Inheritance 与 generics 接口

Question

I'm trying to migrate an application which has to read from various XML, each of them having different models and methods/stuff to do before persisting them into db. For that reason, I've decided to create an interface IEntityController<T> where T: PersistentEntity and a controller implementing it for each entity (XML), this way they all can have different implementations...

    public interface IEntityController<T> where T : PersistentEntity
{
    /// <summary>
    /// Check if the data inserted into the xml is valid and all the requirements are met
    /// </summary>
    /// <exception cref="FooValidationException">When the instance is not valid and can't be saved</exception>
    void Validate(T entity);

    /// <summary>
    /// Implement changes in data before its saved
    /// </summary>
    void PreProcess(T entity);

    /// <summary>
    /// Save or Update the entity
    /// </summary>
    /// <exception cref="System.Data.SqlClient.SqlException"></exception>
    void Persist(T entity);

    /// <summary>
    /// Implement changes in data after the entity is saved
    /// </summary>
    void PostProcess(T entity);
}

After that, I've got a factory method which returns the proper controller based on the xml:

        private IEntityController<PersistentEntity> GetController(XmlFile file)
    {
        Type fileType = file.GetType();

        if(fileType == typeof(FileBoo))
            return (IEntityController<PersistentEntity>) new BooController(DC);
      ......
    }

My problem is: BooController implements the interface of IEntityController<Boo> where Boo inherits PersistentEntity , when trying to cast the controller to IEntityController<PersistentEntity> I get an InvalidCastException , though I have no problems casting BooController to IEntityController<Boo> , It's like C# ignores inheritance of the generic types.

Any ideas on how to solve this/implement it better? Thanks

Answer 1

The simplest thing you can do is to remove the generic type from IEntityController so that it looks like this:

 public interface IEntityController
{
    /// <summary>
    /// Check if the data inserted into the xml is valid and all the requirements are met
    /// </summary>
    /// <exception cref="FooValidationException">When the instance is not valid and can't be saved</exception>
    void Validate(PersistentEntity entity);

    /// <summary>
    /// Implement changes in data before its saved
    /// </summary>
    void PreProcess(PersistentEntity  entity);

    /// <summary>
    /// Save or Update the entity
    /// </summary>
    /// <exception cref="System.Data.SqlClient.SqlException"></exception>
    void Persist(PersistentEntity entity);

    /// <summary>
    /// Implement changes in data after the entity is saved
    /// </summary>
    void PostProcess(PersistentEntity  entity);
}

Since your method

private IEntityController<PersistentEntity> GetController(XmlFile file)

always returns an IEntityController of PersistentEntity and not an IEntityController of T you don't need generics at all.

Remember:

List<string> doesn't inherit from List<object>

otherwise this could happen

public static void DestroyTheWorld(List<object> list) {
    list.add(new object());
}

public static void Main() {
    var list = new List<string>();
    DestroyTheWorld(list); //possible if list<string> inherits from list<object>
    //an object has been added to a list of string
}