Get the id of inserted row using C#

MySqlCommand comm = connect.CreateCommand();
comm.CommandText = insertStatement;  // Set the insert statement
comm.ExecuteNonQuery();              // Execute the command
long id = comm.LastInsertedId;       // Get the ID of the inserted item

[Edit: added "select" before references to last_insert_id()]

What about running "select last_insert_id();" after your insert?

MySqlCommand comm = connect.CreateCommand();
comm.CommandText = insertInvoice;
comm.CommandText += "\'" + invoiceDate.ToString("yyyy:MM:dd hh:mm:ss") + "\', "  
    + bookFee + ", " + adminFee + ", " + totalFee + ", " + customerID +  ");";
    + "select last_insert_id();"

int id = Convert.ToInt32(comm.ExecuteScalar());

Edit: As duffymo mentioned, you really would be well served using parameterized queries like this.

Edit: Until you switch over to a parameterized version, you might find peace with string.Format:

comm.CommandText = string.Format("{0} '{1}', {2}, {3}, {4}, {5}); select last_insert_id();",
  insertInvoice, invoiceDate.ToString(...), bookFee, adminFee, totalFee, customerID);

Use LastInsertedId.

View my suggestion with example here: http://livshitz.wordpress.com/2011/10/28/returning-last-inserted-id-in-c-using-mysql-db-provider/