编了窗体程序，但是运行时出现了些问题，求高人指点迷津
using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

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

        private void Form1_Load(object sender, EventArgs e)
        {
            label1.Text = ("请输入分数：");
            button1.Text = ("转换");
            label2.Text = ("成绩等级：");
        }

        private void button1_Click(object sender, EventArgs e)
        {
            double score;
            score = Convert.ToDouble(textBox1.Text);
            string grade;
            if (score >= 100 || score < 0)
            {
                textBox2.Text += ("输入的分数不合法，请核查！");
            }
            if (score >= 90)
                grade = "优秀";
            else if (score >= 80)
                grade = "良好";
            else if (score >= 70)
                grade = "中等";
            else if (score >= 60)
                grade = "及格";
            else
                grade = "不及格";
            textBox2.Text += grade.ToString();
        }
    }
}

非常感谢呀！

这个不能吧，让他输入的是数字了的。