Step 1:

First create a windows form application and add two combo box one for country and other for city like this:

Step 2:

Now create a xml file and save with “cities.xml” name:

<?xml version="1.0" encoding="utf-8" ?>
<countries>
  <country name='India'>
    <city>Delhi</city>
    <city>Mumbai</city>
    <city>Hyderabad</city>
  </country>
  <country name='Australia'>
    <city>Sydney</city>
    <city>Hobart</city>
    <city>Canberra</city>
  </country>
  <country name='Canada'>
    <city>Toronto</city>
    <city>Niagara</city>
    <city>Victoria</city>
  </country>
 
  <country name='USA'>
    <city>Dallas</city>
    <city>Washington</city>
    <city>Chicago</city>
  </country>
</countries>

Step 3:

By using XDocument object we can load XML documents, writing linq queries on XDocument will return collection. In linq query xdocument.Descendants will return a IEnumarable XElement collection.

Now write the below code in the from load event:

XDocument xdoc = XDocument.Load(“Your xml file path”); //load the xml file
var countries = from country in xdoc.Descendants("countries").Elements("country").Attributes("name")
                            select country.Value; //Get all country from xml 
cbCountry.DataSource = countries.ToList();//Populate country combo box with countries
var cities = from country in xdoc.Descendants("countries").Elements("country")
                         where country.Attribute("name").Value == cbCountry.SelectedValue.ToString()
                         from city in country.Elements("city")
                         orderby city.Value
                         select city.Value; //Get the cities from xml
cbCity.DataSource = cities.ToList();//Populate city combo box with cities

Step 4:

Now write the below code in the selected index changed event of country combo box:

XDocument xdoc = XDocument.Load(“Your xml file path”);
var cities = from country in xdoc.Descendants("countries").Elements("country")
                             where country.Attribute("name").Value == cbCountry.SelectedValue.ToString()
                             from city in country.Elements("city")
                             orderby city.Value
                             select city.Value;//Get cities according to the country currently selected in country combo box
cbCity.DataSource = cities.ToList();

This linq query will populate the city combo box with cities according to

the country selected in the country combo box.

Output