How to retrieve selected number of rows using Linq [duplicate]

The batch processing can be implemented via Linq as follows.

int BatchSize = 1000; IEnumerable<string> Tail = DeviceIds; while (Tail.Count() > 0) { var Head = Tail.Take(BatchSize); // process Head Tail = Tail.Skip(BatchSize); } 

This has already been answered on: Create batches in linq:

public static class MyExtensions { public static IEnumerable<IEnumerable<T>> Batch<T>(this IEnumerable<T> items, int maxItems) { return items.Select((item, inx) => new { item, inx }) .GroupBy(x => x.inx / maxItems) .Select(g => g.Select(x => x.item)); } } 

and the usage would be:

List<int> list = new List<int>() { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; foreach(var batch in list.Batch(3)) { Console.WriteLine(String.Join(",",batch)); } 

OUTPUT:

0,1,2

3,4,5

6,7,8

9

You can use Select method overload which provides element's index to result selector. Then simple grouping by index/batchSize will split all your items into batches of required size:

int batchSize = 1000; var batches = FirebaseNotificationList .Select((fn,i) => new { fn.DeviceID, i }) .GroupBy(x => x.i / batchSize) .Select(g => g.Select(x => x.DeviceID).ToArray()); 

You can also use MoreLINQ Batch to avoid creating anonymous objects which hold information about their index in source list

var batches = FirebaseNotificationList .Select(fn => fn.DeviceID) .Batch(batchSize); // note that batches will be IEnumerable<T> 

If I use below code it will take only first 1000 rows:

string[] deviceIds = FirebaseNotificationList .Select(x => x.DeviceID).Take(1000).ToArray(); 

Yes you will. You can use the overload of Select to index the results. For example, if you have 3345 records, then divide that by your batch size (1000) and you will end up with batches 0, 1, 2, 3. Here is the code with explanation to follow:

int batchSize = 1000; string[] deviceIds = FirebaseNotificationList .Select((x, itemNumber) => new { x.DeviceID, ItemNumber = itemNumber).GroupBy(x => x.ItemNumber / batchSize) .Select(g => g.Select(x => x.DeviceID).ToArray()); 

Explanation:

ItemNumber is an index given to the records. So records with ItemNumber less than 1000, when divided by 1000, will end up with 0 (integer division) and for records with ItemNumber more than 1000 and less than 1999 will end up with 1 and so on. Therefore, you will end up with groups of records using the GroupBy that range from 0 to x, where x will be the number of the last batch. In other words, each group will be contain records for each batch. In the last Select you are selecting records from each group (batch).