从数据库中查询数据后赋给字符串有关问题_ASP.NET

从数据库中查询数据后赋给字符串问题
public string CheckDataYear(string logid)
{
string connect = "User ID=appDB;Initial Catalog=appDB;Data Source=LJJ;Password=appDB123";
SqlConnection con = new SqlConnection(connect);
con.Open();
string sql= "select jyrqy from t_tzsb_ylss_ys1_new where djid='logid'";
SqlCommand com = new SqlCommand(sql,con);
string year = com.ExecuteScalar().Tostring;//出错
return year;
}

我想把查询结果赋给一个字符串，为什么不可以啊

------解决方案--------------------------------------------------------
不过要想让连接能够即时关闭,最好使用using
如下

C# code

public string CheckDataYear(string logid) {  string year="";  string connect = "User ID=appDB;Initial Catalog=appDB;Data Source=LJJ;Password=appDB123";   using(SqlConnection con = new SqlConnection(connect)){     con.Open();     string sql= "select jyrqy from t_tzsb_ylss_ys1_new where djid='logid'";     SqlCommand com = new SqlCommand(sql,con);    SqlDataReader rd=com.ExecuteReader();    if(rd.read()){      year = rd["字段名"].ToString();//出错      }  }    return year; }
------解决方案--------------------------------------------------------
        string year = com.ExecuteScalar().Tostring;//出错  
是  ToString()
------解决方案--------------------------------------------------------
    string year = com.ExecuteScalar().Tostring;//出错  
确定这个地方返回的值不是null?

------解决方案--------------------------------------------------------
探讨
不过要想让连接能够即时关闭,最好使用using 
如下 

C# code
public string CheckDataYear(string logid) 
{
  string year="";
  string connect = "User ID=appDB;Initial Catalog=appDB;Data Source=LJJ;Password=appDB123"; 
  using(SqlConnection con = new SqlConnection(connect)){ 
    con.Open(); 
    string sql= "select jyrqy from t_tzsb_ylss_ys1_new where djid='logid'"; 
    SqlCommand com = n…