Does anyone have any idea how I call a lambda expression from within a lambda expression?
If I have:
public class CourseViewModel
{
public int Id { get; set; }
public string Name { get; set; }
public static Expression<Func<Course, CourseViewModel>> AsViewModel = x =>
new CourseViewModel
{
Id = x.Id,
Name = x.Name,
}
}
public class StudentViewModel
{
public int Id { get; set; }
public string Name{ get; set; }
public string PreferredCheese { get; set; }
public IEnumerable<CourseViewModel> Courses { get; set; }
public static Expression<Func<Student, StudentViewModel>> AsViewModel = x =>
new StudentViewModel
{
Id = x.Id,
Name = x.Name,
PreferredCheese = x.PreferredCheese,
Courses = ???I'd like to call the CourseViewModel.AsViewModel here
}
}
In the code above rather than writing the AsViewModel expression within the StudentViewModel as:
Courses = new CourseViewModel
{
Id = x.Id,
Name = x.Name,
}
I'd like to call CourseViewModel.AsViewModel to allow code re-use and to keep the code converting a Course to a CourseViewModel in the CourseViewModel class. Is this possible?
You can just use x.Courses.Select(c => CourseViewModel.AsViewModel(c)) So your whole expression would be:
public static Expression<Func<Student, StudentViewModel>> AsViewModel = x =>
new StudentViewModel
{
Id = x.Id,
Name = x.Name,
PreferredCheese = x.PreferredCheese,
Courses = x.Courses.Select(c => CourseViewModel.AsViewModel(c))
}
If you want to preserve CourseViewModel.AsViewModel as a nested expression (which you probably do if you're using a LINQ query provider), this gets tricky; you effectively have to build up the AsViewModel expression in StudentViewModel by yourself:
using E = System.Linq.Expressions.Expression; // for brevity
// ...
static StudentViewModel()
{
var s = E.Parameter(typeof(Student), "s");
//
// Quick hack to resolve the generic `Enumerable.Select()` extension method
// with the correct type arguments.
//
var selectMethod = (Expression<Func<Student, IEnumerable<CourseViewModel>>>)
(_ => _.Courses.Select(c => default(CourseViewModel)));
var lambda = E.Lambda<Func<Student, StudentViewModel>>(
E.MemberInit(
E.New(typeof(StudentViewModel)),
E.Bind(
typeof(StudentViewModel).GetProperty("Id"),
E.Property(s, "Id")),
E.Bind(
typeof(StudentViewModel).GetProperty("Name"),
E.Property(s, "Name")),
E.Bind(
typeof(StudentViewModel).GetProperty("PreferredCheese"),
E.Property(s, "PreferredCheese")), // LOL?
E.Bind(
typeof(StudentViewModel).GetProperty("Courses"),
E.Call(
((MethodCallExpression)selectMethod.Body).Method,
E.Property(s, "Courses"),
CourseViewModel.AsViewModel))
),
s);
AsViewModel = lambda;
}
The resulting expression tree is equivalent to:
s => new StudentViewModel {
Id = s.Id,
Name = s.Name,
PreferredCheese = s.PreferredCheese,
Courses = s.Courses.Select(x => new CourseViewModel { Id = x.Id, Name = x.Name })
}
Related
I am looking for a way of optimizing my LINQ query.
Classes:
public class OffersObject
{
public List<SingleFlight> Flights { get; set; }
public List<Offer> Offers { get; set; } = new List<Offer>();
}
public class SingleFlight
{
public int Id { get; set; }
public string CarrierCode { get; set; }
public string FlightNumber { get; set; }
}
public class Offer
{
public int ProfileId { get; set; }
public List<ExtraOffer> ExtraOffers { get; set; } = new List<ExtraOffer>();
}
public class ExtraOffer
{
public List<int> Flights { get; set; }
public string Name { get; set; }
}
Sample object:
var sampleObject = new OffersObject
{
Flights = new List<SingleFlight>
{
new SingleFlight
{
Id = 1,
CarrierCode = "KL",
FlightNumber = "1"
},
new SingleFlight
{
Id = 2,
CarrierCode = "KL",
FlightNumber = "2"
}
},
Offers = new List<Offer>
{
new Offer
{
ProfileId = 41,
ExtraOffers = new List<ExtraOffer>
{
new ExtraOffer
{
Flights = new List<int>{1},
Name = "TEST"
},
new ExtraOffer
{
Flights = new List<int>{2},
Name = "TEST"
},
new ExtraOffer
{
Flights = new List<int>{1,2},
Name = "TEST"
}
}
}
}
};
Goal of LINQ query:
List of:
{ int ProfileId, string CommercialName, List<string> fullFlightNumbers }
FullFlightNumber should by created by "Id association" of a flight. It is created like: {CarrierCode} {FlightNumber}
What I have so far (works correctly, but not the fastest way I guess):
var result = sampleObject.Offers
.SelectMany(x => x.ExtraOffers,
(a, b) => {
return new
{
ProfileId = a.ProfileId,
Name = b.Name,
FullFlightNumbers = b.Flights.Select(f => $"{sampleObject.Flights.FirstOrDefault(fl => fl.Id == f).CarrierCode} {sampleObject.Flights.First(fl => fl.Id == f).FlightNumber}").ToList()
};
})
.ToList();
Final note
The part that looks wrong to me is:
.Select(f => $"{sampleObject.Flights.FirstOrDefault(fl => fl.Id == f)?.CarrierCode} {sampleObject.Flights.FirstOrDefault(fl => fl.Id == f)?.FlightNumber}").ToList()
I am basically looking for a way of "joining" those two lists of the OffersObject by Flight's Id.
Any tips appreciated.
If there will only be a few flights defined in sampleObject.Flights, a sequential search using a numeric key is hard to beat.
However, if the number of flights times the number of offers is substantial (1000s or more), I would suggest loading the list of flights into a dictionary with Id as the key for efficient lookup. Something like:
var flightLookup = sampleObject.Flights.ToDictionary(f => f.Id);
And then calculate your FullFlightNumbers as
FullFlightNumbers = b.Flights
.Select(flightId => {
flightLookup.TryGetValue(flightId, out SingleFlight flight);
return $"{flight?.CarrierCode} {flight?.FlightNumber}";
})
.ToList()
TryGetValue above will quietly return a null value for flight if no match is found. If you know that a match will always be present, the lookup cold alternately be coded as:
SingleFlight flight = flightLookup[flightId];
The above also uses a statement lambda. In short, lambda functions can have either expression or statement blocks as bodies. See the C# reference for more information.
I'd suggest replacing the double .FirstOrDefault() approach with .IntersectBy(). It is available in the System.Linq namespace, starting from .NET 6.
.IntersectBy() basically filters sampleObject.Flights by matching the flight ID for each flight in sampleObject with flight IDs in ExtraOffers.Flights.
In the code below, fl => fl.Id is the key selector for sampleObject.Flights (i.e. fl is a SingleFlight).
var result = sampleObject.Offers
.SelectMany(x => x.ExtraOffers,
(a, b) => {
return new
{
ProfileId = a.ProfileId,
Name = b.Name,
FullFlightNumbers = sampleObject.Flights
.IntersectBy(b.Flights, fl => fl.Id)
.Select(fl => fl.FullFlightNumber) // alternative 1
//.Select(fl => $"{fl.CarrierCode} {fl.FlightNumber}") // alternative 2
.ToList()
};
})
.ToList();
In my suggestion I have added the property FullFlightNumber to SingleFlight so that the Linq statement looks slightly cleaner:
public class SingleFlight
{
public int Id { get; set; }
public string CarrierCode { get; set; }
public string FlightNumber { get; set; }
public string FullFlightNumber => $"{CarrierCode} {FlightNumber}";
}
If defining SingleFlight.FullFlightNumber is not possible/desirable for you, the second alternative in the code suggestion can be used instead.
Example fiddle here.
I'm trying to build a sub-query by using expression-trees. In linq I would write something like:
var single = MyTable
.AsExpandable()
.Select(c => new
{
Childs = Enumerable.Select(
MyTable.VisibleChilds.Invoke(c, dbContext),
cc => Convert(cfg.ChildsConfig).Invoke(dbContext, cc))
});
where the Convert is building an expression like
p => new MyTableSelect {
Id = p.Id,
Name = p.Name
}
depending on the given values from the config (to only read needed data from database).
but I'm struggeling with the second parameter to be passed to the Select call as I need cc to be passed to the Convert-call.
I guess I need something like Expression.Lambda<Func<>> but I don't see it.
Expression.Lambda>(Expression.Invoke(Instance.Convert(cfg.ChildOrganizersFilterConfig), ContextParameter, theEntity));
I am not familiar with your use of Invoke but if you just want to run a 'Converter' in a fluent syntax for use in a Linq Expression I could show you an example of that. Say I have three POCO classes, one parent container, a child container, and a container I want to convert to.
public class POC
{
public int Id { get; set; }
public string Name { get; set; }
public POC(int id, string name)
{
Id = id;
Name = name;
}
}
public class ChildPOC
{
public int ParentId { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public ChildPOC(int parentId, string firstName, string lastName)
{
ParentId = parentId;
FirstName = firstName;
LastName = lastName;
}
}
public class ChildPOCAlter
{
public int ParentId { get; set; }
public string Name { get; set; }
public ChildPOCAlter(string first, string last, int parentId)
{
ParentId = parentId;
Name = $"{first} {last}";
}
}
I could write a converter method to take ChildPOC to ChildPOCAlter like so:
public static Converter<ChildPOC, ChildPOCAlter> ChildPOCOAlter()
{
return new Converter<ChildPOC, ChildPOCAlter>((x) => { return new ChildPOCAlter(x.FirstName, x.LastName, x.ParentId); });
}
I could then populate some data:
var someParents = new List<POC> { new POC(1, "A"), new POC(2, "B") };
var somechildren = new List<ChildPOC> { new ChildPOC(1, "Brett", "x"), new ChildPOC(1, "Emily", "X"), new ChildPOC(2, "John", "Y") };
And then I may want to take these relationships and apply a converter directly on it:
var relationships = someParents.Select(x => new
{
Id = x.Id,
Name = x.Name,
Children = somechildren.Where(y => y.ParentId == x.Id).ToList().ConvertAll(ChildPOCOAlter())
});
How can I achieve the projection on the last select? I need the property defined by the string prop.Name to be selected into the SeriesProjection object.
public override IQueryable<SeriesProjection> FilterOn(string column)
{
//Get metadata class property by defined Attributes and parameter column
var prop = typeof(CommunicationMetaData)
.GetProperties()
.Single(p => p.GetCustomAttribute<FilterableAttribute>().ReferenceProperty == column);
var attr = ((FilterableAttribute)prop.GetCustomAttribute(typeof(FilterableAttribute)));
var param = Expression.Parameter(typeof(Communication));
Expression conversion = Expression.Convert(Expression.Property(param, attr.ReferenceProperty), typeof(int));
var condition = Expression.Lambda<Func<Communication, int>>(conversion, param); // for LINQ to SQl/Entities skip Compile() call
var result = DbQuery.Include(prop.Name)
//.GroupBy(c => c.GetType().GetProperty(attr.ReferenceProperty))
.GroupBy(condition)
.OrderByDescending(g => g.Count())
.Select(group => new SeriesProjection()
{
Count = group.Count(),
Id = group.Key,
//set this navigation property dynamically
Name = group.FirstOrDefault().GetType().GetProperty(prop.Name)
});
return result;
}
For the GroupBy I used the fk property name that's always an int on the Communication entity, but for the select I can't figure out the expression.
[EDIT]
System.Data.Entity.Infrastructure.DbQuery<Communication> DbQuery;
---
[MetadataType(typeof(CommunicationMetaData))]
public partial class Communication
{
public int CommunicationId { get; set; }
public Nullable<int> TopicId { get; set; }
public int CreateById { get; set; }
public virtual Employee CreateByEmployee { get; set; }
public virtual Topic Topic { get; set; }
}
---
public class CommunicationMetaData
{
[Filterable("By Employee", nameof(Communication.CreateById))]
public Employee CreateByEmployee { get; set; }
[Filterable("By Topic", nameof(Communication.TopicId))]
public Topic Topic { get; set; }
}
---
[AttributeUsage(AttributeTargets.Property)]
public class FilterableAttribute : System.Attribute
{
public FilterableAttribute(string friendlyName, string referenceProperty)
{
FriendlyName = friendlyName;
ReferenceProperty = referenceProperty;
}
public string FriendlyName { get; set; }
public string ReferenceProperty { get; set; }
}
---
public class SeriesProjection
{
public int Count { get; set; }
public int Id { get; set; }
public object Name { get; set; }
}
Without some expression helper library, you have to build the whole selector expression manually.
The input of the selector will be a parameter of type IGrouping<int, Communication>, the result type - SeriesProjection, and the body will be MemberInit expression:
var projectionParameter = Expression.Parameter(typeof(IGrouping<int, Communication>), "group");
var projectionType = typeof(SeriesProjection);
var projectionBody = Expression.MemberInit(
// new SeriesProjection
Expression.New(projectionType),
// {
// Count = group.Count(),
Expression.Bind(
projectionType.GetProperty(nameof(SeriesProjection.Count)),
Expression.Call(typeof(Enumerable), "Count", new[] { typeof(Communication) }, projectionParameter)),
// Id = group.Key
Expression.Bind(
projectionType.GetProperty(nameof(SeriesProjection.Id)),
Expression.Property(projectionParameter, "Key")),
// Name = group.FirstOrDefault().Property
Expression.Bind(
projectionType.GetProperty(nameof(SeriesProjection.Name)),
Expression.Property(
Expression.Call(typeof(Enumerable), "FirstOrDefault", new[] { typeof(Communication) }, projectionParameter),
prop.Name))
// }
);
var projectionSelector = Expression.Lambda<Func<IGrouping<int, Communication>, SeriesProjection>>(projectionBody, projectionParameter);
and then of course use simply:
var result = DbQuery.Include(prop.Name)
.GroupBy(condition)
.OrderByDescending(g => g.Count())
.Select(projectionSelector);
I'm trying to get a particular result set for my View to bind. I'm new to Linq expression, so I'm not very sure about the different ways of doing it.
Here is my MenuModel
public class MenuModel : DisposeBase
{
public string ParentID { get; set; }
public string ParentName { get; set; }
public List<MenuItemModel> MenuItems { get; set; }
}
My MenuItemModel
public class MenuItemModel : DisposeBase
{
public string ChildID { get; set; }
public string ChildName { get; set; }
public string PageURL { get; set; }
}
MenuModel is the output type I'm expecting as a result set. I'm getting result set of type DataTable from backend
DataTable dtable = oDatabase.ExecuteAdapter(System.Data.CommandType.StoredProcedure, "SP_GETUSERNAVMENUDATA");
Here is my SQL result set,
My DataTable will looks like this
Now I need to convert this Datatable to type MenuModel.
I tried to Query distinct MenuModel and based on that I'm building MenuItemModel object.
List<MenuModel> lstMenuModel = dtable.DataTableToList<MenuModel>()
.GroupBy(p => new { p.ParentID, p.ParentName })
.Select(g => g.First())
.ToList<MenuModel>();
foreach (MenuModel parentItem in lstMenuModel)
{
List<MenuItemModel> lstUserMenuItemData = dtable.DataTableToList<MenuItemModel>()
.Select(i => new { i.ChildID, i.ChildName, i.PageURL, i.ParentID })
.Where(i => i.ParentID.Equals(parentItem.ParentID))
.ToList<MenuItemModel>();
}
But still I'm getting conversion error while building MenuItemModel. Now I wanted to know, is there any best practice to do this same conversion of these nested class type? I'm sure there should be something simple to do so.
Any help could be appreciated. Thanks!
Note: DataTableToList is a method that will convert DataTable object to specific generic type
Its not clear what your DataTableToList<MenuModel>() method is doing or returning, but it would need to return a collection of a model that contains all 5 properties represented in the data table.
Assuming you have the following model
public class MenuSQLSet
{
public string ParentID { get; set; }
public string ParentName { get; set; }
public string ChildID { get; set; }
public string ChildName { get; set; }
public string PageURL { get; set; }
}
then your query should be
List<MenuModel> lstMenuModel = dtable.DataTableToList<MenuSQLSet>()
.GroupBy(x => new { x.ParentID, x.ParentName })
.Select(x => new MenuModel()
{
ParentID = x.Key.ParentID,
ParentName = x.Key.ParentName,
MenuItems = x.Select(y => new MenuItemModel()
{
ChildID = y.ChildID,
ChildName = y.ChildName,
PageURL = y.PageURL
}).ToList()
}).ToList();
Alternatively you can use .AsEnumerable() on the DataTable and reference the column names
List<MenuModel> lstMenuModel = dtable.AsEnumerable()
.GroupBy(x => new { ParentID = x["ParentID"], ParentName = x["ParentName"] })
.Select(x => new MenuModel()
{
ParentID = x.Key.ParentID,
ParentName = x.Key.ParentName,
MenuItems = x.Select(y => new MenuItemModel()
{
ChildID = y["ChildID"],
....
I have a dynamic selector expression that produces anonymous type. It's working fine in linq to objects, but in linq to entities, it throws:
Attempt 1
NotSupportedException
Only parameterless constructors and initializers are supported in LINQ to Entities.
Expression<Func<User, T>> DynamicSelect<T>(T obj, ParameterExpression userParam)
{
var newExpression = Expression.New(
typeof(T).GetConstructor(typeof(T).GenericTypeArguments),
userParam,
Expression.Constant("X"),
Expression.Constant("Y")
);
return Expression.Lambda<Func<User, T>>(newExpression, userParam);
}
var userParam = Expression.Parameter(typeof(User), "u");
var obj = new { User = new User(), Address = string.Empty, Fax = string.Empty };
var arr = context.Set<T>()
.Select(DynamicSelect(obj, userParam))
.ToArray();
Attempt 2, If I create a custom type, it's working, but I don't want to, because I want to reuse this helper method without creating additional custom type for each entity, I want to be able to pass the type based on consumer.
public class Container
{
public User User { get; set; }
public string Address { get; set; }
public string Fax { get; set; }
}
Expression<Func<User, T>> DynamicSelect<T>(T obj, ParameterExpression userParam)
{
var initExpression = Expression.MemberInit(
Expression.New(typeof(T)),
Expression.Bind(typeof(T).GetProperty("User"), userParam),
Expression.Bind(typeof(T).GetProperty("Address"), Expression.Constant("X")),
Expression.Bind(typeof(T).GetProperty("Fax"), Expression.Constant("Y"))
);
return Expression.Lambda<Func<User, T>>(initExpression, userParam);
}
var userParam = Expression.Parameter(typeof(User), "u");
var arr = context.Set<T>()
.Select(DynamicSelect<Container>(null, userParam))
.ToArray();
Attempt 3, I also tried using Tuple<User, string, string>, but it's not supported too.
NotSupportedException
LINQ to Entities does not recognize the method
'System.Tuple`3[User,System.String,System.String]
Create[User,String,String](User, System.String, System.String)'
method, and this method cannot be translated into a store expression.
Expression<Func<User, T>> DynamicSelect<T>(T obj, ParameterExpression userParam)
{
var createExpression = Expression.Call(
typeof(Tuple),
"Create",
typeof(T).GenericTypeArguments,
userParam,
Expression.Constant("X"),
Expression.Constant("Y"));
return Expression.Lambda<Func<User, T>>(createExpression, userParam);
}
var userParam = Expression.Parameter(typeof(User), "u");
var arr = context.Set<User>()
.Select(DynamicSelect<Tuple<User, string, string>>(null, userParam))
.ToArray();
Please help.
update
I was trying to reuse this helper method in any consumer (User, Customer, Associate, etc) without having specific implementation to each consumer.
The class structure look like.
public class User
{
public int Id { get; set; }
public string UserName { get; set; }
public virtual ICollection<Contact> Contacts { get; set; }
}
public class Customer
{
public int Id { get; set; }
public string CompanyName { get; set; }
public virtual ICollection<Contact> Contacts { get; set; }
}
public class Contact
{
public int Id { get; set; }
public string Type { get; set; }
public string Content { get; set; }
}
public class UserDto
{
public int Id { get; set; }
public string UserName { get; set; }
public ContactDto Contact { get; set; }
}
public class CustomerDto
{
public int Id { get; set; }
public string CompanyName { get; set; }
public ContactDto Contact { get; set; }
}
public class ContactDto
{
public string Email { get; set; }
public string Address { get; set; }
public string Fax { get; set; }
// other contact informations
}
And I have many contacts that could be different for each consumer.
var users = context.Set<User>()
.Select(x => new UserDto
{
Id = x.Id,
UserName = x.UserName,
Contact = new ContactDto
{
Email = x.Contacts.Where(c => c.Type == "Email").Select(c => c.Value).FirstOrDefault()
}
})
.ToArray();
var customers = context.Set<Customer>()
.Select(x => new CustomerDto
{
Id = x.Id,
CompanyName = x.CompanyName,
Contact = new ContactDto
{
Address = x.Contacts.Where(c => c.Type == "Address").Select(c => c.Value).FirstOrDefault(),
Fax = x.Contacts.Where(c => c.Type == "Fax").Select(c => c.Value).FirstOrDefault(),
}
})
.ToArray();
And have refactored the x.Contacts.Where(c => c.Type == "Address").Select(c => c.Value).FirstOrDefault() into expression, but I can't use it directly inside the method like:
var users = context.Set<User>()
.Select(x => new UserDto
{
Id = x.Id,
UserName = x.UserName,
Contact = new ContactDto
{
Email = GetContactExpression("Email").Compile()(x)
}
})
.ToArray();
It will throw error because Invoke method is not supported in linq to expression, so that I need to refactored the whole Select expression, but I need to make it generic (User has UserName, but Customer has CompanyName, and any other information) and probably passing the contact type(s) too after this get solved. The expected result at the moment would be something lile:
var obj = new { User = new User(), Email = "" };
var users = context.Set<User>()
.Select(x => DynamicSelect(obj))
.Select(x => new UserDto
{
Id = x.User.Id,
UserName = x.User.UserName,
Contact = new ContactDto
{
Email = x.Email
}
})
.ToArray();
When I compare your expression with the one created by the compiler for something like u => new { User = u, Address = "X", Fax = "Y" }, the difference is that the latter has filled Members.
I don't quite understand what is the reason for Members to exist at all, but I would try to set it, my guess is that it will fix your problem. The code might look something like:
var constructor = typeof(T).GetConstructor(typeof(T).GenericTypeArguments);
var newExpression = Expression.New(
constructor,
new Expression[] { userParam, Expression.Constant("X"), Expression.Constant("Y") },
constructor.GetParameters().Select(p => typeof(T).GetProperty(p.Name)));