Question

First off im very new to C# im trying to recreate a application i created in Java.

I have 4 Listboxes. Each box will hold a list of values from the xml file.

listBox_year for the <Year>.
listBox_make for the <Make>.
listBox_model for the <Model>.
listBox_subModel is for the <sub-Model>.

So lets say i add all the years to the listBox_year with no duplicate years. Say i click on a year it will bring up all the Make of cars that have that year. Then i click on the Make and it will bring up the models for that Make thats under that Year etc…

with Java i was able to use a HashMap to make this work to where i can have multiple keys of the same name and i can search for what ever key in this case year is selected grab all the Makes or Values that has that year as a key.

Here is the XML format

<?xml version="1.0" encoding="utf-8" ?>
<vehicles>

  <Manufacturer>
    <Make>Subaru</Make>
    <Year>2010</Year>
    <Model>Impreza</Model>
    <Sub-Model>2.0i</Sub-Model>
    <Highway>36 MPG highway</Highway>
    <City>27 MPG city</City>
    <Price>$17,495</Price>
    <Description>
      Symmetrical All-Wheel Drive. 
      SUBARU BOXER® engine. 
      Seven airbags standard. 
      >Vehicle Dynamics Control (VDC). 
    </Description>
  </Manufacturer>

  <Manufacturer>
    <Make>Toyota</Make>
    <Year>2012</Year>
    <Model>Supra</Model>
    <Sub-Model>TT</Sub-Model>
    <Highway>22 MPG highway</Highway>
    <City>19 MPG city</City>
    <Price>$48,795</Price>
    <Description>
      16-inch aluminum-alloy wheels.
      6-speaker audio system w/iPod® control.
      Bluetooth® hands-free phone and audio.
      Available power moonroof.
    </Description>
  </Manufacturer>

  <Manufacturer>
    <Make>Subaru</Make>
    <Year>2011</Year>
    <Model>Impreza</Model>
    <Sub-Model>2.0i Limited</Sub-Model>
    <Highway>36 MPG highway</Highway>
    <City>27 MPG city</City>
    <Price>$18,795</Price>
    <Description>
      16-inch aluminum-alloy wheels. 
      6-speaker audio system w/iPod® control. 
      Bluetooth® hands-free phone and audio. 
      Available power moonroof.
    </Description>
  </Manufacturer>

  <Manufacturer>
    <Make>Subaru</Make>
    <Year>2011</Year>
    <Model>Impreza</Model>
    <Sub-Model>2.0i Limited</Sub-Model>
    <Highway>36 MPG highway</Highway>
    <City>27 MPG city</City>
    <Price>$18,795</Price>
    <Description>
      16-inch aluminum-alloy wheels.
      6-speaker audio system w/iPod® control.
      Bluetooth® hands-free phone and audio.
      Available power moonroof.
    </Description>
  </Manufacturer>

</vehicles>

Answer 1

The closest type to the java hashmap is the Dictionary. Since you need to have multiple items with the same key, I would use a Dictionary<int,List<Item>>.
Here are some basic function you might need:

void AddItem(int key, Item i, Dictionary<int,List<Item>> dict)
{
   if (!dict.ContainsKey(key))
   {
      dict.Add(i,new List<Item>());
   }
   dict[key].Add(i);
}

List<Item> GetList(int key)
{
   if (dict.ContainsKey(key))
   {
      return dict[key];
   }
   else
   {
      return new List<Item>(); // can also be null
   }
}