C# implement interface with properties that are interface.

Question

I am using EF code first to generate my db and I do need concrete property for ICollection of EF entity models. I am writing a data access layer ( Using generic classes) however hit the following road block with using interfaces in my generic class as shown below.

public interface ITestClassProp
{
    int Value { get; set; }
}

public class TestClassProp : ITestClassProp
{
    public int Value { get; set; }
}


public interface ITestClass
{
    ICollection<ITestClassProp> TestProp { get; set; }
}

public class TestClass : ITestClass
{
    // works
    //public ICollection<ITestClassProp> TestProp { get; set; }
    // does not work
    public ICollection<TestClassProp> TestProp { get; set; }
}

Am I totally mis using the interfaces? why cant I use TestClassProp instead of ITestClassProp?

Thanks

Because you are redefining the interface. If you want to use concrete types, you have to define it in the interface. — Ron Beyer
– Ron Beyer, Commented Jun 9, 2015 at 19:42
Because the ITestClass interface has a property of type ICollection<ITestClassProp>. You aren't implementing the interface if your is a ICollection<TestClassProp> — Matt Burland
– Matt Burland, Commented Jun 9, 2015 at 19:43
You could make this a generic interface ITestClass<T> where T : ITestClassProp. Then TestClass : ITestClass<TestClassProp>. — juharr
– juharr, Commented Jun 9, 2015 at 19:45

mason · Accepted Answer · 2015-06-09 19:42:36Z

3

When you implement an interface, you have to implement the methods/properties of that interface with the same signature. Since the interface declares ICollection<ITestClassProp> TestProp { get; set; } then your TestClass must also declare ICollection<TestClassProp> TestProp { get; set; }.

The reason this is necessary is that other classes that know about the interface but not the concrete class are expecting the property to be ICollection<ITestClassProp> and not ICollection<TestClassProp>.

answered Jun 9, 2015 at 19:42

mason

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1 Comment

Farnam Over a year ago

Thanks Mason. I appreciate your answer. i have mark david L as answer as that is how I end up solving the problem and might be useful for others who are looking for quick solutions. Thanks again.

David L · Accepted Answer · 2015-06-09 19:47:03Z

0

As you've currently written your code, you are not satisfying the requirement you've imposed via your ITestClass interface, which is to have a property of ICollection<ITestProp>.

One way around this is to actually make ITestClass generic, but provide a generic constraint of ITestClassProp

public interface ITestClassProp
{
    int Value { get; set; }
}

public class TestClassProp : ITestClassProp
{
    public int Value { get; set; }
}


public interface ITestClass<T> where T : ITestClassProp
{
    ICollection<T> TestProp { get; set; }
}

public class TestClass : ITestClass<TestClassProp>
{
    public ICollection<TestClassProp> TestProp { get; set; }
}

This allows you to provide any concrete type that implements ITestProp to your ICollection.

answered Jun 9, 2015 at 19:47

David L

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1 Comment

Farnam Over a year ago

I end up solving with the generic class as you shown here. Thanks again.

plinth · Accepted Answer · 2015-06-09 19:45:42Z

0

Simply, the interface declares a property of type ICollection, but you implement it as ICollection, which has a totally different signature.

You might want to read up on covariance and contravariance also.

answered Jun 9, 2015 at 19:45

plinth

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