如何创建实现基本类型通用接口的类的实例

Question

Lets say I have this:

public class BaseEntity {}

public class Product : BaseEntity {}

public interface IPreLoad<T> where T : BaseEntity 
{
    void Preload();
}

public class ProductService : IPreload<Product> 
{
    public void Preload()
    {
        //Do something
    }
}

//Returns list of types implementing IPreload<>
var preloaders = typeFinder.FindClassesOfType(typeof(IPreLoad<>)).ToList(); 

foreach (var preloaderType in preloaders)
{
    var preloader = (IPreLoad<BaseEntity>)EngineContext.Current.ResolveUnregistered(preloaderType);
    cacheManager.Get(preloader.PreLoadCacheKey, () => preloader.PreLoad());
}

The issue I have is that I can't cast a ProductService : IPreLoad<Product> to a IPreLoad<BaseEntity> and I don't know how else I can cast this to achieve the same result. Basically, I want to create an instance of this service and call preload.

Not performing the cast works fine and it creates an instance but then as I haven't cast it as anything, I can't make any calls to it based on the interface.

Answer 1

Your interface should be covariant:

public interface IPreLoad<out T> where T : BaseEntity 
{
    void Preload();
}

Then, assigment will be ok:

IPreLoad<BaseEntity> preLoader = new ProductService();

Answer 2

In your example, The Preload Method is not affected by the generic type T, you could use a non generic interface like

public interface IPreLoad
{
    void Preload();
}

public interface IPreLoad<T>: IPreload where T : BaseEntity 
{
   // your other typed methods...
}

then you cast your objects as IPreload instead of IPreLoad<BaseEntity>