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

Lesson objectives

At the end of this lesson, you will know…

• How to ensure all changes for a property go through a shared method of a class

One of the most common requests has been to show how to increase the player’s MaximumHitPoints when they gain a level.

We could take the easy way, and add come code to recalculate the player’s MaximumHitPoints after every line where we add to the player’s experience points. However, that could lead to problems.

We’d need to find every place where we add to the player’s experience points. In a small program like this, that isn’t too difficult. In a larger program, that can be a nightmare. Plus, if we ever decide to change the increase for each level (for example, increase by 5 hit points per level, instead of 10), we’d have to go back through all the code again, and make multiple changes.

If you do this in big programs, you are almost guaranteed to miss one place. Then, you’ll spend hours (or days) trying to track down why your program isn’t working the way you expect.

To make this easy to maintain, we’re going to add the code in one place, and make sure the rest of the program has to go through that one place.

How to ensure all changes for a property go through a common location

Step 1: Open the SuperAdventure solution in Visual Studio and open the Player.cs class. Go to line 43 and insert this new method:

public void AddExperiencePoints(int experiencePointsToAdd)
{
    ExperiencePoints += experiencePointsToAdd;
    MaximumHitPoints = (Level * 10);
}

Step 2: To make sure nowhere else in the program can modify the player’s experience points, we need to make its “set” private. This way, the value can only be set by other methods in the Player class. Go up to the ExperiencePoints property in the Player.cs class (line 13) and change it to this:

public int ExperiencePoints { get; private set; }

Step 3: Find all the places in the program that used to directly update the player’s ExperiencePoints property and change them to use the new method.

Since we have a small program, we can cheat a little and build the solution. We’ll get an error for every line that is currently trying to add to ExperiencePoints. If you haven’t modified the program, this should be in two places in SuperAdventure.cs: line 122, when the player gets experience for completing a quest, and line 347, when the player gets experience for killing a monster.

Make these changes:

Line 122

From:

_player.ExperiencePoints += newLocation.QuestAvailableHere.RewardExperiencePoints;

To:

_player.AddExperiencePoints(newLocation.QuestAvailableHere.RewardExperiencePoints);

Line 347

From:

_player.ExperiencePoints += _currentMonster.RewardExperiencePoints;

To:

_player.AddExperiencePoints(_currentMonster.RewardExperiencePoints);

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

There is a programming principle called “DRY – Don’t Repeat Yourself”.

You generally don’t want to have code that does the same thing in multiple places. With repeated code, it takes longer to make changes, and you’ll often end up with bugs, because you missed the change in one place. Do something like this to ensure all your program uses the same logic.

Source code for this lesson

Source code on GitHub

Source code on Dropbox

Next lesson: Lesson 20.1 – Refactoring the SuperAdventure program

Previous lesson: Lesson 19.5 – Changing dropdown default values

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