Reflection on COM object.

  • Thread starter Thread starter Lars Nielsen
  • Start date Start date
L

Lars Nielsen

Hi everybody,

I've run into a problem when using the reflection assembly on COM objects
and been browsing around the net for some while. Apparently a few has asked
the same question though no answer has ever occoured.

This is what I'm trying to do:

I wish to build a function that crawls any COM object (interop). I did
shortly examine the serializer until I realized it's used for translation
and not reflection). Until now it work's quite fine until it returns an
array of strings:

Public Function ReturnAbstractObject(ByVal o As Object) As XmlDocument
Dim type As System.Type
type = o.GetType()

Dim propertyinfo As System.Reflection.PropertyInfo

For Each propertyinfo In type.GetProperties()

If propertyinfo.PropertyType.FullName = "System.String" Then
Dim s As String
s = propertyinfo.GetValue(o, Nothing).ToString() ' HERE IT GOES
WRONG
End If

Next
:
End Function


When returning an array of strings (mind you, the
propertyinfo.GetType().IsArray returns false) I get the following error:
"Parameter count mismatch".

It's the propertyinfo.GetValue(o, Nothing) that raises an error. I cannot
find an example (that works) where the second parameter are used (this is
the Index parameter). I may be wrong in the entire approch, and would gladly
receive any help you guys may offer.

In advance thanks,

Lars Nielsen
(e-mail address removed)
(If mailing the answer, - please remove the CAPS from the email)
 
Lars,

The second parameter should be used for indexers, which is not what you
want to use, a property that exposes an array of strings is a regular
property, not an indexer. You should not use this overload. What is the
COM signature of the property?
 
Lars,
Until now it work's quite fine until it returns an array of strings:
Click to expand...

It sounds more like you're seeing a parameterized property (a.k.a.
indexer in C#), that takes an index argument and returns a single
string, as opposed to returning an entire array. That would explain
why IsArray for the property type returns false, and why you get the
exception.

If the property takes an integer argument specifying the index, you
should be able to query it like this

s = propertyinfo.GetValue(o, New Object() {index}).ToString()

You can find out more about the parmeters a property takes by checking
PropertyInfo.GetIndexParameters().



Mattias
 
Lars,
However, this does not seem to supply any method to get the length of the
index. Do you, or anybody else, know any way to extract such a value?
Click to expand...

Not all parameterized properties wrap arrays, so index parameters
might not be numeric, and there can be more than one. So it's close to
impossible to write generic code that queries such properties without
any knowledge of the implementation.



Mattias
 
Yes, that's what I feared.

Do you know any way to express an (any) Object as XML ?

I'm curious; Is it possible to get the length (or upperbound) from the
index?

By the way, thanks for the help. It has clearifyed many "dimmed" areas.


-- Lars
 
Lars,
Do you know any way to express an (any) Object as XML ?
Click to expand...

No, I don't. Usually COM objects take care of their own persistance
and exposes it by implementing one or more of the IPersist interfaces.

I'm curious; Is it possible to get the length (or upperbound) from the
index?
Click to expand...

I guess you could look for a property called Count or Length or
something like that. But that wouldn't work in all situations.



Mattias
 
Mattias,

That will off course pose as a problem when making a general COM object
reflection class (that dumps all values). You must then know something about
the object.

Thank you very much for your help. When my class is done, I will do a repost
as a Q - > A as a help to others that tries the same.


Best regards,

Lars Fløe Nielsen
 
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