Why is this generic Interface definition wrong?

Question

I am trying to write an interface that looks something like this

public interface IPropertyGroupCollection
{
    IEnumerable<IPropertyGroup> _Propertygroups { get;}
}

public interface IPropertyGroup
{
    IEnumerable<IProperty<T, U, V>> _conditions { get; }
}

public interface IProperty<T, U, V>
{
    T _p1 { get; }
    U _p2 { get; }
    V _p3 { get; }
}

public class Property<T, U, V> : IProperty<T, U, V>
{
    //Some Implementation
}

I keep on getting a compilation error for the ienumerable definition of _Conditions.

What am i doing wrong? The Idea is the implementing classes will serve a generic property bag collection

Replace all generics with `object` and you are set. – John Alexiou Jul 13 '12 at 06:16 — John Alexiou, Jul 13 '12 at 06:16

Jakub Konecki · Accepted Answer · 2012-07-13T06:52:58.580

7

It's because you haven't declared T, U and V:

public interface IPropertyGroup<T, U, V>
{
    IEnumerable<IProperty<T, U, V>> _conditions { get; }
}

You will have to add generic types to IPropertyGroupCollection as well.

Remember, IProperty<bool,bool,bool> is a different type to IProperty<int,int,int> despite the fact that they came from the same generic 'template'. You can't create a collection of IProperty<T, U, V>, you can only create a collection of IProperty<bool, bool, bool> or IProperty<int int, int>.

UPDATE:

public interface IPropertyGroupCollection
{
    IEnumerable<IPropertyGroup> _Propertygroups { get;}
}

public interface IPropertyGroup
{
    IEnumerable<IProperty> _conditions { get; }
}

public interface IProperty
{
}

public interface IProperty<T, U, V> : IProperty
{
    T _p1 { get; }
    U _p2 { get; }
    V _p3 { get; }
}

public class Property<T, U, V> : IProperty<T, U, V>
{
    //Some Implementation
}

edited Jul 13 '12 at 06:52

answered Jul 13 '12 at 06:03

Jakub Konecki

45,581
7
87
126

Then how can i use the collection as a PropertyBag ? If I add the generic type to the collection interface? – SudheerKovalam Jul 13 '12 at 06:12
Well, you can't use generics the way you want to use them. – Jakub Konecki Jul 13 '12 at 06:15
So Is there a way in which I can define an interface that acts like a property bag? – SudheerKovalam Jul 13 '12 at 06:16
Only if all property values derive from the same type. – John Alexiou Jul 13 '12 at 06:17
I can restrict the property values to basic properties such as int/string/datetime/double .. frankly they are the only datatypes I care about as of now – SudheerKovalam Jul 13 '12 at 06:19
Or loose generics. Create `public interface IProperty : IProperty` ans have a collection of `IProperty`. But then you would need to cast down to concreate types. – Jakub Konecki Jul 13 '12 at 06:20
@JakubKonecki can you elaborate that a bit? I can try that – SudheerKovalam Jul 13 '12 at 06:22
This is doable without downcasting to concrete types if you add properties to your classes one by one in the mixin-way. In such a case `PropertyGroup` ("property bag") is bad design. – Riga Jul 13 '12 at 18:31

Why is this generic Interface definition wrong?

1 Answers1