以接口作为通用参数的通用查询

Question

So I've been fighting against generics lately and don't quite know how to solve this one.

Basically I have a generic query framework defined as IRxQuery<T> . My basic model follows one rule: T needs to be an instance type. Here is the implementation header:

public class RxQuery<T> : IRxQuery<T>
        where T : IQueryableObject

One of the notable methods in this class is the Get method, which will deserialize the object got by the query into a T object. The method kinda looks like that:

    protected virtual T Get(IDeserializableObject row)
    {
        var value = Activator.CreateInstance<T>();

        // fill value 

        return value;
    }

Now, I would like to upgrade my query so it would be working with an interface. I am facing the current problem: I have an ISomething interface, implemented by ObjectA and ObjectB . I'd have a case where the query could be any of them and I am trying to fix it, without going adding inheritance. So I was trying something like:

IRxQuery<ISomething> query;
if (isObjectA)
{
   query = new RxQuery<ObjectA>();
}
else
{
   query = new RxQuery<ObjectB>();
}

And of course, you know that won't compile. The only solution I thought about so far would be to add a property to the IRxQuery type looking like:

public Type InstanceType{ get; set; }

And then update the Get method to this:

protected virtual T Get(IDeserializableObject row)
{
   T value = (T)Activator.CreateInstance(typeof(InstanceType))
}

What approach would you advise me to use to fix this problem?

Thank you!

Answer 1

It sounds like you're trying to make this work:

class Program {
    static void Main(string[] args) {
        //compile error: ISomething must be non-abstract with public
        //               parameterless constructor
        RxQuery<ISomething> something = new RxQuery<ISomething>();
    }
}
interface ISomething { };
interface IRxQuery<T> { }
public class RxQuery<T> : IRxQuery<T> where T:new() /*, IDeserializableObject*/ {
    //Factory Method Pattern
    protected virtual T Get(/*IDeserializableObject row*/) { return new T(); }
}

Get is a factory method and cannot instantiate an interface using new() . Even if you made T covariant ( IRxQuery<out T> ), you're no closer to it working. Your proposed solution of using Activator.CreateInstance (reflection) and having the Type information returned from IRxQuery might work, but regardless it's a poor solution in my opinion (it forfeits some compile-time checks, and requires reflection (obfuscation, if ever needed, could cause this code to crash)).

If T must be able to be an interface , then my suggestion is to find a new design such that RxQuery<T> does not need to construct an object of type T .