<?xml version="1.0" encoding="utf-8"?>
<mx:ComboBox>
   <mx:Script>
      <![CDATA[
         private var _selectedValue:String;
         private var bSelectedValueSet:Boolean = false;
         private var bDataProviderSet:Boolean = false;
 
         // Override committ, this may be called repeatedly 
         override protected function commitProperties():void {
            // invoke ComboBox version 
            super.commitProperties();
 
            // If value set and have dataProvider 
            if (bSelectedValueSet && bDataProviderSet) {
               // Set flag to false so code won't be called until selectedValue is set again 
               bSelectedValueSet=false;
               // Loop through dataProvider 
               for (var i:int=0; i<this.dataProvider.length; i++) {
                  // Get this item's data 
                  var item:String = this.dataProvider[i].data;
 
                  // Check if is selectedValue 
                  if(item == _selectedValue) {
                     // Yes, set selectedIndex 
                     this.selectedIndex = i;
                     break;
                  }
               }
            }
         }
 
         // Trap dataProvider being set 
         override public function set dataProvider(o:Object):void {
            // invoke ComboBox version 
            super.dataProvider = o;
 
            // This may get called before dataProvider is set, so make sure not null and has entries 
            if (o!=null && o.length) {
               // Got it, set flag 
               bDataProviderSet = true;
            }
         }
 
         // set for selectedValue 
         public function set selectedValue(s:String):void {
            // Set flag 
            bSelectedValueSet = true;
            // Save value 
            _selectedValue = s;
            // Invalidate to force commit 
            invalidateProperties();
         }
      ]]>
   </mx:Script>
 
</mx:ComboBox>

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:fx="com.ui.*">
   <fx:ComboBox id="cbx_1″ selectedValue="STRING_VALUE" width="150″ dataProvider="{DATA_PROVIDER_ID_HERE}"/>
<mx:Application>