C# interface property implementation

Question

interface IAlpha
{
    IBeta BetaProperty { get; set; }
}

interface IBeta
{

}

class Alpha : IAlpha
{
    public Beta BetaProperty { get; set; } // error here
}

class Beta : IBeta
{

}

'InterfaceTest.Alpha' does not implement interface member 'InterfaceTest.IAlpha.BetaProperty'. 'InterfaceTest.Alpha.BetaProperty' cannot implement 'InterfaceTest.IAlpha.BetaProperty' because it does not have the matching return type of 'InterfaceTest.IBeta'.

My question is why is a property implementation restricted to the very same type. Why can't I use the derived type instead?

Your class that inherits the interface does return a Beta insteed of a IBeta declared in the interface. — Mattias Josefsson
– Mattias Josefsson, Commented Dec 25, 2012 at 8:12

Jon Skeet · Accepted Answer · 2012-12-25 08:13:42Z

11

You have to implement the exact same interface. For example, this should be valid:

 IAlpha alpha = new Alpha();
 alpha.BetaProperty = new SomeOtherIBetaImplementation();

... but that wouldn't work with your code which always expects it to be a Beta, would it?

You can use generics for this though:

interface IAlpha<TBeta> where TBeta : IBeta
{
    TBeta BetaProperty { get; set; }
}

...

public class Alpha : IAlpha<Beta>

Of course, that may be overkill - you may be better just using a property of type IBeta in Alpha, exactly as per the interface. It depends on the context.

answered Dec 25, 2012 at 8:13

Jon Skeet

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3 Comments

karaxuna Over a year ago

Thanks a lot for the explanation and for the alternative solution. :)

user2697817 Over a year ago

@Jon Keet. Why is it still not possible if the IAlpha interface only defines a getter on the BetaProperty? Any interface implementation would be always guaranteed to return an IBeta.

Jon Skeet Over a year ago

@user2697817: It's just the way .NET and C# are defined. You could use explicit interface implementation to implement IAlpha and then expose a more specific property getter with the same name though.

Yochai Timmer · Accepted Answer · 2012-12-25 08:11:40Z

2

An interface declares a set of methods the class will have, so anyone using that interface knows what to expect.

So, if you're implementing that interface, you must implement the exact interface, so all the other users get what they expected.

answered Dec 25, 2012 at 8:11

Yochai Timmer

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