Lesson 19.2 – Use a calculated value for a property

Lesson objectives

At the end of this lesson, you will know…

• How to return a value in a property, by calculating it from other properties

One thing missing from the game is updating the player’s level, based on their current experience points.

We could do this by having the game re-calculate the level, and reset the value of the Player.Level property every time the player gains experience, but there is a much simpler way. We’re going to have the property automatically calculate its value every time someone “gets” (reads) it.

How to have a property calculate its value from other properties

Step 1: Open the SuperAdventure solution in Visual Studio and open the Player.cs class.

Step 2: Change the line for the Level property from this:

public int Level { get; set; }

To this:

public int Level 
{
    get { return ((ExperiencePoints / 100) + 1); }
}

Notice that we removed the “set;”. That’s because we’re never going to put a value into the Level property – which is what the “set” is for.

We also changed the “get;” Before, since this was an “auto-property”, the “get” would get the value that it was previously “set” to. Now, the program will calculate the value for Level, by dividing the ExperiencePoints by 100.

Since this property is an “int”, it will automatically round the answer down, which is why we add 1 to it – so the player will start out a level 1, and not 0.

Step 3: Now we need to clean up the places where we were setting the Level value, since we removed the “set” option.

In Player.cs:

Remove this line from the constructor:

Level = level;

Since we don’t set the Level value any more, we can also remove it as a parameter from the constructor. So, now the constructor for Player.cs shuld look like this:

public Player(int currentHitPoints, int maximumHitPoints, int gold, int experiencePoints) : base(currentHitPoints, maximumHitPoints)
{
    Gold = gold;
    ExperiencePoints = experiencePoints;
    Inventory = new List<InventoryItem>();
    Quests = new List<PlayerQuest>();
}

Step 4: Now that we removed the level parameter from the Player class, we need to clean up anything that called the constructor.

Open the code for the SuperAdventure.cs screen class.

In the constructor for this class, we populate the “_player” variable by instantiating a new Player object.

_player = new Player(10, 10, 20, 0, 1);

Remove the value passed in for the level, so it looks like this:

_player = new Player(10, 10, 20, 0);

Step 5: Now we need to make sure the Level is updated on the game screen, after every time the player gains experience (by killing a monster or completing a quest).

You could do this by just adding this line after every time the player’s experience changes:

lblExperience.Text = _player.ExperiencePoints.ToString();

However, I decided to make this new function to update all the player’s stats:

private void UpdatePlayerStats()
{
    // Refresh player information and inventory controls
    lblHitPoints.Text = _player.CurrentHitPoints.ToString();
    lblGold.Text = _player.Gold.ToString();
    lblExperience.Text = _player.ExperiencePoints.ToString();
    lblLevel.Text = _player.Level.ToString();
}

Then add a call to that function from the other functions that could update the player’s level (the constructor, btnUseWeapon_Click, and MoveTo). If you want to see exactly where I put those calls, look at the code for this lesson in the link below.

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

It may not be obvious at first, but this is a powerful programming practice.

Before, we needed to manually update both the player’s ExperiencePoints property and their Level property. Why do two things (and possibly forget to do one, making the game act strangely) when you can do one thing and have it automatically update everything else that depends on it?

Source code for this lesson

Source code on GitHub

Source code on Dropbox

Next lesson: Lesson 19.3 – Clean up the source code by converting foreach to LINQ

Previous lesson: Lesson 19.1 – Scroll to the bottom of a rich text box

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