求高手帮助看看

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;namespace StudentManager
{
    public partial class StudentManager : Form
    {
        DataSet set = new DataSet();
        SqlDataAdapter adapter;
        public StudentManager()
        {
            InitializeComponent();
        }        private void StudentManager_Load(object sender, EventArgs e)
        {
            string sql = "select StudentId, Name, StudentNO, Sex, Telephone, Address, Job from Student";
            adapter = new SqlDataAdapter(sql, DBHelper.connection);
            adapter.Fill(set,"table");
            dgvManager.DataSource=set.Tables["table"];
        }        private void btnSearch_Click(object sender, EventArgs e)
        {
            string sql = Getsql();
            adapter = new SqlDataAdapter(sql, DBHelper.connection);
            set.Tables["table"].Clear();
            adapter.Fill(set,"table");
            dgvManager.DataSource = set.Tables["table"];        }        public string Getsql()
        {
            string sql="";
            if (cboSearchSex.SelectedIndex == 0)
            {
                sql = "select StudentId, Name, StudentNO, Sex, Telephone, Address, Job from Student";
            }
            else if (cboSearchSex.SelectedIndex == 1 || cboSearchSex.SelectedIndex == 2)
            {
                sql = string.Format("select StudentId, Name, StudentNO, Sex, Telephone, Address, Job from Student where sex='{0}'", cboSearchSex.Text.Trim());
            }
            return sql;
        }        private void btnSave_Click(object sender, EventArgs e)
        {
            DialogResult result=MessageBox.Show("确实要将修改保存到数据库吗?","操作提示",MessageBoxButtons.OKCancel,MessageBoxIcon.Information);
            if (result == DialogResult.OK)
            {
                SqlCommandBuilder builder = new SqlCommandBuilder(adapter);
                adapter.Update(set,"table");
            }
        }        private void btnResch_Click(object sender, EventArgs e)
        {
            string sql="Select * from Student";
            adapter=new SqlDataAdapter(sql,DBHelper.connection);
            set.Tables["table"].Clear();
            adapter.Fill(set,"table");
            dgvManager.DataSource = set.Tables["table"];
        }
    }
}
初学C#，这是用DataGridView做的一个东西，编译的时候总是报：无法将string类型隐式转换为system.windows.Forms.DataGridViewTextBoxColumn请高手帮助看看，谢谢！