System.Collections.Generic.List'需要& # 39;1 & # 39;类型参数

我在c#中创建了一个代码，但是我有这个错误:

错误1使用泛型类型'System.Collections.Generic.List'C:'Users'Abdelhakim'Desktop'Wind applicationc# 'Exercice4'Exercice4'Form1.cs 9 34 Exercice4

，这是我写的代码:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections.Generic.List;
namespace Exercice4
{
    public partial class Form1 : Form
    {
        List<string> ls = new List<string>();
        string p;
        public Form1()
        {
            InitializeComponent();
        }
        private void label2_Click(object sender, EventArgs e)
        {
        }
        private void r1_CheckedChanged(object sender, EventArgs e)
        {
            p = "+";
            l3.Text = (Convert.ToDouble(t1.Text) + Convert.ToDouble(t2.Text)).ToString();
        }
        private void r2_CheckedChanged(object sender, EventArgs e)
        {
            p = "-";
            l3.Text = (Convert.ToDouble(t1.Text) - Convert.ToDouble(t2.Text)).ToString();
        }
        private void r3_CheckedChanged(object sender, EventArgs e)
        {
            p = "*";
            l3.Text = (Convert.ToDouble(t1.Text) * Convert.ToDouble(t2.Text)).ToString();
        }
        private void r4_CheckedChanged(object sender, EventArgs e)
        {
            p = "/";
            if (Convert.ToDouble(t2.Text) == 0)
                l3.Text = "Erreur";
            else
                l3.Text = (Convert.ToDouble(t1.Text) / Convert.ToDouble(t2.Text)).ToString();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            int i;
            for (i = 0; i < ls.Count; i++)
                ls.Items.Add(ls[i]);
        }
        private void button2_Click(object sender, EventArgs e)
        {
            ls.Add(t1.Text + p + t2.Text + " =" + l3.Text);
        }
    }
}

using System.Collections.Generic.List;

using System.Collections.Generic;

ls.Items.Add(ls[i]);

ls.Add(ls[i]);