Where is the ProcessNewValue event for ComboBoxEditSettings.AddNewButtonPlacement?

The ComboBoxEditSettings class is used to describe a column's editor and does not provide access to a real editor instance that is created at runtime. If you wish to adjust ComboBoxEdit or assign an event handler, you can accomplish this task by handling TableView's ShownEditor event in the following manner:

C#
private void view_ShownEditor(object sender, DevExpress.Xpf.Grid.EditorEventArgs e) {  
    if (e.Column.FieldName == "ComboBoxColumn") {  
        ComboBoxEdit comboBoxEdit = (ComboBoxEdit)e.Editor;  
        if ((string)comboBoxEdit.Tag == "ProcessNewValueHandlerAssigned") return;  
        comboBoxEdit.ProcessNewValue += new ProcessNewValueEventHandler(comboBoxEdit_ProcessNewValue);  
        comboBoxEdit.Tag = "ProcessNewValueHandlerAssigned";  
    }  
}  

void comboBoxEdit_ProcessNewValue(DependencyObject sender, ProcessNewValueEventArgs e) {  
    ....  
}  

An alternative solution is to assign a custom CellTemplate to define the ComboBoxEdit explicitly instead of using the column's EditSettings:

XAML
<dxg:GridColumn FieldName="FirstName">  
    <dxg:GridColumn.CellTemplate>  
        <DataTemplate>  
            <dxe:ComboBoxEdit x:Name="PART_Editor" ProcessNewValue="comboBoxEdit_ProcessNewValue" AddNewButtonPlacement="EditBox"/>  
        </DataTemplate>  
    </dxg:GridColumn.CellTemplate>  
</dxg:GridColumn>  

See also: ID Q317875, Why use EditSettings instead of CellTemplate or EditTemplate/DisplayTemplate
Feel free to post new comments if you need further assistance.