Output Stored Procedure

 1 public static List<string> viewtree(int root)
2 {
3 SqlConnection con = new SqlConnection(mainConnectionString);
4 con.Open();
5 try
6 {
7 List<string> ids = new List<string>();
8 SqlCommand command = new SqlCommand(@"ShowHierarchy2", con);
9 command.Parameters.AddWithValue("@root", root);
10 command.Parameters.Add(new SqlParameter("@outstring", SqlDbType.VarChar));
11 command.Parameters["@outstring"].Direction = ParameterDirection.Output;
12 command.CommandType = CommandType.StoredProcedure;
13 //command.ExecuteScalar();
14 //ids = command.Parameters["@outstring"].Value.ToString();
15
16 SqlDataReader dr = command.ExecuteReader();
17 while (dr.Read())
18 {
19 ids.Add((dr["@outstring"].ToString()));
20 }
21 //command.Parameters.Clear();
22
23 return ids;
24 }
25 finally
26 {
27 con.Close();
28 }
29 }

 
Can someone tell me why i'm getting the following error:
String[1]: the Size property has an invalid size of 0.
Thanks in advance