C#: Using a dictionary of string, interface to reference different classes

Question

I would like to create a dictionary that uses strings as keys to instantiate objects. Here is the start of my dictionary:

Dictionary<string, ITerminalCommand> validInputs = new Dictionary<string, ITerminalCommand>()
{
    {"help",  TerminalCommandHelp},
    {"exit",  TerminalCommandExit},
};

These terminal command classes implement the ITerminalCommand interface as such:

public class TerminalCommandHelp : MonoBehaviour, ITerminalCommand
{
    //contents of class correctly implementing interface
}

The problem is that when I declare and initialize my dictionary, I'm getting an error saying

"TerminalCommandHelp" is a type, which is not valid in the given context.

I thought interfaces could be use abstractly to represent any class that implements from it? Ultimately, when the user looks up a key, I want to create an instance of that particular class. Can someone point out my misunderstanding? Thank you!

You're trying to pass the Type to your dictionary not an initialized object. You need an instance of TerminalCommandHelp and TerminalCommandExit not their names. — Cubicle.Jockey, Aug 03 '20 at 19:00
`I thought interfaces could be use abstractly to represent any class that implements from it` that's correct. But the way you are trying to use is not correct. You need to add objects of classes to the dictionary. Not the class themselves. — Chetan, Aug 03 '20 at 19:00
Following up on the other correct comments, you need to do `new TerminalCommandHelp()` and `new TerminalCommandExit()`. Demo fiddle here: https://dotnetfiddle.net/6TNeTR — dbc, Aug 03 '20 at 19:02
I see -- so is there a way to initialize a class from this dictionary by saying something like: new validEntries["help"]()? Or must I only pass in an initialized object, and use that initialized object? I cannot create more this way? — Vranvs, Aug 03 '20 at 19:02
You are creating a dictionary of `ITerminalCommand` objects, so you need to properly construct each object you add. See [Using Constructors (C# Programming Guide)](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/using-constructors). Or are you really trying to create a dictionary of `ITerminalCommand` **factories**, like the one in [Factory pattern, Avoid same switch case for different interface](https://stackoverflow.com/a/50927254/3744182)? — dbc, Aug 03 '20 at 19:06
I am not formally trained in C#, but I think a factory is what I want. In essence, when I lookup an entry in the dictionary via the "help" key, I want to instantiate a new TerminalCommandHelp object and pass in a few parameters into the constructor. So, I need I guess an abstract reference to the class/type, instead of a pre-initialized object, if that makes sense? — Vranvs, Aug 03 '20 at 19:08
Then please [edit] your question to clarify. Possibly [Factory pattern, Avoid same switch case for different interface](https://stackoverflow.com/q/50927069/3744182) is a duplicate though, just use `Dictionary>` instead of `Dictionary>`. — dbc, Aug 03 '20 at 19:10

score 0 · Accepted Answer · answered Aug 03 '20 at 19:02

//You are trying to pass their type not an instance.
Dictionary<string, ITerminalCommand> validInputs = new Dictionary<string, ITerminalCommand>()
    {
        {"help",  TerminalCommandHelp},
        {"exit",  TerminalCommandExit},
    };

//Initialize your types into objects and put those in your Dictionary.
IDictionary<string, ITerminalCommand> validInputs = new Dictionary<string, ITerminalCommand>()
    {
        {"help",  new TerminalCommandHelp()},
        {"exit",  new TerminalCommandExit()},
    };

C#: Using a dictionary of string, interface to reference different classes

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