返回具有不同泛型类型的泛型接口实现

Question

I have created this simple generic interface:

public interface IInitializerSettings<in ViewerType> where ViewerType : Component
{
    void Apply(ViewerType dataViewer);
}

And added an implementation for it:

public class MenuSettings : IInitializerSettings<CustomGridLayout>
{
    public void Apply(CustomGridLayout dataViewer)
    {
        Debug.Log("Applied");
    }
}

public class CustomGridLayout : CustomLayout
{
    // The implementation code
}

Now I try to use it like that:

public IInitializerSettings<CustomLayout> GetDefaultSettings()
{
    return new MenuSettings();
}

But I get this error "Cannot convert type MenuSettings to return type IInitializerSettings"

I don't understand why it isn't allowed, CustomGridLayout inherits CustomLayout .

All I could find is this question , but this solution doesn't work for me (I can't use the out keyword).

Answer 1

The reason you cannot do this is because for a contravariant interface (specified by your use of in for the generic type parameter) you cannot implicitly convert it to an instance of a less derived type. I think the bullet points in the docs explains it fairly ok, if you think in terms of IEnumerable<T> (covariant) and Action<T> (contravariant).

As Selvin mentions in the comments the Apply method in MenuSettings expects an instance of CustomGridLayout , so trying to cast MenuSettings to IInitializerSettings<CustomLayout> is not possible because public void Apply(CustomGridLayout dataViewer) cannot handle a CustomLayout as input. Let me give an example:

public class CustomLayout
{
    public void SetupCustomLayout() { ... }
}

public class CustomGridLayout : CustomLayout
{
    public void SetupGrid() { ... }
}

public class MenuSettings : IInitializerSettings<CustomGridLayout>
{
    public void Apply(CustomGridLayout dataViewer)
    {
        dataViewer.SetupGrid();
    }
}


// Later in the code...

var menuSettings = new MenuSettings();

// This cast is what GetDefaultSettings() is trying to do
var genericSettings = (IInitializerSettings<CustomLayout>)menuSettings;

var layout = new CustomLayout();

// Looking at the type of 'genericSettings' this following line should be possible
// but 'MenuSettings.Apply()' is calling 'dataViewer.SetupGrid()' which doesn't exist
// in 'layout', so 'layout' is not a valid input
genericSettings.Apply(layout);

So in relation to the docs you have defined IInitializerSettings<ViewerType> as a contravariant interface, but are trying to use it as a covariant interface - which is not possible.