I have a .Net Core 2.2 web app that uses Entity Framework.
In the project, I have this controller that uses Linq to join two tables, and then uses Select to create a node model, like this:
[HttpGet("GameDownloadLinks/{libraryId}")]
public async Task<ActionResult<IEnumerable<GameDownloadLinks>>> GetGameLinksForlibraryAsync(Guid libraryID)
{
var libraryGameLinks = (from gk in _context.GameLinks
join gl in _context.GameList on gk.GameId equals gl.GameId
where gl.libraryId == libraryId
select new GameDownloadLinks
{
LibraryId = gl.libraryId,
LinkText = gk.LinkText,
Price = gk.Price,
GameId = gl.GameId
}).ToList();
var asyncGameDownloadLinks = await Task.WhenAll(libraryGameLinks).toListAsync();
return asyncGameDownloadLinks;
}
I'm trying to make this method asynchronous, but I get this error:
cannot convert from 'System.Collections.Generic.List<Models.GameDownloadLinks>' to 'System.Collections.Generic.IEnumerable<System.Threading.Tasks.Task>'
Some of the answers I found for similar questions indicated this is most likely due to a missing async keyword in the method declaration, but I have included that in mine.
Is there anyway to make this work?
Thanks!
try this
[HttpGet("GameDownloadLinks/{libraryId}")]
public async Task<ActionResult<IEnumerable<GameDownloadLinks>>> GetGameLinksForlibraryAsync(Guid libraryID)
{
var libraryGameLinks = await (from gk in _context.GameLinks
join gl in _context.GameList on gk.GameId equals gl.GameId
where gl.libraryId == libraryId
select new GameDownloadLinks
{
LibraryId = gl.libraryId,
LinkText = gk.LinkText,
Price = gk.Price,
GameId = gl.GameId
}).ToListAsync();
// var asyncGameDownloadLinks = await Task.WhenAll(libraryGameLinks).toListAsync();
return libraryGameLinks ;
}
Related
I want to add a dummy member to an IQueryable and came up with this solution:
IQueryable<Geography> geographies = _unitOfWork.GeographyRepository.GetAll(); //DbSet<Geography>
var dummyGeographies = new Geography[] { new Geography { Id = -1, Name = "All" } }.AsQueryable();
var combinedGeographies = geographies.Union(dummyGeographies);
var test = combinedGeographies.ToList(); //throws runtime exc
But it throws the following exception:
Processing of the LINQ expression 'DbSet
.Union(EnumerableQuery { Geography, })' by 'NavigationExpandingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core.
How could I make it work?!
you can only union on data structure which are the same
IQueryable is only applicable if the query expression not been been expressed (ToList) before its run against db and you want the expression modifiable . aka nothing which which is not going to db as a query needs to be IQueryable (simple explanation better to research and understand this yourself)
List<Geography> geographies = _unitOfWork.GeographyRepository
.GetAll() //DbSet<Geography>
.Select(o => new Geography { Id = o.Id, Name = o.Name })
.ToList();
List<Geography> dummyGeographies = new List<Geography>() {
new Geography[] { new Geography { Id = -1, Name = "All" } }
};
var combinedGeographies = geographies.Union(dummyGeographies);
var test = combinedGeographies.ToList();
I was able to achieve it with the following code:
IQueryable<Geography> geographies = _unitOfWork.GeographyRepository.GetAll().Select(o => new Geography { Id = o.Id, Name = o.Name });
IQueryable<Geography> dummyGeographies = _unitOfWork.GeographyRepository.GetAll().Select(o => new Geography { Id = -1, Name = "All" });
var combinedGeographies = geographies.Union(dummyGeographies);
I have a Document DB repository class that has one get method like below:
private static DocumentClient client;
public async Task<TEntity> Get(string id, string partitionKey = null)
{
try
{
RequestOptions requestOptions = null;
if (partitionKey != null)
{
requestOptions = new RequestOptions { PartitionKey = new PartitionKey(partitionKey) };
}
var result = await client.ReadDocumentAsync(
UriFactory.CreateDocumentUri(DatabaseId, CollectionId, id),
requestOptions);
return (TEntity)(dynamic)result.Resource;
}
catch (DocumentClientException e)
{
// Have logic for different exceptions actually
throw;
}
}
I have two collections - Collection1 and Collection2. Collection1 is non-partitioned whereas Collection2 is partitioned.
On the client side, I create two repository objects, one for each collection.
private static DocumentDBRepository<Collection1Item> collection1Repository = new DocumentDBRepository<Collection1Item>("Collection1");
private static DocumentDBRepository<Collection2Item> collection2Repository = new DocumentDBRepository<Collection2Item>("Collection2");
List<Collection1Item> collection1Items = await collection1Repository.GetItemsFromCollection1(); // Selects first forty documents based on time
List<UIItem> uiItems = new List<UIItem>();
foreach (var item in collection1Items)
{
var collection2Item = await storageRepository.Get(item.Collection2Reference, item.TargetId); // TargetId is my partition key for Collection2
uiItems.Add(new UIItem
{
ItemId = item.ItemId,
Collection1Reference = item.Id,
TargetId = item.TargetId,
Collection2Reference = item.Collection2Reference,
Value = collection2Item.Value
});
}
This works fine. But since it is happening sequentially with foreach, I wanted to do those Get calls in parallel. When I do it in parallel as below:
ConcurrentBag<UIItem> uiItems = new ConcurrentBag<UIItem>();
collection1Items.AsParallel().ForAll(async item => {
var collection2Item = await storageRepository.Get(item.Collection2Reference, item.TargetId); // TargetId is my partition key for Collection2
uiItems.Add(new UIItem
{
ItemId = item.ItemId,
Collection1Reference = item.Id,
TargetId = item.TargetId,
Collection2Reference = item.Collection2Reference,
Value = collection2Item.Value
});
}
);
It doesn't work and uiItems is always empty.
You don't need Parallel.For to run async operations concurrently. If they are truly asynchronous they already run concurrently.
You could collect the task returned from each operation and simply call await Task.WhenAll() on all the tasks. If you modify your lambda to create and return a UIItem, the result of await Task.WhenAll() will be a collection of UIItems. No need to modify global state from inside the concurrent operations.
For example:
var itemTasks = collection1Items.Select(async item =>
{
var collection2Item = await storageRepository.Get(item.Collection2Reference, item.TargetId);
return new UIItem
{
ItemId = item.ItemId,
Collection1Reference = item.Id,
TargetId = item.TargetId,
Collection2Reference = item.Collection2Reference,
Value = collection2Item.Value
}
});
var results= await Task.WhenAll(itemTasks);
A word of caution though - this will fire all Get operations concurrently. That may not be what you want, especially when calling a service with rate limiting.
Try simply starting tasks and waiting for all of them at the end. That would result in parallel execution.
var tasks = collection1Items.Select(async item =>
{
//var collection2Item = await storageRepository.Get...
return new UIItem
{
//...
};
});
var uiItems = await Task.WhenAll(tasks);
PLINQ is useful when working with in-memory constructs and using as many threads as possible, but if used with the async-await technique (which is for releasing threads while accessing external resources), you can end up with strange results.
I would like to share a solution for an issue i saw in some comments.
If you're scared about thread rate limit, and you want to limit this by yourself, you can do something like this, using SemaphoreSlim.
var nbCores = Environment.ProcessorCount;
var semaphore = new SemaphoreSlim(nbCores, nbCores);
var processTasks = items.Select(async x =>
{
await semaphore.WaitAsync();
try
{
await ProcessAsync();
}
finally
{
semaphore.Release();
}
});
await Task.WhenAll(processTasks);
In this example, i called concurrently my "ProcessAsync" but limited to {processor number} concurrent processes.
Hope that's help someone.
NB : You could set the "nbCores" variable as a proper value that satisfy your code condition, of course.
NB 2 : This example is for some use cases, not all of them. I would highly suggest with a big load of task to refer to TPL programming
Hi im too new on entity framework and im using it on windows form app.
using (GezentiEntities GE = new GezentiEntities())
{
var cities = from c in GE.Cities
where c.CountryId == ((Guid)(dgCountry.SelectedRows[0].Cells[0].Value))
select new { c.Id, Şehir = c.Name };
dgCity.DataSource = cities.ToList();
}
on dgCity.DataSource = cities.ToList(); line it gives me error when im using it with where condition and ((Guid)(dgCountry.SelectedRows[0].Cells[0].Value)) that code works fine it gives me ID.
you may try to get the needed ID out of the linq to entities query
var id = (Guid)dgCountry.SelectedRows[0].Cells[0].Value;
var cities = from c in GE.Cities
where c.CountryId == id
select new { c.Id, Şehir = c.Name };
dgCity.DataSource = cities.ToList();
Continued from this solution (thanks Daniel Hilgarth)
return db.Tags.Select(ConstructTagItem());
And the method:
private Expression<Func<Tag, TagItem>> ConstructTagItem()
{
return a => new TagItem {ID = a.Id Name = a.Name };
}
Additional question, how do i use it in this scenario then:
return (from c in db.News_Attributes
select new NewsTagItem
{
NewsID = c.News_Id,
TagID = c.Tag_Id,
Content = c.Content,
Attribute = new TagItem
{
ID = c.Tag.Id,
Name = c.Tag.Name
}
});
I want to reuse the method from the other answer:
private Expression<Func<Tag, TagItem>> ConstructTagItem
{
get { return a => new TagItem {ID = a.Id Name = a.Name }; }
}
To construct something like this:
return (from c in db.News_Attributes
select new NewsTagItem
{
NewsID = c.News_Id,
TagID = c.Tag_Id,
Content = c.Content,
Attribute = ConstructTagItem // TODO: need some way to tell the method that it should use c.Tag
});
I want to use the same construction of my TagItem multiple places. This will make it easier if the object changes, and save lines.
I guess that I somehow have to define that it is c.Tag into ConstructTagItem(), but I really don't know much about expressions yet. So i hope that someone is able to help?
I'm not sure if I have a full handle on what you're trying to do. What does "use it in this scenario" mean? Can you mimic your previous technique with something like this in order to encapsulate creating a NewsTagItem, or is it something else you're trying to achieve?
private Expression<Func<News_Attribute, NewsTagItem>> ConstructNewsTagItem()
{
return c => new NewsTagItem
{
NewsID = c.News_Id,
Name = a.Name
TagID = c.Tag_Id,
Content = c.Content,
Attribute = new TagItem
{
ID = c.Tag.Id,
Name = c.Tag.Name
}
}
});
db.News_Attributes.Select(ConstructNewsTagItem());
UPDATE:
OK, we can't directly re-use your ConstructTagItem() because it returns an expression containing a function. What you need is a MemberInitExpression. It's a little tricky to create by hand, but we can use a trick whereby we create the expression we desire wrapped with a thunk, so that it isn't evaluated, and then grab the body of the thunk to get the expression. See the snippet below:
private Expression GenerateNewTagItem(TagItem c)
{
Expression<Func<TagItem>> expr = () => new TagItem { ID = c.ID, Name = c.Name };
return expr.Body;
}
With this function, we can now do pretty much exactly what you want:
return (from c in db.News_Attributes
select new NewsTagItem
{
NewsID = c.News_Id,
TagID = c.Tag_Id,
Content = c.Content,
Attribute = GenerateNewTagItem(c)
});
Pretty neat right?
I am having trouble to return an IEnumerable and IList, I cant do it!
I am using EF 4 with POCOs
Here's the whole method:
//public IList<Genre> GetGenresByGame(int gameId)
public IEnumerable<Genre> GetGenresByGame(int gameId)
{
using(var ctx = new XContext())
{
var results =
from t0 in ctx.GameGenres
join t1 in ctx.GenreCultureDetails on t0.GenreId equals t1.GenreId
where t0.GameId == gameId && t1.CultureId == _cultureId
select new Genre
{
GenreId = t0.GenreId,
GenreName = t1.GenreName
};
return results.ToList();
}
}
I have tried different ways that I have found on the net.. but can't make it work!
Question 2:
I saw a screencast with Julie something, saying that "You should always return an ICollection" when using EF4.
Any thoughts about that ?
BR
EDIT:
When I load the page in Debug-mode i get these errors: The ObjectContext instance has been disposed and can no longer be used for operations that require a connection. OR The entity or complex type 'XModel.Genre' cannot be constructed in a LINQ to Entities query.
Genre must not be a L2EF type. Try this:
public IEnumerable<Genre> GetGenresByGame(int gameId)
{
using(var ctx = new XContext())
{
var resultList =
from t0 in ctx.GameGenres
join t1 in ctx.GenreCultureDetails on t0.GenreId equals t1.GenreId
where t0.GameId == gameId && t1.CultureId == _cultureId
select new { t0.GenreId, t1.GenreName };
var genres = resultList.AsEnumerable().Select(o => new Genre
{
GenreId = o.GenreId,
GenreName = o.GenreName
});
return genres.ToList();
}
}
First an foremost if Genre is in the database you should select it? If you have FKs from Genre->GenreCultureDetails let me know and I can update the below, but from the looks of it you could do it like this:
using(var ctx = new XContext())
{
var results =
from g in ctx.Genre
join gcd in ctx.GenreCultureDetails on g.GenreId equals gcd.GenreId
where g.GameId == gameId && gcd.CultureId == _cultureId
select g;
return result.ToList();
}
Alternatively continue down your path select them into an annoynmous type, and then copy them. You can use select instead of convertall if you please.
IList<Genre> returnMe = Null;
using(var ctx = new XContext())
{
var results =
from t0 in ctx.GameGenres
join t1 in ctx.GenreCultureDetails on t0.GenreId equals t1.GenreId
where t0.GameId == gameId && t1.CultureId == _cultureId
select new
{
GenreId = t0.GenreId,
GenreName = t1.GenreName
};
returnMe = results.ToList().ConvertAll(x=>new Genre(){
GenreId = x.GenreId,
GenreName = x.GenreName
}
);
}
return returnMe;