I am playing around with an AddIn, just trying to learn the funcationality behind selecting entities that have been named.  Under the UI Callbacks region of my AddIn, I created a new method called CheckName.

Pre-Conditions to using this method, which is called from my AddIn Menu

1.  There is an open part document / model

2.  I have named a face

3.  I select that named face before I execute the CheckName method

Here is the method.  Basically, it starts with a face I have selected.  The face I selected, has been named.  It is the only face in the model that has been named.  I retrieve the name and save to a string.  I then clear the selection list.  Then, using the name save to the string, I get the named entity, add it back to the selection list (which I cleared).  Along the way I throw in a few MSGBox statements to confirm thngs are working. 

public void CheckName()

        {           

            IModelDoc2 iModDoc = (IModelDoc2)iSwApp.ActiveDoc;//Get our active document

            iSwSelect = (ISelectionMgr)iModDoc.SelectionManager;//Gets the Selection Manager for the active doc

            IPartDoc iPart = (IPartDoc)iModDoc;//Get the more specific Part object as well.  This is needed to the GetEntityByName method

            if (iSwSelect.GetSelectedObjectCount2(-1)==1)//If we have ONE and only one component (face, edge, vertex, etc) selected....

            {

                object entity = (object)iSwSelect.GetSelectedObject6(1,-1);//Gets reference variable to the selected face/edge, etc.  We do not know type, so we cast to object

                System.Windows.Forms.MessageBox.Show("Selected Entity Name is " +iModDoc.GetEntityName(entity) + "." );//MSGBox to confirm...

                string retName = iModDoc.GetEntityName(entity);//Save the retrieved name to a string

                iModDoc.ClearSelection2(true);//Clear the selection list

                IEntity iSwEnt = (IEntity)iPart.GetEntityByName(retName, 2);//Get the named entity/face. When we NAMED the entity, we noted the type = 2

                SelectData iSelData; //HERE IS MY Question....Why does Select Data work...but ISelect does not?

                iSelData= iSwSelect.CreateSelectData();//Create the select data object for our named face.

                bool newSelect = iSwEnt.Select4(true, iSelData);//Now, finally, select the named face.  We are right back where we started, with the named face selected.

                if (newSelect)

                {

                    entity = (object)iSwSelect.GetSelectedObject6(1, -1);//Gets reference variable to the selected object

                    System.Windows.Forms.MessageBox.Show("Selected Entity Name is " + iModDoc.GetEntityName(entity) + ".");//Check objects name   

                }

                else System.Windows.Forms.MessageBox.Show("Entity was not successfully selected by ID");

            }

            else System.Windows.Forms.MessageBox.Show("Please select only one face." );

        }

Here is my question.  This code work perfect with

SelectData iSelData;

Originally, I wrote it with the ISelect interface as follows:

ISelectData iSelData;

The former works...ISelectData does not!  Why is this.  Documentation on the help only shows the Interface objects, the the class objects...I can not figure out whey SelectData works...but ISelectData will not. 

Thanks!