สอบถามเรื่องการประกาศ ตัวแปร ที่เดียวแล้วใช้ได้ตลอดหน่อยครับ

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Training1
{
public partial class Form1 : Form
{

public Form1()
{
InitializeComponent();

}

//ประกาศตัวแปรให้ใช้ได้ทั้ง class รึป่าวครับ แบบนี้
int n1 =0;
int n2 = ;
int sum = 0 ;
private void BTN1_Click(object sender, EventArgs e)
{
n1 = Convert.ToInt32(txtnum1.Text);
n2 = Convert.ToInt32(txtnum2.Text);
sum = n1 + n2;

// MessageBox.Show(sum.ToString());
txttotal.Text = (sum.ToString());
}

private void BTN2_Click(object sender, EventArgs e)
{
n1 = Convert.ToInt32(txtnum1.Text);
n2 = Convert.ToInt32(txtnum2.Text);
sum = n1 - n2;

// MessageBox.Show(sum.ToString());
txttotal.Text = (sum.ToString());
}
}
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Training1
{
public partial class Form1 : Form
{

public Form1()
{
InitializeComponent();

}

void SetSum(strng operate)
{
int n1 =0;
int n2 = ;
int sum = 0 ;
n1 = Convert.ToInt32(txtnum1.Text);
n2 = Convert.ToInt32(txtnum2.Text);
if(operate == "+")
{
sum = n1 + n2;
}
else if(operate == "-")
{
sum = n1-n2;
}

// MessageBox.Show(sum.ToString());
txttotal.Text = (sum.ToString());
}

private void BTN1_Click(object sender, EventArgs e)
{
SetSum("+");
}

private void BTN2_Click(object sender, EventArgs e)
{
SetSum("-");
}
}
}