为什么按了计算这个按钮运行起来会错误呢

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 Mark
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        int count = 0; 
        private void button1_Click(object sender, EventArgs e)
        {

           lbScore.Items.Add(txtScore.Text.Trim());
           txtScore.Text = "";
           count++;
        }

        private void btnClear_Click(object sender, EventArgs e)
        {
            lbScore.Items.Clear();
        }

        private void btnCaculate_Click(object sender, EventArgs e)
        {   
            double sum = 0;
            double max = 0;
            double min = 0;
            double average = 0;
            foreach (double c in lbScore.Items)
            {
                sum += c;
                max = Math.Max(max, c);
                min = Math.Max(min, c);
                average = (sum - max - min) /(count-2);
            }
            lblResult.Text = "最大数:" + max.ToString()+"\n" + "最小数:" + min.ToString() +"\n"+ "平均数:" + average.ToString();
        }

  

      
    }
}

 double sum = 0;
            double max = 0;
            double min = 0;
            double average = 0;
            foreach (var item in lbScore.Items)
            {
                Double c = Convert.ToInt32(item.ToString());
                sum += c;
                max = Math.Max(max, c);
                min = Math.Max(min, c);
                average = (sum - max - min) /(count-2);
            }
            lblResult.Text = "最大数:" + max.ToString()+"\n" + "最小数:" + min.ToString() +"\n"+ "平均数:" + average.ToString();