CODEWITHSUNDEEP
c#generics
Twenty
Twenty

Reputation: 5901

Using Generic T of Method to create new instance of another object with the same Generic T

So I have a method which looks something like this

public void Register<T>(TimeSpan interval, ILogger logger) where T : ISchedule
{
    _schedules.Add(new Schedule<T>(interval, logger));
}

I want to create a new Schedule with the T provided to the Register Method.

internal class Schedule<T> : IDisposable where T : ISchedule, new() {}

Here I get the following error: 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'Schedule<T>'

Is this behavior even possible or is there anything I am doing wrong?

Upvotes: 1

Views: 55

Answers (1)

Thomas Flinkow
Thomas Flinkow

Reputation: 5105

You need to apply the where T : ISchedule, new() constraint you specified on the Schedule<T> class to the Register method as well:

public void Register<T>(TimeSpan interval, ILogger logger) where T : ISchedule, new()
{
    _schedules.Add(new Schedule<T>(interval, logger));
}

Consider the following two classes which implement the ISchedule interface.

public class Schedule1 : ISchedule
{
    public Schedule1() { }
}

public class Schedule2 : ISchedule
{
    public Schedule2(string foo) { }
}

The Schedule<T> class has a generic type constraint where T : ISchedule, new() meaning only those types implementing ISchedule and containing a parameterless constructor can be used as the type parameter. Therefore, it is illegal to specify a Schedule<Schedule2> since Schedule2 does not contain a parameterless constructor.

If we do not apply the same type constraint to the Register method, we can call that method with both Schedule1 and Schedule2. As Schedule2 does not conform to Schedule<T>'s type constraint, we now have a problem ('T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'Schedule<T>') that can be solved by applying the same constraint of Schedule<T> to the Register method.

Further reading: Constraints on Type Parameters on Microsoft Docs.

Upvotes: 2

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