Lesson 19.5 – Changing dropdown default values

Lesson objectives

At the end of this lesson, you will know how to…

• Set the default selected item in a dropdown
• Prepare to make a change to an existing program

A reader mentioned a problem they saw when the player has more than one weapon. When the player moves to a new location, the dropdown for weapons always selects the first weapon in the list.

If the player wants to fight with a different weapon, the reader wanted to have the dropdown always select that weapon – unless the player manually changed weapons again.

Here is how to do that.

Planning

Before you change an existing program, it’s good to think about everything it will affect.

This seems like a simple change. However, we should make some additional changes when we do this.

For example, store the currently selected weapon in the save game file. This way, when the player restarts the game, they will have their last weapon selected.

If we think a little more, we realize that the player may already have a saved game file, without a value for the currently selected weapon in it. So, in the code to read the currently selected weapon from the save game file, we need to deal with a file that does not have that value in it.

This is the type of thing to watch out for when you modify a program.

Saving and reloading player information

Step 1: Open the SuperAdventure solution in Visual Studio and select the Player class to modify.

Add a new property to store the player’s current weapon. I added this under the “CurrentLocation” property (although you could add it anywhere outside of a function):

public Weapon CurrentWeapon { get; set; }

Step 2: Open the code file for the SuperAdventure.cs form.

In the lesson where we save and restore the game state in a file, we connected a function to the FormClosing event. This function is an “event handler”, it “handles” the closing “event”. In that lesson, we let Visual Studio connect the event to the function automatically. For this lesson, we need to connect the function manually. You’ll see why in a minute.

Add this function to SuperAdventure.cs:

private void cboWeapons_SelectedIndexChanged(object sender, EventArgs e)
{
   _player.CurrentWeapon = (Weapon)cboWeapons.SelectedItem;
}

This gets the selected item from the cboWeapons dropdown and saves it in the player’s CurrentWeapon property.

Notice that we have “(Weapon)” in front of “cboWeapons.SelectedItem”. That’s because a dropdown can hold different types of objects, and cboWeapons.SelectedItem could be any datatype. By adding “(Weapon)” in front of it, we are “casting” it to the Weapon datatype.

We can do this because the items in the dropdown are Weapon objects, so the casting will work.

We need to do this, because the CurrentWeapon property only holds Weapon objects. It can’t take a generic SelectedItem object.

Step 3: Now we need to change the code that populates the cboWeapons dropdown. We want it to select the player’s CurrentWeapon (if they have one) and connect to the cboWeapons_SelectedIndexChanged function (for when the player changes the value in the dropdown list).

Find the “UpdateWeaponsListInUI” function and change it to the code below:

private void UpdateWeaponListInUI()
{
   List<Weapon> weapons = new List<Weapon>();
 
   foreach(InventoryItem inventoryItem in _player.Inventory)
   {
       if(inventoryItem.Details is Weapon)
       {
           if(inventoryItem.Quantity > 0)
           {
               weapons.Add((Weapon)inventoryItem.Details);
           }
       }
   }
 
   if(weapons.Count == 0)
   {
       // The player doesn't have any weapons, so hide the weapon combobox and "Use" button
       cboWeapons.Visible = false;
       btnUseWeapon.Visible = false;
   }
   else
   {
       cboWeapons.SelectedIndexChanged -= cboWeapons_SelectedIndexChanged;
       cboWeapons.DataSource = weapons;
       cboWeapons.SelectedIndexChanged += cboWeapons_SelectedIndexChanged;
       cboWeapons.DisplayMember = "Name";
       cboWeapons.ValueMember = "ID";
 
       if(_player.CurrentWeapon != null)
       {
          cboWeapons.SelectedItem = _player.CurrentWeapon;
       }
       else
       {
           cboWeapons.SelectedIndex = 0;
       }
   }
}

Look at the lines before and after where we set the cboWeapons.DataSource. These lines connect, or disconnect, a function to the SelectedIndexChanged event of cboWeapons.

In the line before setting the datasource, we remove the function connected to the dropdown’s “SelectedIndexChanged event (the line with the “-=”). That’s because when you set the DataSource property of a dropdown, it automatically calls the function connected to the SelectedIndexChanged event. We don’t want that to happen. We only want that event called when the player manually changes the value.

After the DataSource is set, we add the event handler function back to the SelectedIndexChanged event (with the “+=”). This way, the function will run when the player changes the value.

At the end of this function, we have code to check if the player has a value in their CurrentWeapon property. If they do, we set the dropdown’s SelectedItem to that weapon. If the player doesn’t have a CurrentWeapon, we default to the first one.

Step 4: Now we need to update the save game and load game functions. Go back to editing the Player.cs file.

Find the “ToXmlString” function and add this code after the section for saving the current location:

if(CurrentWeapon != null)
{
   XmlNode currentWeapon = playerData.CreateElement("CurrentWeapon");
   currentWeapon.AppendChild(playerData.CreateTextNode(this.CurrentWeapon.ID.ToString()));
   stats.AppendChild(currentWeapon);
}

This will save the ID of the player’s currently selected weapon (if they have one).

Next, find the “CreatePlayerFromXmlString” function and add this code after the lines to set the CurrentLocation:

if(playerData.SelectSingleNode("/Player/Stats/CurrentWeapon") != null)
{
   int currentWeaponID = Convert.ToInt32(playerData.SelectSingleNode("/Player/Stats/CurrentWeapon").InnerText);
   player.CurrentWeapon = (Weapon)World.ItemByID(currentWeaponID);
}

This will load the player’s currently selected weapon, if one exists in the save game file.

Now, we’re finished. When the player changes their active weapon, that value will stay set when they move to a new location, or if they exit and resume the game. It makes the game a little easier for the player, which is always a good thing to do with your programs.

Check your work

Build the solution and make sure there are no errors in the “output” box at the bottom of Visual Studio. If you see any problems, double-check the changes you made in this lesson.

Summary

The important thing from this lesson is that you need to think before making a change to an existing program. What else could your change affect?

This is why some programmers create unit tests and use continuous integration. With those, you can make a change and quickly know if it broke anything.

Source code for this lesson

Source code on GitHub

Source code on Dropbox

Next lesson: Lesson 19.6 – Increase maximum hit points when the player gains a level

Previous lesson: Lesson 19.4 – Saving and loading the player information

All lessons: Learn C# by Building a Simple RPG Index