c# Interfaces and Generic Types

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Hi,

I am trying to create a generic interface. Just a simple one to start:

C#

interface IFactory<T> where T: IModel
{
    T Get<T>(int id);
}

When I implement this interface I would like it to match this:

C#

public class MemberFactory : IFactory<Member>
{
    Member Get(int id)
    {

    }
}

but all i get is:

C#

public class MemberFactory : IFactory<Member>
{
    T Get<T>(int id)
    {

    }
}

I looked through the object library for examples (i.e. IList<member>) which does implement correctly, but I'm guessing that there is some meta information I'm missing?

Any help would be great. I tried to search t'internets but either my googlefu is not strong or the search terms lead to far too many basic tutorials. Anyway - I'll stop waffling.

Thanks ^_^

Posted 28-Jan-15 5:26am

Andy Lanng

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TechJosh · Accepted Answer · 2015-01-28T05:39:00

Solution 1

In your IFactory<t> interface the generic type parameter is specified at the interface level. However, your Get<t> method also specifies a generic parameter of type T... This is likely the problem. the type parameter T is valid for the whole interface and can be treated as a type. You don't need your Get() method to have a type parameter as well and can force a return type of "T" without it in your interface.

Once done, implementing the interface will be exactly as you desire.

C#

public interface IFactory<T> where T: IModel
{
    T Get(int id);
}

public class Member : IModel
{
}

public class MemberFactory : IFactory<Member>
{
    public Member Get(int id)
    {
        throw new NotImplementedException();  
    }
}

Posted 28-Jan-15 5:39am

TechJosh

Updated 28-Jan-15 5:55am

v4

Comments

PIEBALDconsult 28-Jan-15 11:41am

~~I'm pretty sure MemberFactory will need to be generic.~~

Richard Deeming 28-Jan-15 11:43am

Why? It's quite normal for a non-generic class to implement a generic interface - think IEquatable<T>, for example.

TechJosh 28-Jan-15 11:51am

I don't see why that would be necessary. You're not trying to make a factory that can return instances of any type that matches the type parameter... your making a factory that returns a specific type. And your specific type is declared in the class declaration.

You could support multiple types in the same class if you wish, multiple specific types by implementing the IFactory<t> interface for each type. <pre lang="c#">public class MemberRoleFactory : IFactory<Member>, IFactory<Role> If you want to support all types (that match the IFactory<t> type constraints) you could make it generic but I don't think thats what is being asked here.

PIEBALDconsult 28-Jan-15 11:53am

I wrote that before you posted V2.

TechJosh 28-Jan-15 11:56am

Yeah, somehow my copy & paste munged <member> into an html tag.

PIEBALDconsult 28-Jan-15 12:00pm

Yeah, usually I use Improve to see the actual markup of something with generics, but CP refused to show it this time.

Richard Deeming 28-Jan-15 11:44am

You'll either need to make the Get member public, or make it an explicit implementation:
Member IFactory<Member>.Get(int id)

TechJosh 28-Jan-15 11:54am

You are correct... I'll update my answer. Also I'd make the interface public or internal, private interfaces are of limited use and generally only when implementing child classes with a common interface.

Andy Lanng 28-Jan-15 11:44am

Oic! Because using the second <t> meant that I was using a new instance of a generic type! Eureka!
I knew it's be something simple :) thanks

Sergey Alexandrovich Kryukov 28-Jan-15 12:37pm

I up-voted the answer with my 4. It needs some essential addition. Please see Solution 2.
—SA

Sergey Alexandrovich Kryukov · Accepted Answer · 2015-01-28T06:32:00

Solution 1 explains the implementation of interface using implicit implementation where public modifier is essentially required (which you missed). However, this is only good if you really need to use this members in some other ways, not only through the interface (and it's best to avoid, in most cases).

In other cases, it's better to use the explicit implementation of the interface. It gives the access to the implemented member only throw the interface reference, which is clearer. Here is how:

C#

public class Member : IModel { /* ... */ }

public class MemberFactory : IFactory<Member>
{
    Member IFactory<Member>.Get(int id)
    {
        //...
    }
}

I also want to add that MemberFactory itself can better be generic. If some functionality depends on the specific generic types, you can implement all common functionality (common to generic parameters), make it abstract, and override the specifics in derived classes. This is how:

C#

public interface IModel { };
public interface IFactory<Member> where Member: IModel { Member Get(int id); }

public class Member : IModel { /* ... */ }

public class MemberFactory<Member> : IFactory<Member> where Member: IModel {
    Member IFactory<Member>.Get(int id) {
        return default(Member); //replace with real code
    }
    //overide is derived classes to create specialization:
    protected abstract void Something(Member member);
}

—SA