How to access a dynamically changing variable in another class

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4.00/5 (2 votes)

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VS2010

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Hi EveryOne!

I'm having trouble accessing the contents of variable defined in another class which changes dynamically.

It is something like this

namespace CvSharp
{
    public class DetectEye
    {
        CvPoint cursorpos = new CvPoint();
                
        public IplImage DetectMyEye(IplImage sendImg)
        {
                        ......//
                        for (int i = 0; i < eyes.Total; i++)
                        {
                            CvRect r = eyes[i].Value.Rect;
                            CvPoint center = new CvPoint
                            {
                                X = Cv.Round((r.X + r.Width * 0.45) * Scale),
                                Y = Cv.Round((r.Y + r.Height * 0.45) * Scale)
                            };
                            cursorpos = center;
                            //MessageBox.Show(cursorpos.X + "\n" + cursorpos.Y);
                              ........//
                            
                    }
                    return imgROI;
                }
            }
        }
/*as cursorpos is global and I
can't have two return values in same function I'm doing this*/
        public CvPoint GetCursorPos()
        {
                return cursorpos;
        }
    }

And I'm accessing it another class as

namespace CvSharp
{
        public partial class Form1 : Form
        {
            DetectEye pupilobj = new DetectEye();
            private void BtnPowerUrEyes_Click(object sender, EventArgs e)
            {
            newcurpos = pupilobj.GetCursorPos();
            int x = newcurpos.X;
            int y = newcurpos.Y;
            MessageBox.Show("X=" + x + "\n" + "Y=" + y + "\n");
            }

I know I'm doing wrong! But don't know how to correct it :(

Please use your expertise to help me!

Thanks.

Posted 21-May-11 20:08pm

AmarjeetAlien

Updated 21-May-11 20:09pm

v3

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Comments

Sergey Alexandrovich Kryukov 22-May-11 2:41am

Despite of a big mistake in code, the question is not too bad, as it serves as OP uses it as an adequate tool to learn things appropriate to OP's current level of knowledge. My 4.
--SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-05-21T20:38:00

First of all, you do not access the variable (field or other member) of the class. As you never use any static members here, you're trying to access a member of an instance of the class; and this is the key.

You access the instance CvPoint cursorpos correctly, but your problem is modification of it. I don't know if CvPoint a class or a structure. Is this is a structure, modification won't work. Why? Structure (struct) is a value type. When you assign newcurpose, you get a copy of your instance's data and then you modify a copy. No wonder, the data of the instance pupilobj is not modified. To make is working as is, you need to make the type CvPoint a class.

Using the class CvPoint instead of structure is one solution. Another solution would be using structure but writing back the of the data to the pupilobj. In other words, create one more method called SetCursorPos. But don't rush. Let's make another improvement.

You should better make the CursorPos a property. You should better make is a property in all cases, even if you make CvPoint a class:

public class DetectEye {
    public CvPoint CursorPosition { get; set; } //now you can write, not only read 
} //DetectEye

or perhaps better

public class DetectEye {
    public CvPoint CursorPosition { //property
       get { return mCursorPosition; }
       set {
           mCursorPosition;
           DoSomething(); //may be you need side effect (rendering?), so setter is here for you
       }
    } //CursorPosition
    CvPoint mCursorPosition = new CvPoint(); //private member behind the property
} //DetectEye

In this way, you will be able to modify the field from the caller accessing the instance of DetectEye.

Now, couple of useful notes. Why you always use public? Public is to make access from another assembly is well. Are you sure you need it? If not, use internal instead of public, internal protected instead of protected. Just check up if you yield more access than really required.
Don't use names like "Form1", "label3". They are auto-generated and actually violate (good) Microsoft naming conventions. When you start working with such objects, always rename them to something semantic. Visual Studio re-factoring engine is great help here.

—SA