Thanks for all the help.  I now face the following:

1) The dropdown frequently blanks and no amount of erasure of previous input will bring it back.

2) I only seem to be able to see within the first 60 or so rows - there are over 1k rows in my dataset.  Attempts to search for rows beyond this results in grid blanking.

 ComboTypeAhead is set to "suggest".

randrews said:

DataSource is nothing/a null reference when InitializeDataSource is fired.

In this case, you have to ensure that you get or create an instance of what you want to bind to the combo, populate that object with data, and set this value to the DataSource property of the combo, during this event.

randrews said:

EnableXmlHTTP is being set automatically in the control during the Load handler before calling the superclass OnLoad method- is this too late to be setting this attribute?

This sounds to be too late in the page lifecycle, yes.  Move this to the page's (or control's) Init event, or set the property at design-time.

No, DataSource is nothing/a null reference when InitializeDataSource is fired.

EnableXmlHTTP is being set automatically in the control during the Load handler before calling the superclass OnLoad method- is this too late to be setting this attribute?

This is part of a class library, so I'd like to handle my housekeeping issues within my subclass, rather than requiring multple calls from my consuming UI classes.

randrews said:

Okay, but in my code's workflow, I set the datasource externally with MyCombo.DataSource= <some datasource>.  I really don't see anything that needs to be done here that I'm not already doing.

Does your data source actually have data at this point?  It's important to note that InitializeDataSource will be raised very early in the page's lifecycle.

randrews said:

Am I supposed to be filtering the datasource based on user input criteria (e.g. if they entered "c", set DataSource equal to a view of all the data that begins with c), or is this something that the WebCombo will handle for me.

If you've turned on filtering in WebCombo, then it will do this for you.  You need to provide the entire set of data, and the combo will take care of the rest.

randrews said:

Am I to understand that DataBind ought not be called if I'm using AJAX like this?

You understand correctly.  If you call DataBind() in the process of the InitializeDataSource event, then at best, you'll be making the combo DataBind twice and thus will reduce your page's performance.  Doing this might also introduce undesired behaviors.

Okay, but in my code's workflow, I set the datasource externally with MyCombo.DataSource= <some datasource>.  I really don't see anything that needs to be done at InitializeDataSource that I'm not already doing.

 Caveat - I'm extending the ComboBox and adding functionality, so I'm implementing this handler within the subclass.  I am expecting consumers of this class to set DataSource themselves and then call DataBind.  I don't know if this will affect your answer.

Am I supposed to be filtering the datasource based on user input criteria (e.g. if they entered "c", set DataSource equal to a view of all the data that begins with c), or is this something that the WebCombo will handle for me?  This is really the crux of my question- I don't understand the workflow of the control.

 Am I to understand that DataBind ought not be called if I'm using AJAX like this?

 Thanks,