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Q&A

Declaring interface members with generic return types: "The type parameter [generic type] cannot be used with type arguments"

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I'm trying to write an interface to define the set of operations I expect my repositories' Unit of Work implementations to have, and I want this interface to be fulfilled by EF Core's DbContext class.

So far this is what I got: (I know the name is awful, suggestions accepted):

public interface IDbContextable<TContext, TEntityEntry>
    where TContext : class, IDisposable
    where TEntityEntry : class
{
    public TEntityEntry Add([NotNullAttribute] object entity);

    public TEntityEntry<TEntity> Add<TEntity>([NotNullAttribute] TEntity entity)
        where TEntity : class;

    public int SaveChanges();
}

The first Add method's signature from EF Core is: public virtual EntityEntry Add([NotNullAttribute] object entity); - and it works fine.

I'm having trouble with the second. The compiler complains:

Error CS0307 The type parameter 'TEntityEntry' cannot be used with type arguments

For completeness, my context class looks like this:

public partial class EntityFrameworkContext : DbContext, IDbContextable<EntityFrameworkContext, EntityEntry>
{
    //... some extra methods here
}

How to properly declare the interface to get rid of the error, and so that the signature: public virtual EntityEntry<TEntity> Add<TEntity>([NotNullAttribute] TEntity entity) where TEntity : class; from EF Core becomes valid?

Thanks.

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10 comments

Not sure I follow the Q. TEntityEntry type isn't generic. Why can't you just write public TEntityEntry Add<TEntity>([NotNullAttribute] TEntity entity) where TEntity : class;? FoggyFinder‭ 28 days ago

@FoggyFinder I tried, but then in my class that inherits from DbContext I get: Error CS0738 'EntityFrameworkContext' does not implement interface member 'IDbContextable<EntityFrameworkContext, EntityEntry>.Add<TEntity>(TEntity)'. 'DbContext.Add<TEntity>(TEntity)' cannot implement 'IDbContextable<EntityFrameworkContext, EntityEntry>.Add<TEntity>(TEntity)' because it does not have the matching return type of 'EntityEntry'. Marc.2377‭ 28 days ago

... then the only valid implementation would be: public EntityEntry Add<TEntity>(TEntity entity) where TEntity : class - but I want the implementation that already exists in the base class (DbContext) to be valid. And its signature returns EntityEntry<TEntity>. Marc.2377‭ 28 days ago

ah, the goal is avoiding something like that EntityEntry IDbContextable<EntityFrameworkContext, EntityEntry>.Add<TEntity>(TEntity entity) => base.Add(entity); in the EntityFrameworkContext class? FoggyFinder‭ 28 days ago

@FoggyFinder yes, the interface must be applicable to the method already defined in the base class, which in this case is part of the framework. Marc.2377‭ 28 days ago

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2 answers

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The short answer is that it can't be done. C# doesn't support type functions. https://github.com/dotnet/csharplang/issues/339 is probably the issue to follow if you want to track their progress.

Workarounds are going to require another type parameter and probably a factory method: e.g.

    public TEntityEntry_TEntity Add<TEntity, TEntityEntry_TEntity>(
            [NotNull] TEntity entity,
            [NotNull] Func<TEntity, TEntityEntry_TEntity> wrap)
        where TEntity : class
        where TEntityEntry_TEntity : TEntityEntry;
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6 comments

Thanks. Could you give an example of what the factory method implementation would look like? Marc.2377‭ 27 days ago

... and/or, how the implementation of this interface should be done. Marc.2377‭ 27 days ago

I think I got it, if you may have a look at my answer below, tell me if that makes sense and is correct. Marc.2377‭ 27 days ago

@Marc.2377, your example looks like it will compile. I don't know whether the example signature is the best one for your situation, because I'm very short on context. This may be a situation where you should iterate two or three steps of asking "Why do I want to do that?" and then formulate a new question aimed at solving the higher level problem. E.g. is the underlying problem how to do unit testing of layer XYZ? Peter Taylor‭ 27 days ago

My final goal is to have an interface to be able to use EF Core in my application services layer while enforcing that whatever custom DbContext-like implementations an user might want to implement in the future provides the same set of functionality in order to require minimal changes to the application. Marc.2377‭ 27 days ago

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I believe an implementation of the answer by Peter Taylor above would be:

public override EntityEntry<TEntity> Add<TEntity>([NotNullAttribute] TEntity entity)
    where TEntity : class
{
    return Add(entity, base.Add);
}

public TEntityEntry_TEntity Add<TEntity, TEntityEntry_TEntity>([NotNullAttribute] TEntity entity,
    [NotNullAttribute] Func<TEntity, TEntityEntry_TEntity> wrap)
    where TEntity : class
    where TEntityEntry_TEntity : EntityEntry
{
    return wrap(entity);
}
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