How to re-use SqlCommand object for a CommandText after executing a query based on stored procedure in C#?

The problem is that you have reused a SqlCommand that CommandType is StoredProcedure but you want to execute a normal sql query with CommandType.Text.

"In this case, I have to create a new instance of SqlCommand object ?"

I would suggest to do so to avoid such confusions. Creating a SqlCommand is not so expensive that you need to reuse it. Actually the constructor does not more than setting the properties.

From ILSpy:

// System.Data.SqlClient.SqlCommand
// this() does only call _SuppressFinalize
public SqlCommand(string cmdText, SqlConnection connection) : this()
{
    this.CommandText = cmdText;
    this.Connection = connection;
}

aCommand.CommandType = CommandType.StoredProcedure;
aCommand.Parameters.AddWithValue("@book_id", bookID);
aCommand.Parameters.AddWithValue("@user_id", userID);

1. specify the stored procedure name you call

   aCommand.CommandText=yourstoredprocedurename;
   aCommand.ExecuteNonQuery();
   

2. then call your select and sqlreader to get result

   bCommand.CommandType = CommandType.Text
   bCommand.CommandText = "SELECT * FROM aTABLE";
   SqlDataReader rdr = bCommand.ExecuteReader();
   

You have to construct new Command object to erase previously set parameters and value.

aCommand=new SqlCommand();
aCommand.Connection=cn;
aCommand.CommandText = "SELECT * FROM aTABLE";
SqlDataReader reader=aCommand.ExecuteReader();

Call ExecuteReader() method instead of ExecuteNonQuery to fetch the database result.