Question

I wrote some extensions methods to handle the css class like how you would do it in jQuery. Since I’m using ASP.NET the framework has two extension points i need to handle.

The first one is System.Web.UI.WebControls.WebControl

public static bool HasCssClass(this WebControl control, string className)
public static void AddCssClass(this WebControl control, string className)
public static void RemoveCssClass(this WebControl control, string className)

and the second one is System.Web.UI.IAttributeAccessor

public static bool HasCssClass(this IAttributeAccessor accessor, string className)
public static void AddCssClass(this IAttributeAccessor accessor, string className)
public static void RemoveCssClass(this IAttributeAccessor accessor, string className)

the dilemma here has to do with that the CssClass property of WebControl doesn’t seem to reflect accordingly when you access the property through the IAttributeAccessor interface.

i.e.

WebControl c;
c.CssClass = "hello-world";
Debug.Assert(((IAttributeSelector)c).GetAttribute("class") != "hello-world");

I did some digging around with the .NET Reflector and came to the conclusion that the assertion will hold. I can modify the same underlying HTML property class in two distinct ways. Thus my problem, which overload do I use, or rather, which overload does the compiler use?

Now, System.Web.UI.WebControls.WebControl implements System.Web.UI.IAttributeAccessor and this is why I ask the question, which overload will be used? Or is the call ambiguous between the two?

I always imagine that the compiler/run-time calculated the distance to pick one type over the other.

Say TextBox, which derives from WebControl that implements IAttributeAccessor would call the WebControl overload because the distance to WebControl is shorter, strictly speaking.

I found some stuff on MSDN about this, however, it doesn’t say what it means to be better than or worse than. So what’s going to happen when I call.

TextBox c;
c.AddCssClass("hello-world");

?

EDIT

I ended up changing the IAttributeAccessor to HtmlControl instead, becuase it still covers most cases and would be less confusing for other developers on my team.

IntelliSense actually displayed both as possible overloads, while only one, in fact, is possible.

Answer 1

You were nearly there – you wanted section 7.4.2.3 instead – “better conversion”. It’s not a matter of conversion “distance” though. (In the C# 3 spec this is 7.4.3.4 in case you’re interested.)

The conversion from TextBox to WebControl is better than the conversion to IAttributeSelector, because:

If an implicit conversion from T1 to T2 exists, and no implicit conversion from T2 to T1
exists, C1 is the better conversion.

There’s an implicit conversion from WebControl to IAttributeSelector but not vice versa.

Now that we know which conversion is better, we can apply the section you referenced to determine that the overload with WebControl will be picked. This can easily be demonstrated with a test application of course:

using System;

public interface IFoo {}
public class Bar : IFoo {}
public class Baz : Bar {}

public static class Extensions
{
    public static void Extension(this IFoo foo)
    {
        Console.WriteLine("Extension(IFoo)");
    }

    public static void Extension(this Bar bar)
    {
        Console.WriteLine("Extension(Bar)");
    }
}

public class Test
{
    static void Main()
    {
        Baz b = new Baz();
        b.Extension();
    }
}

This prints “Extension(Bar)” – corresponding to the extension method via WebControl in your real case.

Of course, if you cast to b to IFoo then the overload printing “Extension(IFoo)” will be picked – because the compiler doesn’t know that the other overload will be applicable. Remember that overloading is only performed at compile time (leaving aside dynamic in C# 4).