c#求解SQL连接问题

未能找到命名空间，我已添加引用求解怎么改

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WindowsFormsApplication5
{
    class Dao
    {
        SqlConnection sc;
        public SqlConnection connect()
       {
            string str = @"Data Source = .;Initial Catalog = yangtest;Integrated Security = True";
            SqlConnection sc = new SqlConnection(str);
            sc.Open();
            return sc;         
       }
       public SqlCmmand command(string sql)
       {
            SqlCommand cmd = new SqlCommand(sql, connect());
            return cmd;
       }
        public int Execute(string sql)
       {
            return command(sql).ExecutNonQuery();
       }
        public SqlDataReader read(string sql)
       {
            return command(sql).ExecuteReader();
       }
        public void DaoClose()
       {
            sc.Close();
       }
    }
}

using System.Data.SqlClient
数据库链接的命名空间是System.Data.SqlClient

关于C#连接SQL Server数据库，可参考：