子类问题的通用方法约束

Question

First here's the break down of the class hierarchy:

public class Entity {
}

public abstract class AuditBase<T> {
   protected abstract void SomeMethod();
}

public class EntityAudit : AuditBase<Entity> {
   protected override void SomeMethod() {
   }
}

Then, I have a method with a constraint like so:

public async Task<IEnumerable<T>> Query<T>(int id) where T : AuditBase<T>
{
  var instance = Activator.CreateInstance<T>();
  var refTable = instance.RefTable;

  var collection = _context.Database.GetCollection<T>("Audit");
  var filter = String.Join("{ RefTable: '", refTable, "', Object: { Id: {'$eq': ", id.ToString(), "} } }");

  return await collection.FindSync<T>(filter).ToListAsync();
}

However, I get an error when I use the following code:

var result = await Query<EntityAudit>(5);

The error is:

The type 'EntityAudit' cannot be used as type parameter 'T' in the generic type of method 'Audit.Query<T>(int)'. There is no implicit reference conversion from 'EntityAudit' to 'AuditBase<EntityAudit>'.

I kind of understand the issue. My goal is really to restrict the returned IEnumerable to only contain objects that are children of AuditBase . Currently, it's trying to return items of type 'Entity' due to T .

I can't do the constraint as so: ... where T: Audit as Audit "requires one parameter."

Any thoughts? How do I implement the constraint?

EDIT:

So I ended up doing the following:

public async Task<IEnumerable<T>> Query<T,U>(int id) where T: AuditBase<U>
{
  ...
}

var result = await Query<EntityAudit, Entity>(5)

This works.

But, is there a better way?

Answer 1

create a non generic audit base base class

//...

public abstract class AuditBase {
 //...
}

public abstract class AuditBase<T> : AuditBase {
   protected abstract void SomeMethod();
}

//...

and use that for the constraint

public async Task<IEnumerable<T>> Query<T>(int id) where T : AuditBase {
    //...
}