Can I return a collection of multiple Derived Type

问题:

I have a class structure similar to this:

public abstract class Device
{
    public int DeviceId { get; set; }
    //Additional Properties
}

public class DeviceA : Device
{
    //Specific Behaviour
}

public class DeviceB : Device
{
    //Specific Behaviour
}

I need to retrieve a list of Devices, or a single Device which is instantiated as the appropriate derived type (based upon a Type value in the Device Record in the DB). That is, the collection of Device objects should contain a number of objects with different Types, all of which are derived from Device.

I have implemented this the following way, but something just doesn't feel right about it.

public static IEnumerable<Device> AllDevices()
{
    using (var connection = CreateConnection())
    {
        connection.Open();
        return connection.Query<dynamic>("SELECT * FROM Device").Select<dynamic, Device>(d =>
            {
                Device device = null;
                if (d.DeviceTypeID == 1)
                    device = new DeviceA();
                else if (d.DeviceTypeID == 2)
                    device = new DeviceB();
                else throw new Exception("Unknown Device");
                device.DeviceId = d.DeviceID;
                return device;
            });
    }
}

Is this the correct way to achieve this using Dapper, or is there a better approach?

回答1:

In the current build that is probably the only option (especially since the base-type is abstract). However, it wouldn't be unreasonable to think of ways of suggesting a discriminated inheritance system. It isn't something we've done so far simply because it hasn't come up - but it doesn't sound impossible. The biggest problem I can see (other than IL-wrangling, obviously) is simply how we express the relationship.

回答2:

I've came up with this solution:

using (IDbConnection db = new MySqlConnection(ConfigurationManager.ConnectionStrings["yourConnection"].ConnectionString))
        {
            return db.Query<dynamic, DeviceA, DeviceB, Device>(@"
                Select
                    Discriminator,
                    ...
                From Device", (d, da, db) =>
                {
                    if (p.Discriminator == "DeviceA")
                    {
                        return new DeviceA();
                    }
                    else if (p.Discriminator == "DeviceB")
                    {
                         return new DeviceB();
                    }
                    return d;
                });       

Sounds tricky, but it does work!

Hope it can help you. }