A Reflection Puzzle:(

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Laser Lu - 05 May 2005 09:58 GMT

HI, all,
I just want to invoke an internal method named 'ResolveClientUrl', which
is defined in class System.Web.UI.Control, using an instance object of a
type that derives from Control.

Let's see the following code snippet (class MyControl is a sub class that
derives directly from System.Web.UI.Control):
    //...
    MyControl instance = new MyControl();
    Type type = instance.GetType();
    object[] args = new object[] {"images/test.gif"};
    string path = (string)type.InvokeMember("ResolveClientUrl",
        BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance |
BindingFlags.GetProperty,
        null, instance, args);
    //...

Actually, I intended to use the type object of MyControl to invoke the inherited
method 'ResolveClientUrl', but an Exception occured, described as "Method
LaserWeb.Presentation.MyControl.ResolveClientUrl not found." It seems that
reflection didn't search in the inherited members up the class hierarchy,
since 'ResolveClientUrl' is a declared method in the base class Control.

This action, I guess, should only be taken when BindingFlags.DeclaredOnly
is specified for invoking a method, otherwise inherited members derived from
base class should be considered. However, in this sample code, it didn't
work:( I did not pass the BindingFlags.DeclaredOnly flag. Then why? How could
it happen? I was puzzled:( Help!

Regards,
Laser Lu

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Jon Skeet [C# MVP] - 05 May 2005 19:33 GMT

> HI, all,
> I just want to invoke an internal method named 'ResolveClientUrl', which
[quoted text clipped - 24 lines]
> work:( I did not pass the BindingFlags.DeclaredOnly flag. Then why? How could
> it happen? I was puzzled:( Help!

Even if you could find the member, you shouldn't be able to invoke it,
if it's internal to a separate assembly.

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Laser Lu - 05 May 2005 20:04 GMT

Hello Jon Skeet, actually, the following code runs well:
//...
MyControl instance = new MyControl();
Type type = typeof(System.Web.UI.Control); // here use the Type object of
Control, not of MyControl.
object[] args = new object[] {"images/test.gif"};
string path = (string)type.InvokeMember("ResolveClientUrl",
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance

| BindingFlags.GetProperty,

null, instance, args);
//...

The only change, that I've made on the code, is that I used the Type object
of Control instead. And the Control class is the declaring type of the member
'ResolveClientUrl'.

Maybe I didn't understand what do you mean in your post. However, the above
code runs well without any exceptions, and the method member gets invoked
correctly, even it is internal to my assembly.

Still, My puzzle is how to invoke inherited members using derived types?
It seems that there's no difference in cases whether you have a BindingFlags.DeclareOnly
specified or not:( Why is it so strange?

>> HI, all,
>> I just want to invoke an internal method named 'ResolveClientUrl',
[quoted text clipped - 31 lines]
> Even if you could find the member, you shouldn't be able to invoke it,
> if it's internal to a separate assembly.

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Jon Skeet [C# MVP] - 05 May 2005 20:50 GMT

> Hello Jon Skeet, actually, the following code runs well:
> //...
[quoted text clipped - 15 lines]
> code runs well without any exceptions, and the method member gets invoked
> correctly, even it is internal to my assembly.

It may in some trust environments, but:

a) It won't in others
b) There's no guarantee that that method will be in future versions of
the framework
c) Internal methods should *not* be seen as part of the public
interface which you should call. If they were meant to be called by the
"outside world", they'd have been made protected or public.

> Still, My puzzle is how to invoke inherited members using derived
> types? It seems that there's no difference in cases whether you have
> a BindingFlags.DeclareOnly specified or not:( Why is it so strange?

Well, just looking at your code again, you're using
BindingFlags.GetProperty, which probably isn't what you're after for a
method call.

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Laser Lu - 06 May 2005 02:23 GMT

Jon Skeet wrote:

> Well, just looking at your code again, you're using
> BindingFlags.GetProperty, which probably isn't what you're after for a
> method call.

So sorry for the mistake I made during copying code to this post!! The original
code snippet should be:
//...
MyControl instance = new MyControl();
Type type = instance.GetType();
object[] args = new object[] {"images/test.gif"};
string path = (string)type.InvokeMember("ResolveClientUrl",
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance

| BindingFlags.InvokeMethod,

null, instance, args); // Exception is thrown, withou no such member
found in the derived class.
//...

The reason why I made this mistake is actually I've wrote a helper class
as the following:
// code starts
using System;
using System.Reflection;

namespace LaserWeb.Presentation.Common
{
    internal sealed class Reflector
    {
        private Reflector() {}

        public static object GetProperty(object instance, string name)
        {
            Type type = instance.GetType();
            return type.InvokeMember(name,
                BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance

| BindingFlags.GetProperty,

                null, instance, null);
        }

        public static object GetProperty(Type type, object instance, string name)
        {
            return type.InvokeMember(name,
                BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance

| BindingFlags.GetProperty,

                null, instance, null);
        }

        public static object InvokeMethod(object instance, string name, object[]
args)
        {
            Type type = instance.GetType();
            return type.InvokeMember(name,
                BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance

| BindingFlags.InvokeMethod,

                null, instance, args);
        }

        public static object InvokeMethod(Type type, object instance, string name,
object[] args)
        {
            return type.InvokeMember(name,
                BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance

| BindingFlags.InvokeMethod,

                null, instance, args);
        }
    }
}
// code ends
When I posted the puzzle, the original code snippet is not handy (I have
removed it from my project), so I made the wrong code snippet by copying
the code fragment from Reflector.GetProperty() and modifing base on that
code. That's the reason why I made the mistake, sorry again for misleading
all of you in your reading!

If you use the above helper class as the following, you will still find the
problem I described:
// ..
MyControl instance = new MyControl();
object args = new objet[] {"images/test.gif"};
Reflector.InvokeMethod(typeof(System.Web.UI.Control), instance, "ResolveClientUrl",
args); // This will works fine.
Reflector.InvokeMethod(instance, "ResolveClientUrl", args); // An exception
will be thrown here, indicating member not found.
// ...

I don't know why I had encountered such a strange problem, and got puzzled.
You may make the similar sample code and have a try, to see whether what
I said would happen. Thanks:)

Laser Lu.

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Jon Skeet [C# MVP] - 06 May 2005 06:56 GMT

> Jon Skeet wrote:
> > Well, just looking at your code again, you're using
[quoted text clipped - 13 lines]
> found in the derived class.
> //...

<snip>

Okay, well I can reproduce the problem, but not solve it. However, as
I've said before, this isn't something you should be doing anyway. I
urge you to change your design so that you don't need to call internal
methods in other assemblies.

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Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
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Laser Lu - 06 May 2005 07:16 GMT

Okay, thank you very much!
And I will try to avoid invoking internal or private members in ther future,
as doing this will certainly violate the design principles:)

> Okay, well I can reproduce the problem, but not solve it. However, as
> I've said before, this isn't something you should be doing anyway. I
> urge you to change your design so that you don't need to call internal
> methods in other assemblies.

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A Reflection Puzzle:(

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