Object-oriented channel referencing

[Simon_Aldworth]

Hi,

I'm just getting used to object-oriented referencing and I'm finding it difficult to learn any shortcuts, not assisted by the highly confusing object model help pages (the page Internal Data > Objects seems exceptionally unhelpful). This is resulting in long-winded code, far longer than when using the previous variable set (ChD, CL, etc).

I understand that if there are channels present in the data file for significant time then it's wise to use a variable set to reference that channel using GetChannel. However, I have channels that existing for a fleeting moment so it's a waste of time to set variables to reference them just to extract a simple piece of information.

So, my question is, if MyChannel is unique and I know it's group name or index, then how do I reduce the following line of code: 

Data.Root.ChannelGroups(1).Channels(MyChannel).Size

 ...to something shorter and simpler, such as:

Channels(MyChannel).Size

I don't understand the need to have to use Data.Root.ChannelGroups(1) everytime I want to reference a temporary channel.

Edit: things are more complicated than I thought as my first line of code above doesn't actually work. I am now unable to find in the help pages a way of referencing a specific channel. What do I put after ChannelGroups(1) to reference a unique channel by it's name? Once the long version is established, is there a shorter version that can be used for temporary channel referencing?

Second Edit: I now find that using the following doesn't work:

Data.Root.ChannelGroups(1).Channels(2).Size

And nor does the following when MyChannel is a text variable with the value [1]/HW Chn:

Data.Root.ChannelGroups(1).Channels(MyChannel).Size

I'm totally baffled as to how to reference a specific and unique channel by name. All help gratefully received.

Thanks, Simon.

[winner]

Hi Simon,

I think what you are looking for is the method GetChannel. Please have a look on the help: http://zone.ni.com/reference/en-XX/help/370858P-01/inavidata/methods/diacmpnt_method_getchannel_inav...

Hope this helps.

Winfried

[Simon_Aldworth]

Hi Winfried,

I'm afraid that's not what I'm after. In the early part of my post I said that I am familiar with GetChannel and understand it's benefits for long term channels. However, it's longwinded for momentary channels. So, I'm asking for the quickest/shortest reference to a channel that avoids setting a variable or using Data.Root.ChannelGroups.etc.etc.

Thanks.

[Brad_Turpin · Accepted solution]

Hi Simon,

First of all, for fleeting channels where you know their name and which group they're in, there's no problem with continuing to use the older "GroupRef/ChanRef" string approach.  Also, ChnLength() still works, even with the object variable as the argument.  I find that there are corner case situations where ChnLength(Channel) is more accurate than Channel.Size, so I still use it.  Also, you can use ChnLength(Channel) to set the channel length, whereas Channel.Size is read-only.

That said, you absolutely CAN reference a particular Channel by passing its name in a string variable or string constant or its index as an integer, and I do strongly endorse using Channel objects over the older "GroupRef/ChanRef" paradigm in DIAdem VBScripts that manipulate data channels.  I also recommend creating a Group object variable so you (and the script engine) don't have to keep repeating the Data.Root.ChannelGroups(Ref) part of the code over and over again.  Here are 3 examples:

Dim Group, AngleChnName, AngleChn
AngleChnName = "Angle"
Set Group = Data.Root.ChannelGroups(1)
Set AngleChn = Group.Channels(AngleChnName)
Set AngleChn = Group.Channels("Angle")
Set AngleChn = Group.Channels(2)
MsgBox AngleChn.Name & " channel size = " & AngleChn.Size

Brad Turpin

Senior Technical Support Engineer

National Instruments