Question

I have some classes which implement an interface as follows:

class C1 : IpropTemplate { ... }

class C2 : IpropTemplate { ... }

:

Also there is some other class which implements another interface:

class C3 : IclassTemplate { ... }

Now, I need to specify the signature of a property into the IclassTemplate that forces the C3 to have a property which is implemented from IpropTemplate. (such as C1 or C2, etc.)

I tried this:

interface IclassTemplate
{
    ...
    IpropTemplate prop1 { get; set; }
}

class C3 : IclassTemplate
{
    ...
    public C1 prop1
    {
        get;
        set;
    }
}

In this case, compiler produces an error indicating that C3 does not implement interface member IclassTemplate.prop1 and that the C3.prop1 cannot implement IpropTemplate.prop1 because it does not have the matching return type of IpropTemplate.

What should I do?
Thanks

Answer 1

You can’t make your C3 implementation only deal in C1 – after all, you’ve guarantee that this will be valid:

IClassTemplate c3 = new C3();
c3.Template = new C2();

If you only need to be able to read the property, it’s slightly easier:

interface IClassTemplate
{
    IPropTemplate Template { get; }
}

public class C3 : IClassTemplate
{
    private readonly C1 c1 = new C1();

    IPropTemplate Template { get { return c1; } }
}

That lets the code in C3 know that it’s really a C1, but still implement the interface.

Another option is to make your interface generic:

interface IClassTemplate<T> where T : IPropTemplate
{
    T Template { get; set; }
}

public class C3 : IClassTemplate<C1>
{
    public C1 Template { get; set; }
}

Hopefully one of these will meet your need – if not, please give more details about what you’re really trying to achieve – the bigger picture.