Implementing interface properties in interfaces

Question

I need to create a dll file which contains all the interfaces of the classes but doesn't contain any class. Because I use these interfaces for a lot of classes it's must be like that:

public interface IClassA
{
    string Word { get; }
}

public interface ITest<TClassA> where TClassA : IClassA
{
    TClassA A { get; }
}

Example of two classes that implements these interfaces the way I want:

public class ClassA : IClassA
{
    public string Word
    {
        get;
        private set;
    }

    public string Sentence
    {
        get;
        private set;
    }

    public ClassA(string word, string sentence)
    {
        this.Word = word;
        this.Sentence = sentence;
    }
}

public class Test : ITest<ClassA>
{
    public ClassA A
    {
        get;
        private set;
    }

    public Test(ClassA a)
    {
        this.A = a;
    }
}

And I want to do something like that in the main program:

public static void Main(string[] args)
{
    ClassA a = new ClassA("hey", "hey world!");
    Test t = new Test(a);

    Print((ITest<IClassA>)t);        
}

public static void Print(ITest<IClassA> t)
{
    Console.WriteLine(t.A.Word);
}

But this casting: (ITest<IClassA>)t makes a run time error. How can I solve it? thanks!

Thomas Levesque · Accepted Answer · 2016-10-16 09:59:29Z

3

You should declare Test as

public class Test : ITest<IClassA>

instead of ITest<ClassA>.

Or declare ITest<TClassA> to be covariant on TClassA:

public interface ITest<out TClassA> where TClassA : IClassA

answered Oct 16, 2016 at 9:59

Thomas Levesque

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Comments

Jan Köhler · Accepted Answer · 2016-10-16 09:59:54Z

0

The Test-class implements the concrete ClassA (public class Test : ITest<ClassA>).

So you're trying to cast an ITest<ClassA> to ITest<IClassA> which obviously fails.

If you let the Test-class implement IClassA, the cast works:

public class Test : ITest<IClassA>
{
    public IClassA A
    {
        get; private set;
    }

    public Test(IClassA a)
    {
        this.A = a;
    }
}

answered Oct 16, 2016 at 9:59

Jan Köhler

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2 Comments

GalRoimi Over a year ago

I want the Test class to have a property of ClassA and not IClassA, It's not possible?

Jan Köhler Over a year ago

Yes, it is. Take a look @Thomas' answer: If ITest<TClassA> is covariant, it's possible: dotnetfiddle.net/KKkktG

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