求教DataView发生的一个错误 急！！

刚写了个.net程序 ，系统错误提示：“DataView是命名空间，在此被当作类型使用”，我想问下这是哪里错误了，望各位高人指教下哈，谢谢了，代码如下：
namespace DataView
{
    public partial class Form1 : Form
    {
        string str = "data source=(local); initial catalog=pubs; user id=sa; password=fly";
        string strsql = "select * from authors";
        SqlDataAdapter da;
        DataSet ds;
        DataView dv;
        public Form1()
        {
            InitializeComponent();
        }        private void Form1_Load(object sender, EventArgs e)
        {
            SqlConnection cn = new SqlConnection(str);
            cn.Open();
            SqlCommand cd = new SqlCommand();
            cd.Connection = cn;
            cd.CommandText = "select distinct city from authors";
            SqlDataReader rdcity = cd.ExecuteReader();
            while (rdcity.Read())
            {
                comboBox1.Items.Add(rdcity["city"]);
            }
            rdcity.Close();
            cn.Close();
        }        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            ds = new DataSet();
            da = new SqlDataAdapter(strsql,str);
            da.Fill(ds, "authors");
        
            dv = new DataView(ds.Tables["authors"]);
         
            dv.RowFilter = "city='" + comboBox1.Text.Trim() + "'";
            dataGridView1.DataSource= dv;
            radioButton1.Enabled = true;
            radioButton2.Enabled = true;
        }        private void radioButton1_CheckedChanged(object sender, EventArgs e)
        {
            dv.Sort = "au_id desc";
        }        private void radioButton2_CheckedChanged(object sender, EventArgs e)
        {
            dv.Sort = "au_id asc";
        }
    }
}