How to get Dropdown selected value in codebehind from a DropDownList in infopath

Hi
I have dropdown list.
Values are populated from a xml-file.

private void bindDropdown()        
{            
using (XmlReader reader = XmlReader.Create(@"C:\Users\Administrator\sharepoint\MCW - Documents\XMLFile1.xml"))            
{                
XPathNavigator nav = this.CreateNavigator().SelectSingleNode("/my:myFields/my:InspectionSection", this.NamespaceManager);                
while (reader.Read())                
{                    
if (reader.IsStartElement()
)                    
{                        
switch (reader.Name.ToString()
)                        
{ 
case "SectionNo":                                
XPathNavigator newNode = null;                                
newNode = nav.Clone();                                
newNode.SelectSingleNode("/my:myFields/my:InspectionSection/my:Displayname", this.NamespaceManager).SetValue(reader.ReadString());                                
newNode.SelectSingleNode("/my:myFields/my:InspectionSection/my:Value", this.NamespaceManager).SetValue(reader.ReadString());                                
nav.InsertAfter(newNode);                                
newNode = null;                                
break;                        
}                    
}                
}                
nav.DeleteSelf();                
nav = null;            
}        
}

I want to get SELECTED-value of the dropdown in code behind.
I am trying to do as follows. But I am always getting blank.

XPathNavigator ddl1 = this.MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:ddlSection", this.NamespaceManager);            
String ddlSectionSelectedValue = ddl1.Value;            
MessageBox.Show(ddlSectionSelectedValue);

Please help me to get the selected value in code
thanks in advance