Unusual behavior when retrieving a system.__comobject

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I am currently working with a 3rd party dll, which return a System.__ComObject. So under a usual circumstance I usually use a foreach statement that can iterate through this object in order for me to get the data I need.

For example

C#

CWMetaData metadata = app.Clients.GetMetaData(userPath);
foreach(var data in metadata)
{
   var getvalue = data;
}

I am getting a System.__ComObject as I iterate through that object. Which I think is weird because this System.__ComObject has a different behavior with a for statement.

For example:

C#

for(int x =0; x< metadata.Count;x++)
{
   var getvalue = metadata[x].Name;
}

In this for statement I am able to actually to drill down into the System.__ComObject with which even provides intellisense for the System.__ComObject. I am not sure what the difference is, but I am sure that both example iterating through the same object.

My question is, why is why is there a different behavior between example one and example two. If both loops are doing the same.

What I have tried:

I have read documentation on Marshaling, but I think it does not apply to this case

Posted 27-Feb-17 3:41am

erick manuel

Updated 27-Feb-17 13:12pm

v3

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F-ES Sitecore 27-Feb-17 9:53am

What's your question?

erick manuel 27-Feb-17 10:03am

My Question is, why is why is there a different behavior between example one and example two. If both loops are doing the same.

[no name] 27-Feb-17 10:05am

No in fact they are not doing the same thing. In the foreach you are making a variable assignment. In the second you are accessing the properties of an object. 2 completely different things. Stop using var might make it more clear.

erick manuel 27-Feb-17 10:11am

thank you

[no name] 27-Feb-17 9:58am

I don't see why you would think this is "unusual behavior". Your code snippets are doing two different things.

erick manuel 27-Feb-17 10:00am

right, I know but with the foreach loop I get a comObject and other hand, with the for statement I am drilling down into the object.

[no name] 27-Feb-17 10:03am

Okay... and? That is what *I* would expect. How is this in any way unusual behavior?

erick manuel 27-Feb-17 10:08am

well just that I am able to traverse the com_object only with a for statement, and gain the intellisense. But I guess it is normal.

2 solutions

Solution 2

If you look at the definition for foreach (in C# Reference)[^]:

The foreach statement repeats a group of embedded statements for each element in an array or an object collection that implements the System.Collections.IEnumerable or System.Collections.Generic.IEnumerable<T> interface. The foreach statement is used to iterate through the collection to get the information that you want, but can not be used to add or remove items from the source collection to avoid unpredictable side effects. If you need to add or remove items from the source collection, use a for loop.

Without any more information on the object being called and based on the information above, I would conclude that the System.Collections.IEnumerable or System.Collections.Generic.IEnumerable<T> interface is not implemented for C#.

Posted 27-Feb-17 13:12pm

Graeme_Grant

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Midi_Mick · Accepted Answer · 2017-02-27T08:08:00

To understand better, you need to look at the inner workings of COM. A COM object that allows enumeration in .NET (foreach) actually returns an IEnumVariant implementation, so each value returned is a natural VARIANT, which when marshalled into .NET, becomes a type object. The fact that this object is of type VT_UNKNOWN or VT_DISPATCH allows .NET to cast it to a ComObject, but the actual COM object being pointed to by this object is not known. The Item property, on the other hand, will generally return a pointer to a specific COM interface implementation, and so the type can be determined at compile time.