这是一个关于sender的问题

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

        private void Form1_Resize(object sender, EventArgs e)
        {
            if (this.WindowState == FormWindowState.Maximized)
            {
                this.WindowState = FormWindowState.Normal;
            }
        }

        private void txtNum1_KeyPress(object sender, KeyPressEventArgs e)
        {
            TextBox currentTextBox = sender as TextBox;
            if (e.KeyChar < '0' || e.KeyChar > '9')     //不是数字
            {
                e.Handled = true;
            }
            if (e.KeyChar == 8)
            {
                e.Handled = false;
            }
            if (e.KeyChar == 46)
            {
                if (currentTextBox.Text.IndexOf(".") == -1)
                {
                    if (currentTextBox.SelectionStart > 0)
                    {
                        e.Handled = false;
                    }
                }
            }

        }

        private void btnCompute_Click(object sender, EventArgs e)
        {
            double num1 = Convert.ToDouble(txtNum1.Text);
            double num2 = Convert.ToDouble(txtNum2.Text);
            switch (cboOperator.Text)
            { 
                case "+":
                    lblResult.Text = (num1 + num2).ToString();
                    break;
                case "-":
                    lblResult.Text = (num1 - num2).ToString();
                    break;
                case "*":
                    lblResult.Text = (num1 * num2).ToString();
                    break;
                case "/":
                    lblResult.Text = (num1 / num2).ToString();
                    break;
            }
        }

        
    }
}

Textbox cureetTextbox=sender as Textbox