c# - 对象引用未设置为对象的实例(不知道我应该做什么)

Question

这是错误:

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

它停在这里:con.Open();

这是代码:

SqlConnection con = new SqlConnection(DBHelper.connection);
    SqlCommand com = new SqlCommand();
    con = com.Connection;
    con.Open();
    com.CommandType = CommandType.Text;
    com.CommandText = "select catname,catdescription,photo from category where catid=" + catselectddl.SelectedValue ;
     SqlDataReader dr= com.ExecuteReader();
     DataTable dt = new DataTable();
     dt.Load(dr);
     DataRow drr;
     drr=dt.Rows[0];
     con.Close();

错误:

Line 19:         SqlCommand com = new SqlCommand();
Line 20:         con = com.Connection;
Line 21:         con.Open(); // here the error
Line 22:         com.CommandType = CommandType.Text;
Line 23:         com.CommandText = "select catname,catdescription,photo from category where catid=" + catselectddl.SelectedValue 

Answer 1

第三行是错误的。应该是

  com.Connection = con;