OK I've worked out what object(s) manage(s) single/multi part settings. This doesn't explain why the interface disables the basic command when posted on a toolbar but does give you a way to add this functionality to your own controls.You derive IMultiPartTextElement from IElement:
(Please forgive the ragged bits in the following development code)
Private Sub ToggleSingleMulti()
On Error GoTo lblError
Dim pDoc As IMxDocument
Set pDoc = ThisDocument
Dim pMap As IMap
Set pMap = pDoc.FocusMap
Dim pFeatSel As IEnumFeature
Set pFeatSel = pMap.FeatureSelection
Dim pFeat As IFeature
Set pFeat = pFeatSel.Next
Dim pAnnoFeat As IAnnotationFeature2
Dim pElem As IElement
Dim pTextEl As ITextElement
Dim pMultTextEl As IMultiPartTextElement
Dim pAnnoClassExt As IAnnotationClassExtension2
Set pAnnoClassExt = GetAnnoFeaClsExt(pMap, pDoc)
If pAnnoClassExt Is Nothing Then
Exit Sub
End If
Do While Not pFeat Is Nothing
If TypeOf pFeat Is IAnnotationFeature2 Then
Set pAnnoFeat = pFeat
Set pElem = pAnnoFeat.Annotation
If Not pElem Is Nothing Then
Set pMultTextEl = pElem
If pMultTextEl.IsMultipart Then
pMultTextEl.ConvertToSinglePart
Else
pMultTextEl.ConvertToMultiPart pAnnoClassExt.Display(pElem)
End If
pAnnoFeat.Annotation = pElem
Set pFeat = pAnnoFeat
pFeat.Store
End If
End If
Set pFeat = pFeatSel.Next
Loop
Exit Sub
lblError:
Debug.Print Err.Description
Exit Sub
End Sub
Public Function GetAnnoFeaClsExt(pMap As IMap, pMxdoc As IMxDocument) As IAnnotationClassExtension
On Error GoTo lblError
'Dim pMxDoc As IMxDocument
'Set pMxDoc = Nothing
Dim pID As UID 'Get a handle to the Editor extension
Set pID = New UID
Dim pAnnoExt As IAnnotationEditExtension
Dim pAnnoID As New UID
pAnnoID = "esriEditor.AnnotationEditExtension"
Dim pAnnoClass As IAnnotationClassExtension
pID = "esriEditor.Editor"
Dim pEditor As IEditor2
Set pEditor = Application.FindExtensionByCLSID(pID)
Dim pEditLayers As IEditLayers
If pEditor.EditState = esriStateNotEditing Then
MsgBox "Please start the Editor to select points."
Set GetAnnoFeaClsExt = Nothing
Exit Function
End If
Set pEditLayers = pEditor 'QI to grab CurrentLayer property
If pEditLayers Is Nothing Then
return nothing
End If
Dim iLayerCount As Integer
Dim pFeatureLayer As IFeatureLayer
For iLayerCount = 0 To pMap.LayerCount - 1
If TypeOf pMap.Layer(iLayerCount) Is IFeatureLayer Then
Set pFeatureLayer = pMap.Layer(iLayerCount)
s = pFeatureLayer.Name
pEditLayers.SetCurrentLayer pFeatureLayer, 0
Debug.Print pEditLayers.CurrentLayer.Name
End If
Next iLayerCount
Set pAnnoClass = pEditLayers.CurrentLayer.FeatureClass.Extension
Set GetAnnoFeaClsExt = pAnnoClass
Exit Function
lblError:
Debug.Print Err.Description
Exit Function
End Function
