尝试使用通用扩展方法时没有隐式引用转换

Question

Even having browsed stack overflow for similar answers I'm struggling to understand the concepts behind why I'm experiencing issues.

My code is below:

class Program {
    static void Main(string[] args) {
        HomePage page = new HomePage();
        page.Load(); //No Error
        LoginPage page2 = new LoginPage();
        page2.Load(); //No Error
        ApprenticePage page3 = new ApprenticePage();
        page3.Load(); //No implicit reference conversion
        ApprenticeEPA page4 = new ApprenticeEPA();
        page4.Load(); //No implicit reference conversion
        Console.ReadLine();
    }
}

public abstract class BasePage<T> where T : BasePage<T> {
    public abstract bool EvaluateLoad();
}

public class HomePage : BasePage<HomePage>
{
    public override bool EvaluateLoad()
    {
        Console.WriteLine("In HomePage");
        return true;
    } 
}

public class LoginPage : BasePage<LoginPage>
{
    public override bool EvaluateLoad()
    {
        Console.WriteLine("In LoginPage");
        return true;
    }
}

public class ApprenticePage : HomePage
{
    public override bool EvaluateLoad()
    {
        Console.WriteLine("In ApprenticePage");
        return false;
    }
}

public class ApprenticeEPA : ApprenticePage
{
    public override bool EvaluateLoad()
    {
        Console.WriteLine("In ApprenticeEPA");
        return false;
    }
}

public static class Helper {
    public static T Load<T>(this T page) where T : BasePage<T> {
        if (page.EvaluateLoad()) {
            Console.WriteLine("It's true!");
            return page;
        } else {
            Console.WriteLine("It's false!");
            return default(T);
        }        
    }
}

I'm getting the following error:

The type 'IndexerTest.ApprenticePage' cannot be used as type parameter 'T' in the generic type or method 'Helper.Load(T)'. There is no implicit reference conversion from 'IndexerTest.ApprenticePage' to 'IndexerTest.BasePage<IndexerTest.ApprenticePage>'.

In my mind, as ApprenticePage inherits from HomePage , which inherits from BasePage<T> , using the load method would be okay but obviously it isn't.

How would I fix my Load method so that it could be called on all instances of the classes in my code with that classes specific implementation of EvaluateLoad() ?

Answer 1

If you examine the error closely, it says the following:

There is no implicit reference conversion from 'ApprenticePage' to 'BasePage<ApprenticePage>'

This is correct; ApprenticePage is a BasePage<HomePage> , it's not a BasePage<ApprenticePage>

What you're trying to do is create an extension method which works on anything that implements BaseType<T> . So, your extension method should look like this:

public static BasePage<T> Load<T>(this BasePage<T> page) where T : BasePage<T>
{
    if (page.EvaluateLoad())
    {
        Console.WriteLine("It's true!");
        return page;
    }
    else
    {
        Console.WriteLine("It's false!");
        return null; // No need for default() here - default() of a reference type is
                     // always going to be null
    }
}