string compare in c#

[datapham]

Hi everyone , i try to validate a [string ID] when user input it in a textBox(C#)for searching datarows in database , but it does not send a message "False Input" if i input a wrong string ID which is different from database .

protected void button_search(object sender, EventArgs e)
{
  string str = TextBox1.Text;
  Dataset ds = Class.Search(str);
  for(i=0; i<ds.Tables[0].Rows.Count; i++)
    {
      if(str == ds.Tables[0].Rows[i][0].ToString())// column[0] is the "string ID" //Column in database
        {
          GridView1.DataSource = ds;
          GridView1.DataBind();
          GridView1.Visible = true;
          Label1.Visible = false; 
        }
      else
        {
          Label1.Text = "False Input";
          Label1.Visible = true;
          GridView1.Visible = false;
        }        

    }

}