not suitable method found to override generic

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public class SomeRepository<TContext> : IDisposable
            where TContext : DbContext, new()
        {
            protected TContext context;
            protected SomeRepository()
            { }

        public virtual void Create<T>(T item) where T : class, new()
        {
            ...
        }
        }

    internal class SomeCrud : SomeRepository<SomeContext>
    {
        public override void Create(ConcreteType item)
        {
            ....
        }
    }
}

Hello I have base class like SomeRepository with four standart methods.Each one this method can work with different types.That works good.
But I got a situation that I need to add some instruction to one of those methods in derived class.That's why I signed like virtual in base class and tried to override one of this method with concrete type and I got a error
not suitable method found to override on deriveed overriden method please help me!

Posted 19-Jun-12 9:39am

thomas_wingfield

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[no name] 19-Jun-12 15:58pm

public override void Create<T>(T item) { base.Create<T>(item); }

thomas_wingfield 19-Jun-12 16:04pm

Wes Aday. If I want that derived Create methd should seems on Create in base class, but with one different instruction in derived method.If I would call base
I can't add this instruction.I need Create method with own logic in derived class without logic of base class method

Karthik. A 19-Jun-12 16:06pm

Exactly, Wes Aday and my answer's are just stubs. You just have to remove the call base.Create<t>(item) and add your own implementation.

[no name] 19-Jun-12 16:09pm

That make absolutely no sense at all. Are you overriding Create or not?

thomas_wingfield 19-Jun-12 16:16pm

Yes I'm overriding but it should seems to Create of base class but a little bit another

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Sergey Alexandrovich Kryukov · Accepted Answer · 2012-06-19T11:41:00

This is something which could called generic specialization, by the analogy with C++ template specialization.

It looks like your efforts are useless until you work with generic methods; it is not possible to specialize the implementation via overriding for a concrete type. I want to give you an alternative idea: move a generic parameter from a method to a type. Just think about it:

C#

class Base<T> {
    internal protected virtual void Create(T item) { /*...*/ } // maybe, pseudo-abstract
} //class Base<T>

struct ConcreteType {/*...*/}

//this is something you really can do
class Derived : Base<ConcreteType> {
    internal protected override void Create(ConcreteType item) { /*...*/ }
} //class Derived

Pretty similar to what you wanted, but in this way it can work.

More typically, the first method would be abstract:

C#

abstract class Base<T> {
    internal protected abstract void Create(T item)
} //class Base<T>

class Derived : Base<ConcreteType> {
    internal protected override void Create(ConcreteType item) { /*...*/ }
} //class Derived

Note that you don't have to specialize all the generic parameters in the derived type, but still can override the method with a specialized generic parameter; this is how to do it:

C#

abstract class Base<SomeType, ParameterType> {
    internal protected abstract void Create(ParameterType item);
    // use SomeType in some other way
} //class Base<T>

class Derived<SomeType> : Base<SomeType, ConcreteType> { //second generic parameter "specialized out"
    internal protected override void Create(ConcreteType item) { /*...*/ }
    // use SomeType in some other way
} //class Derived

Are you getting it? This way you can get yourself enough flexibility which could suit your goals. Again, think about it.

—SA

Karthik. A · Accepted Answer · 2012-06-19T09:51:00

Solution 1

That's because the signature of the Create method is wrong. Change it to the following

C#

internal class SomeCrud : SomeRepository<SomeContext>
{
    public override void Create<T>(T item)
    {
        base.Create<T>(item);
    }
}

Shortcut to do this is, in the SomeCrud class, enter override first, then give a space. Now you will get a list of items that can be overridden. Just choose one, you will get the stub w/ a call to the base class's method.

Hope this helps!

Posted 19-Jun-12 9:51am

Karthik. A

Comments

thomas_wingfield 19-Jun-12 15:57pm

Sorry but it doesn't help.But thanks for post

Karthik. A 19-Jun-12 16:03pm

Sorry about that. But are you saying you get an error if you do this? You cannot just override a generic method with a concrete type, if you are trying to achieve something in the effect of what you have posted. Can you pls. elaborate your issue?

[no name] 19-Jun-12 16:03pm

What do you mean "it doesn't help"?

thomas_wingfield 19-Jun-12 16:05pm

check my comment for my question

Karthik. A 19-Jun-12 16:06pm

Pls. check my reply for your comment.

thomas_wingfield 19-Jun-12 16:13pm

My reply:)
I have logic that adds a Something to database in Create method of base class(Just simple crud).In derived class when I add class I have to attach some another class.Check my source code:
public virtual void Create<t>(T item) where T : class, new()
{
DbSet<t> set = GetDbSet(typeof(T)).GetValue(context, null) as DbSet<t>;
using (context)
{
set.Add(item);
Save(context);
}
}
That works good
But like this no:
public override void Create(Product item)
{
DbSet<Product> set = GetDbSet(typeof(Product)).GetValue(context, null) as DbSet<Product>;
DbSet<user> set1 = GetDbSet(typeof(User)).GetValue(context, null) as DbSet<user>;
using (context)
{
set1.Attach(item.User);
set.Add(item);
Save(context);
}
}

Karthik. A 19-Jun-12 16:20pm

This is not an override at all. Any override to Create must also be generic. Once you start having concrete item, there is no point overriding.

thomas_wingfield 19-Jun-12 16:15pm

I tried like this:
If i changed signature Product on T I can't see User property, If I change like
override void Create<Product>(Product item) my IntelSense can't see Product like a type.

Karthik. A 19-Jun-12 16:22pm

You are totally missing the point - an override of a constrained method should also be constrained. Only when you call this method, you get to pass a concrete type. Like, instance.Create<Product>(product);

thomas_wingfield 19-Jun-12 16:22pm

Can u provide some example?How I will see in item.User if it will be a T?

Karthik. A 19-Jun-12 16:26pm

You could have 2 constraints and have the method in the base class itself. Like this:

public virtual void Create<t,>(T tItem, U uItem)
where T : class, new()
where U : class, new()
{
throw new NotImplementedException();
}

Now you can attach the 2nd item and add the 1st item.

thomas_wingfield 19-Jun-12 16:29pm

Yes but if I will be use Signature generic like u said I can't see User in item
Yes I did it like u said with base Create method, but how I can do with derived?

Karthik. A 19-Jun-12 16:31pm

You don't even need the derived class, if you have this method in the base class, isn't it?

thomas_wingfield 19-Jun-12 16:30pm

I tried with a 2 constraints but it not comfortable.

Karthik. A 19-Jun-12 16:31pm

Not sure what you mean by that... what's not comfortable?

thomas_wingfield 19-Jun-12 16:40pm

Karthik I'm very tahnkfull for your help man.But I have to use only one type constriaint in method.
Look I tried right now this:
internal class ProductCrud<t> : AuctionRepository<auctioncontext> where T : Product
public override void Create(T item){}

That's helped me right now with error
But I'm new in programming with EF and WCF.I have a question.
new ProductCrud<crud>().Create(some).It' not bad architecture?Van I resolve it on ur opinion?

Karthik. A 19-Jun-12 16:47pm

You are welcome! I wouldn't say it's bad. But if it seems like you are doing the same for various other types (w/ nested types), I would advice having a generic method w/ 2 constraints. Because now you have tied the concrete type to your repository which is not that best method to do this in my opinion. As you are saying that you cannot have multiple constraints, I cannot think of any other better way right away.

thomas_wingfield 19-Jun-12 17:00pm

any way thanks a lot, if it will be bad idea I will try again with 2 constraints.
Did u mean add second constraint to AuctionRepository like class?

Karthik. A 19-Jun-12 17:06pm

You're welcome and no, I meant adding a constraint to the Create method - T and U, as I mentioned in an earlier comment.

thomas_wingfield 19-Jun-12 17:05pm

that was a bad solution, didn't help:(

Karthik. A 19-Jun-12 17:07pm

Then, I would definitely advice using 2 constraints.

Sergey Alexandrovich Kryukov 19-Jun-12 17:43pm

Those were correct explanations and arguments, but don't you think there should be some positive solution? The idea would be to modify the approach, but not giving up using OOP with the generic. I voted 4.

Want to see what's my idea? Please see my answer.
--SA