使用LINQ将一行变成两个对象吗？

Question

I have a single item array that I compare to an updated item array to determine the distinct list of changed columns. It's working.

But I need to build two objects out of that one row because I need to send two objects to the API I'm using to send the email. One for the old value and one for the new value. I've tried it two different ways:

var updates = currentRow.ItemArray
    .Select((o, i) => new { Row = o, Index = i })
    .Where(r => (r.Row == null && updatedRow[r.Index] != null)
            || (r.Row != null && updatedRow[r.Index] != null
            && !r.Row.Equals(updatedRow[r.Index])))
        .Select(r => new AppServices.NotificationData[]
        {
            new AppServices.NotificationData
            {
                Key = string.Format("{0}OldValue",
                    columns[r.Index].ColumnName.EmailTemplateName(type)),
                Value = Convert.ToString(currentRow[r.Index])
            },
            new AppServices.NotificationData
            {
                Key = string.Format("{0}NewValue",
                    columns[r.Index].ColumnName.EmailTemplateName(type)),
                Value = Convert.ToString(updatedRow[r.Index])
            }                        
        });

but this one actually, honestly as expected, gives me an enumerable of arrays. I've also tried it like this:

var updates = currentRow.ItemArray
    .Select((o, i) => new { Row = o, Index = i })
    .Where(r => (r.Row == null && updatedRow[r.Index] != null)
            || (r.Row != null && updatedRow[r.Index] != null
            && !r.Row.Equals(updatedRow[r.Index])))
        .Select(r => new AppServices.NotificationData
        {
            Key = string.Format("{0}OldValue",
                columns[r.Index].ColumnName.EmailTemplateName(type)),
            Value = Convert.ToString(currentRow[r.Index])
        })
        .Select(r => new AppServices.NotificationData
        {
            Key = string.Format("{0}NewValue",
                columns[r.Index].ColumnName.EmailTemplateName(type)),
            Value = Convert.ToString(updatedRow[r.Index])
        });

but the problem here is that the second Select passes in the NotificationData object from the previous select, so r isn't actually the item array.

Here currentRow and updatedRow are DataRow objects representing the old and new data respectively.

I'm sure there's a way to get what I'm after, I just don't know how.

So, how do I get a Select , which is built to return a single type, to return two instances of the same object so that the enumerable is ultimately IEnumerable<NotificationData> ?

Answer 1

Use SelectMany instead of Select in the first query:

var updates = currentRow.ItemArray
    .Select((o, i) => new { Row = o, Index = i })
    .Where(r => (r.Row == null && updatedRow[r.Index] != null)
            || (r.Row != null && updatedRow[r.Index] != null
            && !r.Row.Equals(updatedRow[r.Index])))
        .SelectMany(r => new AppServices.NotificationData[]
        {
            new AppServices.NotificationData
            {
                Key = string.Format("{0}OldValue",
                    columns[r.Index].ColumnName.EmailTemplateName(type)),
                Value = Convert.ToString(currentRow[r.Index])
            },
            new AppServices.NotificationData
            {
                Key = string.Format("{0}NewValue",
                    columns[r.Index].ColumnName.EmailTemplateName(type)),
                Value = Convert.ToString(updatedRow[r.Index])
            }                        
        });

It will flatten your IEnumerable<IEnumerable<T>> into IEnumerable<T> .

Answer 2

If what you want is to have two objects created per row , and then be able to send these two objects together to your API, then I think that the best way would be to use an anonymous type.

var updates = currentRow.ItemArray
    .Select((o, i) => new { Row = o, Index = i })
    .Where(r => (r.Row == null && updatedRow[r.Index] != null)
            || (r.Row != null && updatedRow[r.Index] != null
            && !r.Row.Equals(updatedRow[r.Index])))
        .Select(r => new {
            NotificationForServiceA = new AppServices.NotificationData({
                Key = string.Format("{0}OldValue",
                    columns[r.Index].ColumnName.EmailTemplateName(type)),
                Value = Convert.ToString(currentRow[r.Index])
            }),
            NotificationForServiceB = new AppServices.NotificationData({
                Key = string.Format("{0}NewValue",
                    columns[r.Index].ColumnName.EmailTemplateName(type)),
                Value = Convert.ToString(updatedRow[r.Index])
            })
        });

Then you could use it that way:

var oneItem = updates.FirstOrDefault();
myApi(oneItem.NotificationForServiceA, oneItem.NotificationForServiceB);

If what you want is to create two NotificationData per row and have them merged/flattened in one enumerable then you should use SelectMany to perform that. See MarcinJuraszek answer to do that.