Manipulate Infopath Dropdown List Through Managed Code

You cannot manipulate the form control directly, you can only manipulate the data source the control is bound. In the following example I have a data connection to get data from a SharePoint List, and the dropdown list has been configured to bind to the data connection. In the button click event I manually created a new item, append it to the list and then select that item.

public void CTRL6_5_Clicked(object sender, ClickedEventArgs e)
{
DataSource ds = this.DataSources["Customers"];
ds.QueryConnection.Execute();
XPathNodeIterator nodes = ds.CreateNavigator().Select("/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW",this.NamespaceManager);
XPathNavigator lastNode = ds.CreateNavigator().SelectSingleNode("/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW[" + nodes.Count.ToString() + "]", this.NamespaceManager);

XmlDocument doc = new XmlDocument();
XmlElement newNode = doc.CreateElement("d:SharePointListItem_RW", this.NamespaceManager.LookupNamespace("d"));
newNode.AppendChild(doc.CreateElement("d:Title", this.NamespaceManager.LookupNamespace("d"))).InnerText = "Jeff";
newNode.AppendChild(doc.CreateElement("d:ID", this.NamespaceManager.LookupNamespace("d"))).InnerText = "99";

lastNode.InsertAfter(newNode.OuterXml);

this.MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:field1", this.NamespaceManager).SetValue("Jeff");
}