Where to instantiate in winform?

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hi, I am learning C#, i have a simple MathClass would like to use in Winform.
it works but something not clear.

Is it Okay or conventional to instantiate at here?

private void button1_Click(object sender, EventArgs e)
     {
         MathClass obj = new MathClass(x, y);
         textBox3.Text = obj.Add().ToString();
     }

class MathClass
    {
        private int _x;
        private int _y;

      public MathClass(int x,int y)
        {
            _x = x;
            _y = y;
        }
      public int Add()
        {
            return _x + _y;
        }
    }

What I have tried:

I try to instantiate :
1. Outside the button1_Click method, but inside partial class Form1, it show error.

public partial class Form1 : Form
   {
       public Form1()
       {
           InitializeComponent();
       }

       int x = 0;
       int y = 0;

       MathClass obj = new MathClass(x, y);

2. inside

private void InitializeComponent()

no error, but obj cannot use inside Form1 anymore.

Posted 15-May-18 16:54pm

Member 13462842

Updated 11-May-22 11:39am

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3 solutions

Solution 1

You can't pass parameters outside a method. Declare it where you are, outside a method, and create it in the constructor

Posted 15-May-18 17:03pm

Christian Graus

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OriginalGriff · Accepted Answer · 2018-05-15T19:00:00

Don't add it to InitializeComponent: that is to prepare controls and such like that are managed by the Visual Studio Designer - generally, you don't need to edit that at all.
This is perfectly valid:

private void button1_Click(object sender, EventArgs e)
     {
         MathClass obj = new MathClass(x, y);
         textBox3.Text = obj.Add().ToString();
     }

And we instantiate objects all the time:

private void button1_Click(object sender, EventArgs e)
     {
         SaveFileDialog sfd = new SaveFileDialog();
         if (sfd.ShowDialog() == DialogResult.OK)
         {
             SaveFile(sfd.FileName);
         }
     }

but the naming could use some work: "obj" is not a good name for anything as it implies you don't know what the type is:

MathClass mc = new MathClass(x, y);
textBox3.Text = mc.Add().ToString();

You can't do it like this outside a method:

public Form1()
{
    InitializeComponent();
}

int x = 0;
int y = 0;

MathClass obj = new MathClass(x, y);

because the creation of the MathClass object relies on other variables (x and y) and there is no guarantee that they have been initialized when it tries to create the instance. Think about it - what if you did this:

C#

int x = y + 2;
int y = z - 3;
int z = 9;

That requires z to have been created and initialized before x, which isn't obvious unless you read y first!
Class level initializations must be simple: anything more complex needs to be in the form constructor which ios only executed once the "basic variable" init is complete.

BillWoodruff · Accepted Answer · 2018-05-16T08:20:00

I think your design needs re-working: there should be no need to instantiate the class more than once in order to invoke its methods:

class MathClass
{
        public MathClass(){}
        
        public int Add(int x, int y))
        {
            return x + y;
        }

        public int Sub(int x, int y))
        {
            return x - y;
        }
}

Use: I assume you need to get your x,y values from two TextBoxes:

// in the Form
MathClass mathClassInstance = new MathClass();
'
// in the Buttom Click Event
private void button1_Click(object sender, EventArgs e)
{
    int x, y;
    
    if (Int32.TryParse(textBox1.Text, out x))
    {
        if (Int32.TryParse(textBox2.Text, out y))
        {
            textBox3.Text = mathClassInstance.Add(x, y).ToString();
            return;
        }
    }

    throw new ArgumentException("Bad input");
}

Where to instantiate in winform?

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