Create instance of generic class c#

Question

So here is the code:

  public interface IObserver<T> where T : StackEventData<T>
    {
        void HandleEvent(StackEventData<T> eventData);
    }

    public class Observer<T> : IObserver<T> where T : StackEventData<T>, new()
    {
        public StringBuilder Log = new StringBuilder();

        public void HandleEvent(StackEventData<T> eventData)
        {
            Log.Append(eventData);
        }
    }

public class StackOperationsLogger
    {
        private readonly Observer<T> observer = new Observer<T>();
    }

I need to initialize Observer observer in StackOperationsLogger without making StackOperationsLogger generic. Any suggestions?

I'm not sure I understand. What does Observer<T> mean to you if StackOperationsLogger isn't generic? How would you use it? — Kevin Gosse
– Kevin Gosse, Commented Mar 17, 2019 at 10:41
You just need to replace T with the appropriate type, which seems difficult because you need a type that is a StackEventData of the same type... what is StackEventData<T> and why is it generic? — oerkelens
– oerkelens, Commented Mar 17, 2019 at 10:41
Make sure you look at System.IObserver<T> just for reference because your version is so different. Personally I've been studying the pattern for a couple years now and it is really useful. — ChaosPandion
– ChaosPandion, Commented Mar 17, 2019 at 18:40

ChaosPandion · Accepted Answer · 2019-03-17 18:43:40Z

2

One huge problem is the original generic definition of the interface.

public interface IObserver<T> where T : StackEventData<T>

Since the type constraint is defined recursively you can't use it in the way you want. Essentially type T has to be something like this: StackEventData<StackEventData<StackEventData<StackEventData<T>>>> etc. I don't think that is what you are looking for.

I have a feeling that this is what you are looking for.

public interface IObserver<T> 
{
    void HandleEvent(StackEventData<T> eventData);
}

public class Observer<T> : IObserver<T>
{
    public StringBuilder Log = new StringBuilder();

    public void HandleEvent(StackEventData<T> eventData)
    {
        Log.Append(eventData);
    }
}

public class StackOperationsLogger
{
    private readonly Observer<DataObject> observer = new Observer<DataObject>();
}

edited Mar 17, 2019 at 18:43

answered Mar 17, 2019 at 11:03

ChaosPandion

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Comments

Christos · Accepted Answer · 2019-03-17 11:04:14Z

2

Provided that you have some class definitions like the following:

public class ConcreteType : StackEventData<ConcreteType>
{
}

public class StackEventData<T>
{
}

You could try this:

public class StackOperationsLogger
{
    private Observer<ConcreteType> observer = new Observer<ConcreteType>();
}

Apparently you have to design your class correspondingly. Above just highlighted how should be the "name" of the classes and what is expected from the ConcreteType class you will define -bad name..you have to change also this.

edited Mar 17, 2019 at 11:04

answered Mar 17, 2019 at 10:42

Christos

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3 Comments

oerkelens Over a year ago

Except that Observer<int> doesn't compile, because int cannot be cast to StackEventData<T> - whatever that is. So this doesn't seem to answer the question...

oerkelens Over a year ago

But that doesn't compile either, because StackEventData<int> is not a StackEventData<StackEventData<int>>. That seems to be the actual problem in the OP's code.

Christos Over a year ago

@oerkelens Now fixed and tested :) Apparently I had to check it more carefully. Thanks !!

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