If we do a .ToString() on a Linq expressiontree, we get a reprensentation of the actual code. Similarly is there a way to get the actual linq expression if you have access to an implementation of IQueryable?
In other words suppose I have this
var cars = new List<Car> { new Car { name = "Ford" }, new Car { name = "Merc"}, new Car { name = "Toyota" } };
var myCar = cars.Where(c => c.name =="Ford").AsQueryable();
How do I do this
var originalQuery=myCar.ToString() // I expect cars.Where(c => c.name =="Ford")
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 am using Dynamic Linq helper for grouping data. My code is as follows :
Employee[] empList = new Employee[6];
empList[0] = new Employee() { Name = "CA", State = "A", Department = "xyz" };
empList[1] = new Employee() { Name = "ZP", State = "B", Department = "xyz" };
empList[2] = new Employee() { Name = "AC", State = "B", Department = "xyz" };
empList[3] = new Employee() { Name = "AA", State = "A", Department = "xyz" };
empList[4] = new Employee() { Name = "A2", State = "A", Department = "pqr" };
empList[5] = new Employee() { Name = "BA", State = "B", Department = "pqr" };
var empqueryable = empList.AsQueryable();
var dynamiclinqquery = DynamicQueryable.GroupBy(empqueryable, "new (State, Department)", "it");
How can I get back the Key and corresponding list of grouped items i.e IEnumerable of {Key, List} from dynamiclinqquery ?
I solved the problem by defining a selector that projects the Key as well as Employees List.
var eq = empqueryable.GroupBy("new (State, Department)", "it").Select("new(it.Key as Key, it as Employees)");
var keyEmplist = (from dynamic dat in eq select dat).ToList();
foreach (var group in keyEmplist)
{
var key = group.Key;
var elist = group.Employees;
foreach (var emp in elist)
{
}
}
The GroupBy method should still return something that implements IEnumerable<IGrouping<TKey, TElement>>.
While you might not be able to actually cast it (I'm assuming it's dynamic), you can certainly still make calls on it, like so:
foreach (var group in dynamiclinqquery)
{
// Print out the key.
Console.WriteLine("Key: {0}", group.Key);
// Write the items.
foreach (var item in group)
{
Console.WriteLine("Item: {0}", item);
}
}
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?
Assume I have generic list L of some type in c#. Then, using linq, call OrderBy() on it, passing in a lambda expression.
If I then re-assign the L, the previous order operation will obviously be lost.
Is there any way I can 'save' the lambda expression I used on the list before i reassigned it, and re-apply it?
Use a Func delegate to store your ordering then pass that to the OrderBy method:
Func<int, int> orderFunc = i => i; // func for ordering
var list = Enumerable.Range(1,10).OrderByDescending(i => i); // 10, 9 ... 1
var newList = list.OrderBy(orderFunc); // 1, 2 ... 10
As another example consider a Person class:
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
}
Now you want to preserve a sort order that sorts by the Name property. In this case the Func operates on a Person type (T) and the TResult will be a string since Name is a string and is what you are sorting by.
Func<Person, string> nameOrder = p => p.Name;
var list = new List<Person>
{
new Person { Id = 1, Name = "ABC" },
new Person { Id = 2, Name = "DEF" },
new Person { Id = 3, Name = "GHI" },
};
// descending order by name
foreach (var p in list.OrderByDescending(nameOrder))
Console.WriteLine(p.Id + ":" + p.Name);
// 3:GHI
// 2:DEF
// 1:ABC
// re-assinging the list
list = new List<Person>
{
new Person { Id = 23, Name = "Foo" },
new Person { Id = 14, Name = "Buzz" },
new Person { Id = 50, Name = "Bar" },
};
// reusing the order function (ascending by name in this case)
foreach (var p in list.OrderBy(nameOrder))
Console.WriteLine(p.Id + ":" + p.Name);
// 50:Bar
// 14:Buzz
// 23:Foo
EDIT: be sure to add ToList() after the OrderBy calls if you need a List<T> since the LINQ methods will return an IEnumerable<T>.
Calling ToList() or ToArray() on your IEnumerable<T> will cause it to be immediately evaluated. You can then assign the resulting list or array to "save" your ordered list.
Is there an equivalent of a SQL IN statement in LINQ to objects?
Yes - Contains.
var desiredNames = new[] { "Jon", "Marc" };
var people = new[]
{
new { FirstName="Jon", Surname="Skeet" },
new { FirstName="Marc", Surname="Gravell" },
new { FirstName="Jeff", Surname="Atwood" }
};
var matches = people.Where(person => desiredNames.Contains(person.FirstName));
foreach (var person in matches)
{
Console.WriteLine(person);
}
(In LINQ to SQL this ends up as an "IN" query.)
Note that in LINQ to Objects the above isn't really very efficient. You'd be better off with a join:
var matches = from person in people
join name in desiredNames on person.FirstName equals name
select person;
(This could still be done with dot notation of course, but it ends up being somewhat messier.)
I will go for Inner Join in this context. If I would have used contains, it would iterate 6 times despite if the fact that there are just two matches. I just want to emphasize here that I will go for Joins instead of IN predicate.
var desiredNames = new[] { "Pankaj" };
var people = new[]
{
new { FirstName="Pankaj", Surname="Garg" },
new { FirstName="Marc", Surname="Gravell" },
new { FirstName="Jeff", Surname="Atwood" }
};
var records = (from p in people join filtered in desiredNames on p.FirstName equals filtered select p.FirstName).ToList();