在配置文件web.config里添加
<Connectionstring > 
<add name="strConn" connectionString="server=服务器地址;database=数据库名;uid=登录名称;pwd=密码" / > 
</Connectionstring > 

    //取得连接字符串
    string strCn = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;

    //获得数据
    public DataTable getData()
    { 
        DataTable dt = new DataTable();
        SqlConnection sqlCn = new SqlConnection(strCn );
        SqlCommand sqlCmd = new SqlCommand("select * from 表名",sqlCn );
        SqlDataAdapter sqlAda = new SqlDataAdapter(sqlCmd );
        try
        {
            sqlCn.Open();
            sqlCmd.ExecuteNonQuery();
            sqlAda.Fill(dt);
            sqlCn.Close();
        }
        catch (Exception ex)
        {
            throw (ex);
        }
        return dt;
    }
要是用存储过程并有参数的话.
    //如获得图书信息
    public DataTable getBook(int bookid)
    { 
        DataTable dt = new DataTable();
        SqlConnection sqlCn = new SqlConnection(strCn );
        SqlCommand sqlCmd = new SqlCommand("getRecored",sqlCn );
        sqlCmd.CommandType=CommandType.StoredProcedure;
        sqlCmd.Parameters.Add("@bookid",SqlDbType.Int,4);
        sqlCmd.Parameters["@bookid"].Value=bookid;
        SqlDataAdapter sqlAda = new SqlDataAdapter(sqlCmd );
        try
        {
            sqlCn.Open();
            sqlCmd.ExecuteNonQuery();
            sqlAda.Fill(dt);
            sqlCn.Close();
        }
        catch (Exception ex)
        {
            throw (ex);
        }
        return dt;
    }

------解决方案--------------------
写C#代码前要引用
using System.Data.SqlClient;
命名空间.