Gigel does not attack tree life

Question

1.33/5 (3 votes)

See more:

        Sprite gigel = new Sprite(WindowsFormsApplication2.Properties.Resources.Gigel_img);
        Sprite tree = new Sprite(WindowsFormsApplication2.Properties.Resources.tree_img);

        void Screen_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.E)
            {
                gigel.Atack(tree.life);
                tree.LifeUpdate();
            }
            gigel.Reposition();
            Refresh();
        }

...
inside Sprite class 
        public void Atack(int targetLife)
        {
            targetLife -= damage;
        }

'targetLife' is decrementing conform 'damage'
but is not passing the value OUT to tree.life from "gigel.Atack(tree.life);"
tree.life is not getting the value from process ! Why? Or what i am doing wrong?
Thank you.

What I have tried:

//I hope now is more clear: 

            if (e.KeyCode == Keys.E)
            {
                int num = 20;
                gigel.Atack(num);
                tree.life = num; // num is 20 and not 7

                tree.LifeUpdate();
            }
...
inside Sprite class 
        public void Atack(int targetLife) //targetLife = 20
        {
            targetLife -= damage; // 20-13 targetLife=7
            //from this point (num) must take targetLife value of 7
            //but (num) remains at 20.
        }

Posted 31-Jul-18 14:17pm

_Q12_

Updated 1-Aug-18 12:30pm

v4

Add a Solution

Comments

[no name] 31-Jul-18 21:24pm

You haven't shown what "LifeUpdate" does.

Also, "gigel" seems to be sucking its own life.

_Q12_ 31-Jul-18 21:39pm

1- tree.life does not update, so the other classes (LifeUpdate) after are irrelevant.
2- not really... it's a single class called [Sprite] from which I created 2 objects - gigel and tree. The 2 objects have same base class from which they derive. But it seems something is not working right.

Eric Lynch 1-Aug-18 0:18am

Having some trouble following your question. My guess is that you are asking why tree.life, which is passed as a parameter, is not updated when you call Attack.

If this is the case, it is because the data type int is a value type. This means that when you pass it as a parameter, you pass a copy of the value (unless you specify the ref or out keyword). When you decrement targetlife, you are decrementing a copy. Since you do nothing with that copy, its value is discarded after it passes out of scope.

Assuming "life" is a property of tree, this poses further difficulties. You cannot simply pass properties by reference (with a ref/out keyword).

So, you have a few possible options. Expose a "Damage" property from the Sprite, and then: tree.life -= gigel.Damage.

Or, change the signature of the Attack method: public int Attack(targetlife) { return targetlife - damage; } and then: tree.life = gigel.Attack(tree.life).

_Q12_ 1-Aug-18 12:43pm

I updated " What I have tried:" section with more clear example of what i have tried other than original post.

Eric Lynch 1-Aug-18 13:15pm

Read the comment a bit more carefully. This has the exact same problem. When you pass "num" to "Attack", you pass a COPY of the value. This is how value types (e.g. int) work. Then you increment the "targetlife" (which is a copy NOT the original). After this, you return. Since "targetlife" passes out of scope, when you return, it ceases to exist.

I recommended some possible solutions in the earlier comment. You may also want to re-think your object model a bit. There is not enough code for me to be certain, but my intuition tells me the model could be improved to make this less difficult.

For example, if Sprite has both life and damage, you could do the following:

public void Attack(Sprite target)
{
target.life -= damage;
}

gigel.Attack(tree);

To better understand the difference between value types (int, struct, etc.) and reference types (class), you may want to read here:

http://www.albahari.com/valuevsreftypes.aspx

_Q12_ 1-Aug-18 15:36pm

My dear @Eric-Lynch, you are a genius!
It worked as you sugested ! I couldn't see it without your help.
Thank you very much and dont forget to make this as a solution to give you all the credit. You certainly deserve it.

Eric Lynch 1-Aug-18 18:32pm

You're very welcome. I have done as you suggested and added a solution. I will shamelessly accept any credit you deem appropriate :)

I highly recommend taking a bit of extra time to explore the difference between value and reference types in the provided link. This is a fairly common problem encountered by people as they become more familiar with C#. Its likely to cause you problems, in other situations, until you become comfortable with it.

CHill60 1-Aug-18 7:19am

I'm intrigued as to how "ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd" tells us what you have done! That section is for your code and the body of the question is where you explain your problem

_Q12_ 1-Aug-18 12:44pm

I updated " What I have tried:" section with more clear example of what i have tried other than original post.

CHill60 2-Aug-18 3:49am

Thank you - I see you have also benefitted, as @Eric-Lynch has posted a great answer :-)

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Eric Lynch · Accepted Answer · 2018-08-01T12:30:00

So, here is the basic problem: value type versus reference type. For more details, read:

Value vs Reference Types in C#[^]

When you pass a value type (e.g. int) like num or tree.life as a parameter, you are passing a copy of that value. In your method (Attack), you are then decrementing the copy of the value (targetlife), not the original. When you return from the Attack method, that copy (targetlife) passes out of scope and is discarded.

In modern versions of C#, it is possible to pass a value type, by reference, using the ref or out keyword. For details, see the following links:

ref (C# Reference) | Microsoft Docs[^]
out parameter modifier (C# Reference) | Microsoft Docs[^]

However, in your case, there may be additional considerations. While the source code is not shared, I suspect life may be a property. Regrettably, it is not currently possible to pass a property using the ref/out keyword.

So, I would consider slightly re-modeling your Sprite class, which is common to both gigel and tree. In your code, it seems that gigel is attacking tree, so the method signature for Attack should probably more closely resemble this action.

I would suggest you change the method signature of Attack as follows:

public void Attack(Sprite target) =>
  target.life -= damage;

Then, to have a gigel attack a tree, you would do the following:

gigel.Attack(tree);