Dfyjkt

I'm trying to count total data that grouped in my list.

So here is my code:

List<InventoryViewModel> temp =
 InventoriesByLocation.GroupBy(s => s.ProductId).Select(group => group.First()).ToList();

I want to count how many productId that already groupby().

So you have a sequence of InventoryViewModels, where every InventoryViewModel has a ProductId.

You want to group all InventoryViewModels into groups, where every groups contains all InventoryViewModels with the same ProductId. After that you want to count the number of InventoryViewModels in each group, and possibly also some other items. Your example code takes the first element in the group.

var groupsWithSameProductId = inventoryViewModels
 .GroupBy(inventoryViewModel => inventoryViewModel.ProductId)

Now every group has a Key, which contains the ProductId that is the same for all elements in your group. The group itself is a sequence of all InventoryViewModels that have the ProductId equal to the Key.

To get the number of InventoryViewModels that have this ProductId, just Count() them. If you want other items, just Select them:

// continue the LINQ statement
.Select(group => new

 ProductId = group.Key,
 NrOfInventroyViewModelsWithThisProductId = group.Count(),

 // only if you want other items:
 FirstInventoryViewModel = group.First(),
 Names = group.Select(groupItem => groupItem.Name).ToList(),
);

Note, that every groupItem is an InventoryViewModel with ProductId equal to group.Key

There is an overload of Enumerable.GroupBy that combines the grouping and selecting in one statement:

var result = inventoryViewModels
 .GroupBy(inventoryViewModel => inventoryViewModel.ProductId, // make groups with same productId
 (commonProductId, inventoryViewModelsWithThisCommonProductId) => new
 
 ProductId = commonProductId,
 Count = inventoryViewModelsWithThisCommonProductId.Count(),
 Names = inventoryViewModelsWithThisCommonProductId
 .Select(inventoryViewModelWithThisCommonProductId.Name)
 .ToList(),
 );

Something like this:

var count = InventoriesByLocation.GroupBy(s => s.ProductId).Count();

Basically, GroupBy returns a type roughly equivalent to IDictionary<int, InventoryViewModel> in your case. So, length of an individual dictionary entry will be the count of a unique ProductId.