sqldatareader in asp.net

Replace your code with...

SqlConnection con = Database.GetConnection();
        SqlCommand com = new SqlCommand("select * from category where article_allow = 1", con);
        SqlDataReader rs = new SqlDataReader();
        SqlDataReader rst = new SqlDataReader();
        
        string article_visit = "0";
        rs = com.ExecuteReader();
        while (rs.Read())
        {
           
            SqlCommand cmd = new SqlCommand("SELECT article_visit FROM article_list where cat_ID=" + rs["cat_ID"] + " and article_status=1", con);
            rst = cmd.ExecuteReader();
            while (rst.Read())
            {
               
                article_visit = rst["article_visit"] + article_visit;
            }
            
            rst.Close();
 
            
            cmd.Clone();
        }
        rs.Close();

Now whats the problem in your code : Notice that rs("cat_ID") is a method call, while it should be like this rs["cat_ID"]. Similarly for rst("article_visit") is rst["article_visit"]. also the + sign in place of &

Hope you got the point ....... :)

How you create NEW instance of SqlDataReader ?
Do not create new instance of SQLDatareader, it will give you complie time error.

may be your older version of code running.

In addition to the two postings above..

1. You have a typo in your first command string.. (seelct instead of select)
2. Do yourself a favor and use parameterized calls, always, even if your var is read from a database.
3. When I'm not wrong

SqlDataReader rs = new SqlDataReader()

should result in an error.

See code below...

SqlConnection con = Database.GetConnection();
SqlCommand com = new SqlCommand( "SELECT * FROM category WHERE article_allow = 1", con );
string article_visit = "0";
SqlDataReader rs = com.ExecuteReader();

 /* create the second connection here
  * SqlConnection secondCon = new SqlConnection ( connectionString );
  * or if your method Database.GetConnection() returns a new object use this instead
  * SqlConnection secondCon = Database.GetConnection();
  */
while ( rs.Read() )
{
    /* use second connection here */
    SqlCommand cmd = new SqlCommand( "SELECT article_visit FROM article_list WHERE cat_ID = @catID AND article_status=1", secondCon );
    cmd.Parameters.AddWithValue( "catID", rs[ "cat_ID" ] );
    SqlDataReader rst = cmd.ExecuteReader();
    while ( rst.Read() )
    {
        article_visit = rst[ "article_visit" ] + article_visit;
    }
    
    rst.Close();
    rst.Dispose();
    cmd.Dispose();
}

rs.Close();
rs.Dispose();

Edit.
Sorry, my fault.
It is not possible to use the same connection for the second reader while the first is still open.
You have to use another connection to your database for the second reader.

So create a new connection and modify the second SqlCommand to use the new connection..

Edit 2/3
Updated Code example