CODEWITHSUNDEEP
c#generics
rookie
rookie

Reputation: 2923

Generic container: why do I need an interface?

Say I have the following

public interface IInterval<T>
{
    T Start { get; }
    T Stop  { get; }
}

public class DateTimeInterval : IInterval<DateTime>
{
    private DateTime _start;
    private DateTime _stop;

    public DateTimeInterval(DateTime start, DateTime stop)
    {
        _start = start; _stop = stop;
    }

    public DateTime Start
    {
        get { return _start; }
    }
    public DateTime Stop
    {
        get { return _stop; }
    }
}

public class SortedIntervalList<T> 
    where T : IInterval<T>, IComparable<T>
{
}

If I were to now try to instantiate the container

var test = new SortedIntervalList<DateTimeInterval>();

I get a compilation error

The type 'Test' cannot be used as type parameter 'T' in the generic type or method TestContainer<T>. There is no implicit reference conversion from 'Test' to ITest<Test>.

Why is this?

Note on edit history

For clarity, classes for the original question are included below 

public interface ITest<T>
{
    int TestMethod();
}

public class Test : ITest<bool>
{

    public int TestMethod()
    {
        throw new NotImplementedException();
    }
}

public class TestContainer<T> 
    where T : ITest<T>
{ 

}

Upvotes: 2

Views: 114

Answers (2)

Patrick Hofman
Patrick Hofman

Reputation: 157038

Because you expect your T in TestContainer<T> to be ITest<T>. that doesn't make sense. I think you meant :

public class TestContainer<C, T> 
where C : ITest<T>
{

}

For your updated code in your question:

public class SortedIntervalList<C, T> 
    where C : IInterval<T>, IComparable<T>
{ }

With:

test = new SortedIntervalList<DateTimeInterval, DateTime>();

Upvotes: 2

SLaks
SLaks

Reputation: 887767

where T : ITest<T>

Your class inherits ITest<bool>, which is not ITest<T> for your T (Test).
As the error is trying to tell you, that does not meet your generic constraint, so you can't do that.

Upvotes: 4

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