string strSelect = "select field1 from table1";
SqlConnection sqlConn = new SqlConnection("数据库连接字符串");
SqlCommand sqlCmd = new SqlCommand(strSelect,sqlConn);
sqlConn.Open();
SqlDataAdapter sqlDr = sqlCmd.ExecuteReader();
while(sqlDr.Read())
{
   TextBox1.Text = sqlDr["field1"].ToString();
}
sqlDr.Close();
sqlConn.Close();