole logic out in my head while trying to port it from some old AS3 code). — BadmintonCat, Feb 23 '19 at 15:01
@JonSkeet can this be accomplished with a covariant generic interface, such as `ICommand where T: someKnownBaseType`? I am not familiar enough with the topic, unfortunately, but it sounds like the right use case. — CoolBots, Feb 23 '19 at 16:24
Not if there are going to be multiple types in the same dictionary. But you could create a *non-generic* interface, and have `Dictionary`. Note that that would box each of the values though. — Jon Skeet, Feb 23 '19 at 18:21

score 2 · Accepted Answer · answered Feb 23 '19 at 16:35

2

If you want your dictionary to be type specific:

private Dictionary<string, Command<my_type>> _commandMap;

If you want everything in one dictionary, then don't use a generic for the class.
You wrote, that you want to store a class type property to instantiate it later. Yet in your code you store a class instance, not a class type.
I think this is what you are looking for:

internal struct Command
{
    internal string trigger;
    internal Type clazz;
    internal string category;
}

or

internal struct Command
{
    internal string trigger;
    internal IMyInterface clazz;
    internal string category;
}

and finally:

private Dictionary<string, Command> _commandMap;

answered Feb 23 '19 at 16:35

G. B.

528
2
15

Yes, I think `Type` is what I was looking for, thanks! But one question... how do I instantiate `clazz` later? It seems `var c = new command.clazz()` isn't the way it works in C#. – BadmintonCat Feb 23 '19 at 17:25
I think the question may need some clarification, but, from context and comments, I am pretty certain OP wants to store an instance of a generic type `T` within the `Command` struct. – CoolBots Feb 23 '19 at 17:25
@CoolBots no, not an instance. I want to store the actual class type so I can later instantiate new objects from it. – BadmintonCat Feb 23 '19 at 17:26
object instance = Activator.CreateInstance(type); like here: https://stackoverflow.com/questions/11958825/c-sharp-creating-instance-of-class-and-set-properties-by-name-in-string – G. B. Feb 23 '19 at 17:29
https://riptutorial.com/csharp/example/15938/creating-an-instance-of-a-type – G. B. Feb 23 '19 at 17:31
@G.B. Thanks for the links! That's going to helpful! – BadmintonCat Feb 23 '19 at 17:35
If what you want to store is a reference value then you can add a constraint for `T` like `internal struct Command where T : new()` then you can do `var instance = new T()` – Diego Osornio Feb 23 '19 at 18:56

Generic type struct in dictionary

1 Answers1