In C#, can you put an Or in an "where" interface constraint?

Question

if i have this code:

public interface IJobHelper
{
    List<T> FilterwithinOrg<T>(IEnumerable<T> entities) where T : IFilterable;
}

is there anything that support doing something like this:

public interface IJobHelper
{
    List<T> FilterwithinOrg<T>(IEnumerable<T> entities) where T : IFilterable or ISemiFilterable
}

so it will accept anything that supports one of two interfaces. I am basically trying to create an overload.

Why not make a generic filterable class, then inherit that class with more specific filterable classes. You can then use the where clause with the generic filterable class and constrain to multiple classes. — Austin Brunkhorst, Apr 22 '13 at 03:36
http://stackoverflow.com/questions/3679562/generic-methods-and-method-overloading — Turbot, Apr 22 '13 at 03:40

score 3 · Answer 1 · answered Apr 22 '13 at 03:54

3

As far as I know, you can use AND logic not OR.

AND (in this case, T must be child of IFilterable and ISemiFilterable)

public interface IJobHelper
{
    List<T> FilterwithinOrg<T>(IEnumerable<T> entities) where T : IFilterable, ISemiFilterable
}

answered Apr 22 '13 at 03:54

Vano Maisuradze

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score 2 · Answer 2 · answered Apr 22 '13 at 03:33

2

No, the language does not support that.

answered Apr 22 '13 at 03:33

Gjeltema

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score 2 · Answer 3 · answered Apr 22 '13 at 03:36

2

The easy way I would say, is if both your interfaces where children of the same parent interface.

public interface IFilterable { }

public interface IFullyFilterable : IFilterable { }

public interface ISemiFilterable : IFilterable { }

... where T : IFilterable { }

answered Apr 22 '13 at 03:36

LightStriker

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This is not **OR** logic. If you have class which is child of *ISemiFilterable*, then it means, this class is also child of *IFilterable*. – Vano Maisuradze Apr 22 '13 at 04:12
@VanoMaisuradze Yes? And the result would be the same? – LightStriker Apr 22 '13 at 14:29
Which result? What you wrote, isn't OR. It's AND – Vano Maisuradze Apr 22 '13 at 14:48
@VanoMaisuradze Of course not! If `T` is `ISemiFilterable`, it's not `IFullyFilterable`. So, it's not an AND. – LightStriker Apr 22 '13 at 15:03

Daniel Imms · Accepted Answer · 2013-04-22T03:50:10.620

The language doesn't support 'oring' together interfaces/classes in the where clause.

You need will need to state them separately with different method names so the signatures are different.

public interface IJobHelper
{
    List<T> FilterwithinOrg<T>(IEnumerable<T> entities) 
        where T : IFilterable
    List<T> SemiFilterwithinOrg<T>(IEnumerable<T> entities) 
        where T : ISemiFilterable
}

Alternatively you can implement a common interface on both interfaces. This is not the same thing as above though as it may require a cast when you receive the object back if you need a specific interface that isn't contained within IBaseFilterable.

public interface IBaseFilterable { }
public interface IFilterable : IBaseFilterable { }
public interface ISemiFilterable : IBaseFilterable { }

public interface IJobHelper
{
    List<T> FilterwithinOrg<T>(IEnumerable<T> entities)
        where T : IBaseFilterable
}

I don't know the context but the above is probably what you're looking for.

I tried the first thing by just listing it twice in the interface with different Where constraints but i get "same method is already declared" error . . any ideas? — leora, Apr 22 '13 at 03:42
Ah true, I'll fix the answer. You'll need to give them different names — Daniel Imms, Apr 22 '13 at 03:49

score 0 · Answer 5 · answered Apr 22 '13 at 03:38

You can just add another empty base interface that both these interfaces are derived from...

 interface baseInterface {}
 interface IFilterable: baseInterface {}
 interface ISemiFilterable: baseInterface {}

and then demand that the type in the generic type constraint be the base interface

 List<T> FilterwithinOrg<T>(IEnumerable<T> entities) where T : baseInterface

The only downside is the compiler will not allow you to use methods from either of the derived interfaces without casting...

In C#, can you put an Or in an "where" interface constraint?

5 Answers5