c# - How can I design this better? (Avoiding a switch statement with Object Oriented design)

Question

I know a little bit about Object Oriented design, but I'm not sure how to go about using those principles in my code. Here's what I'm working on:

    public void Query(Agency agency, Citation queryCitation) {
        queryCitation.AgencyCode = agency.AgencyCode;

        switch (agency.ClientDb.Type) {
            case "SQL":
                QueryOracle(agency, queryCitation);
                break;
            case "PIC":
                QueryPick(agency, queryCitation);
                break;
        }
    }

(Most of these are objects from NHibernate. I'm working with a legacy database system and am refactoring parts of it into a code library.) Clearly, I could do something differently here so that I don't need duplicate functions for different database queries that have the same input. It should just know based off of the agency object whether to use an Oracle database or a Pick database connection. (If you've never heard of a Pick database, well I hadn't either until I started working here. We make queries to it through HTTP requests, so it's not SQL.)

Should I make an interface, for example called "ClientDbConnection" and then make two classes that implement that interface, move the code to query the database to those and then have something like "agency.clientDb.Query(queryCitation)" replace this whole function? I guess I'm thinking aloud here, but any input on this would be appreciated.

Answer 1

Is Agency a class you control? If so do something like this:

public abstract class GenericDb
{
    public abstract void Query(parms);
}

In your Agency Class, you could have

public GenericDb ClientDb {get; set;}

Then have a SqlDb class like:

public class SqlDb : GenericDb
{
    public void Query(parms);
}

public class PicDb : GenericDb
{
    public void Query(parms);
}

Then this code:

public void Query(Agency agency, Citation queryCitation) {
        queryCitation.AgencyCode = agency.AgencyCode;

        switch (agency.ClientDb.Type) {
            case "SQL":
                QueryOracle(agency, queryCitation);
                break;
            case "PIC":
                QueryPick(agency, queryCitation);
                break;
        }
    }

becomes

public void Query(Agency agency, Citation queryCitation) {
        queryCitation.AgencyCode = agency.AgencyCode;
        agency.ClientDb.Query(queryCitation);
    }

Because of inheritance, it will know that ClientDb has a base class of GenericDb. It will know by the type of the ClientDb parameter whether it should run the SqlDb or the PicDb or Oracle etc.