C# :error There is already an open DataReader associated with this Command which must be closed first

Question

I used to call several functions with this connection string:

class Solders_DB
{
        SqlConnection connection;
        SqlCommand query;
        String command;

        public Solders_DB() 
        {
            connection = new SqlConnection();
            connection.ConnectionString = "Server=localhost;Database=Scheduling_Employee ;Trusted_Connection=True;MultipleActiveResultSets=True;";

            query = new SqlCommand();
         }

As you see I used this MultipleActiveResultSets=True; in my connection but in this function :

    command = @"SELECT [GID]   FROM [Scheduling_Employee].[dbo].[Solder] where [ID]=@ID";

    query.CommandText = command;
    query.Parameters.Clear();
    query.Parameters.AddWithValue("@ID", id);

    Object o= query.ExecuteScalar();

I faced with this error:

There is already an open datareader associated with this command which must be closed first

Answer 1

The code in your question is not complete. Please explain yourself better for further assistance.

When using a SqlConnection is recommended to use the 'using' statement like this:

using (SqlConnection SqlConn = new SqlConnection(ConnString))
    {
         try
         {
             SqlConn.Open();
         }
         catch (Exception ex)
         {
             Debug.WriteLine(ex.ToString());
             return null;
         }

}

You're trying to read data from the first execution of your query, please update your question and put the complete code and the error stack.

Answer 2

Its usually when there was another query before that one and has not yet stopped executing inside the database engine (maybe it was a heavy script). You could try query = new SqlCommand();, query.Cancel() or

while(query.Connection.State != System.Data.ConnectionState.Open){ Threading.Sleep(100);}