Question

I have the following Linq query. transactionData is an IEnumerable.

var totalTransactions = 0;
viewModel.GroupedTransactions = transactionData
    .GroupBy(x => new { DocumentId = x.DocumentId ?? "Un Documented" })
    .Select(x => new GroupedTransaction
    {
        DocumentId = x.Key.DocumentId,
        Transactions = x.Select(y => new Transaction
        {
            Amount = y.CommitAmount,
            ActivityType = y.ActivityType,
            Number = totalTransactions++
        })
    })
    .OrderBy(x => x.DocumentId);

where I’m trying to set the Number on the Transaction record to be an incremented number.
This doesn’t work, leaving gaps in the numbers.

I also tried the following after the query.

foreach (var item in viewModel.GroupedTransactions.SelectMany(x => x.Transactions))
{
     item.Number = totalTransactions;
     totalTransactions++;
}

This didn’t even update the Number value.
What am I doing wrong, or is there a simpler way, with a neat linq extension method?

Answer 1

The problem is that you are closing over the variable totalTransactions, you have to create a local copy to use. Check Closing over the loop variable considered harmful for a more detailed explanation.

Something like this should work:

var totalTransactions = 0;
viewModel.GroupedTransactions = transactionData
    .GroupBy(x => new { DocumentId = x.DocumentId ?? "Un Documented" })
    .Select(x => 
    {
      new GroupedTransaction()
      {
        DocumentId = x.Key.DocumentId,
        Transactions = x.Select(y => 
        {
          var currentTransactionId = totalTransactions;
          totalTransactions++;

          return new Transaction
          {
            Amount = y.CommitAmount,
            ActivityType = y.ActivityType,
            Number = currentTransactionId 
          }
        })
      }
    })
    .OrderBy(x => x.DocumentId);

For your second approach with the foreach loop – you are actually creating a new enumeration with SelectMany() that you subsequently just throw away:

foreach (var item in viewModel.GroupedTransactions.SelectMany(x => x.Transactions))
{
     item.Number = totalTransactions;
     totalTransactions++;
}

Instead you have to force eager evaluation of your collection by using ToList() to create a collection you can safely modify.

var transactions = viewModel.GroupedTransactions
                            .SelectMany(x => x.Transactions)
                            .ToList();
foreach (var item in transactions)
{
     item.Number = totalTransactions;
     totalTransactions++;
}