error 'there is already an open datareader associated with this command which must be closed first'

Question

runtime error \'there is already an open datareader associated with this command which must be closed first\'

objCommand = new SqlCommand(\"SELECT field1, fi         


        

Answer 1

How about pulling the data into a DataSet via Fill and then iterate through that to perform your insert via NonQuery?

IDbDataAdapter da;
IDbCommand selectCommand = connection.CreateCommand();
selectCommand.CommandType = CommandType.Text;
selectCommand.CommandText = "SELECT field1, field2 FROM sourcetable";
connection.Open();
DataSet selectResults= new DataSet();
da.Fill(selectResults); // get dataset
selectCommand.Dispose();
IDbCommand insertCommand;

foreach(DataRow row in selectResults.Tables[0].Rows)
{
    insertCommand = connection.CreateCommand();
    insertCommand.CommandType = CommandType.Text;
    insertCommand.CommandText = "INSERT INTO tablename (field1, field2) VALUES (3, '" + row["columnName"].ToString() + "'";   
}
insertCommand.Dispose();
connection.Close();

Answer 2

Your best bet would be to read the information you need into a list and then iterating the list to perform your inserts like so:

        List<String> values = new List<String>();
        using(SqlCommand objCommand = new SqlCommand("SELECT field1, field2 FROM sourcetable", objConn)) {
            using(SqlDataReader objDataReader = objCommand.ExecuteReader()) {
                while(objDataReader.Read()) {
                    values.Add(objDataReader[0].ToString());
                }
            }
        }
        foreach(String value in values) {
            using(SqlCommand objInsertCommand = new SqlCommand("INSERT INTO tablename (field1, field2) VALUES (3, '" + value + "')", objConn)) {
                objInsertCommand.ExecuteNonQuery();
            }
        }

Answer 3

What version of SQL Server are you using? The problem might be with this:

(from http://msdn.microsoft.com/en-us/library/9kcbe65k.aspx)

When you use versions of SQL Server before SQL Server 2005, while the SqlDataReader is being used, the associated SqlConnection is busy serving the SqlDataReader. While in this state, no other operations can be performed on the SqlConnection other than closing it. This is the case until the Close method of the SqlDataReader is called.

So, if this is what's causing your problem, you should first read all the data, then close the SqlDataReader, and only after that execute your inserts.

Something like:

objCommand = new SqlCommand("SELECT field1, field2 FROM sourcetable", objConn);

objDataReader = objCommand.ExecuteReader();

List<object> values = new List<object>();
while (objDataReader.Read())
{
    values.Add(objDataReader[0]);
}

objDataReader.Close();

foreach (object value in values)
{
    objInsertCommand = new SqlCommand("INSERT INTO tablename (field1, field2) VALUES (3, '" + value + "')", objConn);
    objInsertCommand.ExecuteNonQuery();
}

Answer 4

Adding this to connection string should fix the problem.

MultipleActiveResultSets=true

Answer 5

INSERT INTO tablename (field1, field2)
    SELECT 3, field1 FROM sourcetable

A single SQL statement instead of one per insert. Not sure if this will work for your real-life problem, but for the example you provided, this is a much better query than doing them one at a time.

On a side note, make sure your code uses parameterized queries instead of accepting strings as-is inside the SQL statement - your sample is open to SQL injection.

Answer 6

Several suggestions have been given which work great, along with recommendations for improving the implementation. I hit the MARS limit due to existing code not cleaning up a reader so I wanted to put together a more respectable sample:

const string connectionString = @"server=.\sqlexpress;database=adventureworkslt;integrated security=true";
const bool useMARS = false;
using (var objConn = new System.Data.SqlClient.SqlConnection(connectionString + (useMARS ? ";MultipleActiveResultSets=True" : String.Empty)))
using (var objInsertConn = useMARS ? null : new System.Data.SqlClient.SqlConnection(connectionString))
{
 objConn.Open();
 if (objInsertConn != null)
 {
  objInsertConn.Open();
 }

 using (var testCmd = new System.Data.SqlClient.SqlCommand())
 {
  testCmd.Connection = objConn;
  testCmd.CommandText = @"if not exists(select 1 from information_schema.tables where table_name = 'sourcetable')
                          begin
                           create table sourcetable (field1 int, field2 varchar(5))
                           insert into sourcetable values (1, 'one')
                           create table tablename (field1 int, field2 varchar(5))
                          end";
  testCmd.ExecuteNonQuery();
 }

 using (var objCommand = new System.Data.SqlClient.SqlCommand("SELECT field1, field2 FROM sourcetable", objConn))
 using (var objDataReader = objCommand.ExecuteReader())
 using (var objInsertCommand = new System.Data.SqlClient.SqlCommand("INSERT INTO tablename (field1, field2) VALUES (3, @field2)", objInsertConn ?? objConn))
 {
  objInsertCommand.Parameters.Add(new System.Data.SqlClient.SqlParameter("field2", String.Empty));
  while (objDataReader.Read())
  {
   objInsertCommand.Parameters[0].Value = objDataReader[0];
   objInsertCommand.ExecuteNonQuery();
  }
 }
}