Finding which interfaces an instance implements

[amaca]

Given:

interface ICompass{}
interface INorth : ICompass{ ... stuff ...}
interface ISouth : ICompass{ ... stuff ... }
interface IEast : ICompass { .. stuff ...}
interface IWest : ICompass { .. stuff ... }

interface Box{ ... stuff ... };
interface Carton { ... stuff ...};
interface Bag { ... stuff ...}

and a whole series of classes that implement them, such as

class Thing : INorth, ICarton { ... stuff ... }
class AnotherThing : ISouth, IBag { ... stuff ... }

how can I find out whether two instances have the same Compass interface
without resorting to

if (left is INorth)
return right is INorth
else if (left is ISouth)
return right is ISouth
else ...

TIA,

Andrew

[Paul E Collins]

Re: Finding which interfaces an instance implements

amaca wrote:

how can I find out whether two instances have the
same Compass interface without resorting to
if (left is INorth)
return right is INorth
[...]

You could do some fancy tricks with reflection to navigate the
class/interface hierarchy at runtime - e.g. comparing the items in
left.GetType(). GetInterfaces() and right.GetType() .GetInterfaces( ).

Failing that, there are simpler hacks that aren't entirely hateful.
For example, you could include a method GetUniqueValue in the Compass
interface, ensure that each implementor really does have a unique
value for it (perhaps its own class name), and then just test
left.GetUniqueV alue() == right.GetUnique Value().

Eq.

[Nicholas Paldino [.NET/C# MVP]]

Re: Finding which interfaces an instance implements

You can do this easily with the IsAssignableFro m method on the Type
class. Basically, you can create the following methods to do this for you:

static bool InstancesImplem entCompassInter face(object instanceA, object
instanceB, Type compassInterfac eType) {
// Check instanceA and instanceB, make sure they are not null.
...

// Call the overload.
return InstancesImplem entCompassInter face(instanceA. GetType(),
instanceB.GetTy pe(),
compassInterfac eType);
}

static bool InstancesImplem entCompassInter face(Type typeA, Type typeB, Type
compassInterfac eType)
{
// Check compassInterfac eType here, make sure that it derives from
ICompass.
if (!typeof(ICompa ss).IsAssignabl eFrom(compassIn terfaceType))
{
// Throw an exception here.
}

// Check the types for null here.

// See if the instances are assignable to the interface type.
return (compassInterfa ceType.IsAssign ableFrom(typeA) &&
compassInterfac eType.IsAssigna bleFrom(typeB)) ;
}

Hope this helps.


--
- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard. caspershouse.co m

"Paul E Collins" <find_my_real_a ddress@CL4.orgw rote in message
news:8PGdne4Y_a jlFU7ZnZ2dnUVZ8 qOdnZ2d@bt.com. ..

amaca wrote:
>

>how can I find out whether two instances have the
>same Compass interface without resorting to
>if (left is INorth)
> return right is INorth
>[...]

>
You could do some fancy tricks with reflection to navigate the
class/interface hierarchy at runtime - e.g. comparing the items in
left.GetType(). GetInterfaces() and right.GetType() .GetInterfaces( ).
>
Failing that, there are simpler hacks that aren't entirely hateful. For
example, you could include a method GetUniqueValue in the Compass
interface, ensure that each implementor really does have a unique value
for it (perhaps its own class name), and then just test
left.GetUniqueV alue() == right.GetUnique Value().
>
Eq.
>
>

[amaca]

Re: Finding which interfaces an instance implements

Thank you both, both for the way of doing it using reflection and for the
not-so-hateful hacks. I could use either, but implementing a CompassType for
each class turns out to be helpful for working out what's going on in the
debugger, and so that's the way I'll go.

Andrew