从索引 1 处开始，初始化字符串的格式不符合规范。什么意思？

[xiaoan233]

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.OleDb;
using System.Data.SqlClient;

public partial class _1 : System.Web.UI.Page
{
protected void Button1_Click(object sender, EventArgs e)
{
//连接指定的Access数据库
string content = TextBox1.Text;
String ConnString ="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=1.mdb";
string sql = string.Format("select count(*) from 表1 where username='{0}' ", content);
SqlCommand cmd = new SqlCommand(sql);
string connString = @" 连接字符串";
SqlConnection conn = new SqlConnection(connString);
cmd.Connection = conn;
conn.Open();
object obj = cmd.ExecuteScalar();
conn.Close();
int count = (int)obj;
if (count == 1)
{
// 说明该字段存在于数据库
//进行相关操作
Label1.Text = "233";
}
else
{
//说明该字段在数据库 中不存在
//进行相关操作 
Label1.Text = "22233";
}

}

}

这行出错SqlConnection conn = new SqlConnection(connString);

[slwsss]

var conn=new OleDbConnection(connString);

[xiaoan233]

CS0234: 命名空间“System.Xml”中不存在类型或命名空间名称“Linq”(是缺少程序集引用吗?) 这个又是什么意思？

[slwsss]

OleDbConnection conn=new OleDbConnection(connString);

[hsx49949046]

var conn=new OleDbConnection(connString);