We Keep Coding

C# linq Select objects in list looking inside another list of objects

I have a class like this:
public class MailMessage
{
public string From {get; set; };
public string To {get; set; };
public ICollection<Attachment> Attachments { get; set; }
}
public class Attachment
{
public string Name {get; set;}
public byte[] Content {get;set;}
}
I would like to get all attachments in Attachments collection whose name ends with .pdf.
I tried the following:
List<MailMessage> list = new List<MailMessage>();
List<attachment> pdfAttachmentsCollection = list.Where(x => x.Attachments
.Where(attachment =>
attachment.Name.EndsWith(".pdf")).Select(y => y.Content));
However, this doesnt work. Any suggestions. It gives error:
Cannot implicitly convert type
'System.Collections.Generic.IEnumerable ' to 'bool'

A couple issues:
You need to SelectMany from the MailMessage object to get all attachments in a single collection
You're selecting y.Content which is byte[] but trying to get a list of Attachment
LINQ returns IEnumerables, not Lists
Rewritten, you probably want to do something like:
List<MailMessage> list = new List<MailMessage>();
IEnumerable<Attachment> pdfAttachmentsCollection =
list.SelectMany(message =>
message.Attachments
.Where(attachment => attachment.Name.EndsWith(".pdf")));
This is also a good candidate to write as a query expression:
IEnumerable<Attachment> pdfAttachmentsCollection =
from message in list
from attachment in message.Attachments
where attachment.Name.EndsWith(".pdf")
select attachment;
Both of these do the same thing, just with different syntax.
If you want this as a List<Attachment> you can also do .ToList() at the end of either query. As an IEnumerable, it won't be evaluated until you start iterating through it (and depending on how you use it, you might never have the full collection in memory at any time). However, if you do multiple iterations or operations on it, it'll cause it to be evaluated multiple times -- so in those cases .ToList() makes sense for sure.

How to use a copy-constructor in a LINQ to Entities query?

I'm working on a Linq expression in which I get an object from a DBContext, and I want to make it a custom ViewModel object
my ViewModel receives as parameter an object obtained from the DBContext to work the information and return it completely
This is a little example
public class Obj1 // Object i get from database
{
public int id { get; set; }
public string Param { get; set; }
public string Param2 { get; set; }
public string Random { get; set; }
}
public class Obj2 //ViewModel
{
public string ParamFormateado { get; set; }
public string Random { get; set; }
public Obj2(Obj1 parametro)
{
ParamFormateado = parametro.Param + parametro.Param2;
Random = parametro.Random;
}
}
What I'm trying to do is get an Obj2 with a Linq expression who returns an Obj1 without transforming the information in the linq expression, since in my case it becomes a basically illegible expression
I was try something like this
Obj2 objeto = db.Obj1.Where(x => x.id == "0").Select(x => new Obj2(x)).FirstOrDefault();
Is it possible to perform a Linq query similar to the one I am proposing? since otherwise, I end up having extremely long Linq expressions to format this information, but what would be the best alternative in these cases?

You can't do that because only parameterless constructors are supported. But you can do it with Linq-To-Objects which can be forced with AsEnumerable:
Obj2 objeto = db.Obj1
.Where(x => x.id == "0")
.AsEnumerable() // <--- here
.Select(x => new Obj2(x))
.FirstOrDefault();
So only the filter with Where will be executed in the database, the remaining record(s) are processed in-process.
https://codeblog.jonskeet.uk/2011/01/14/reimplementing-linq-to-objects-part-36-asenumerable/

Dont do it with Linq like that. you have to create a method that takes obj1 as parameter, maps properties and then returns obj2. Or use Automapper from nuget repository.
do it like this
public obj2 Map(obj1 source)
{
var destination = new obj2();
destination.param1 = source.param1;
//
return destination;
}
if you want to pass a collection of objects then do just that and just foreach through the list and return a list of mapped objects. But i would advise you to use Automapper since it automates the proces and you dont have to write a long mapping code.

Member is "not supported in LINQ to Entities"

So I am new to C#, LINQ, and MVC. I am trying to get a list of Ages, but it says
The specified type member 'Age' is not supported in LINQ to Entities.
Only initializers, entity members, and entity navigation properties
are supported.
For a previous tutorial, they use this exact same logic, except they check a string, not an int (Age). Why is this giving me a fit, and how can I fix it?
public ActionResult SearchIndex(string ageValue, string searchString)
{
if (!string.IsNullOrEmpty(ageValue))
{
var AgeList = new List<string>();
var AgeListQry = from d in db.Actors orderby d.Age select d.Age.ToString();
AgeList.AddRange(AgeListQry.Distinct());
}
// other stuff
}
I want to learn what is going on, so that I can avoid this in the future!
Entity Model code
public class Actor
{
public int ID { get; set; }
public string Name { get; set; }
public DateTime BirthDate { get; set; }
public int Age
{
get {
return (int)(DateTime.Now - BirthDate).TotalDays / 365;
}
}
public decimal NetValue { get; set; }
}
public class ActorDBContext : DbContext
{
public DbSet<Actor> Actors { get; set; }
}

As mentioned in the comments, you can't call ToString() in a Linq to Entities query. Instead do it like this:
var AgeList = new List<string>();
//retrieve as whatever type Age is, no conversion in SQL Server
var AgeListQry = (from d in db.Actors orderby d.Age select d.Age).ToList();
//convert them after the fact, using Linq to Objects
AgeList.AddRange(AgeListQry.Select(a => a.ToString()).Distinct());
EDIT
I saw your latest update that does show that Age is not a database column. You are then required to do something like this (assuming BirthDate is properly mapped):
var AgeList = new List<string>();
//retrieve BirthDate from SQL Server and use ToList() to get it to run immediately
var AgeListQry = (from d in db.Actors orderby d.BirthDate select d.BirthDate).ToList();
//convert them after the fact, using Linq to Objects
AgeList.AddRange(AgeListQry.Select(bd => ((int)(DateTime.Now - bd).TotalDays / 365).ToString()).Distinct());
Linq to Entities maps your expressions to SQL statements and there is nothing for it to map to when you use your Age property. Instead, you need to get what you can from SQL Server (BirthDate) and then do the translation to Age yourself. You could replace the inline code with a method call like this if you'd rather:
AgeList.AddRange(AgeListQry.Select(bd => CalculateAge(bd)).Distinct());
//...
private string CalculateAge(DateTime birthday)
{
return ((int)(DateTime.Now - bd).TotalDays / 365).ToString();
}

You haven't the Age in you DB scheme and it is impossible to convert LINQ to DB query.
You must order the Age collection in client side or add calculated column to your table.

There is another way. Have a converter file, where you pass the object, works with the birthdate and produces the age, returns the same object. That also means, that you can't search the database for the age column

Hydrate property via joining another List<Property> on unique Id

I have two lists, one which is a list of Equipment and one which is a list of WorkflowItems with an Equipment property. The Equipment property within the List<WorkflowItem> list only has one of it's values hydrated, ProcessId. The List<Equipment> has two properties hydrated, ProcessId and Name. I want to hydrate the List<WorkflowItem>.Equipment.Name with the value from the single Equipment record in the List<Equipment>
This LINQ query below will select a generic item out doing basically what I'm looking for, but I would rather just fill in the original list.
var list = from item in workflowItems
join equipment in barcodeEquipmentList on
item.Equipment.ProcessId equals equipment.ProcessId
select new
{
ProcessId = item.Equipment.ProcessId,
EquipmentName = equipment.Name
};
Edit
The list is going to be relatively small, even doing something like this would be fine (aside from the fact that this does not work)
workflowItems.ForEach(x => x.Equipment = from e in barcodeEquipmentList
where e.Process.Id == x.Equipment.Process.Id
select e
);
...final edit
but this does work:
workflowItems.ForEach(x => x.Equipment = barcodeEquipmentList
.Where(e => e.Process.Id == x.Equipment.Process.Id)
.FirstOrDefault());

This piece of code should match your needs:
public class Equipment {
public int ProcessId { get; set; }
public string Name { get; set; }
}
public class WorkflowItem {
public Equipment { get; set; }
public void LoadEquipmentFrom(IEnumerable<Equipment> cache){
var equipment = cache.FirstOrDefault(e => e.ProcessId == Equipment.ProcessId);
if(equipment != null)
Equipment.Name = equipment.Name;
}
}
You could also assign the instance from cache to the existing one, it wouldn't matter since both must have the same Identifier Equipment = equipment;. That would be easier if you have more properties to set. To optimize further use an IDictionary<int, Equipment> instead of that IEnumerable<Equipment>, because you'll be reading that collection very often.
I'm guessing you are implementing a kind of ORM, in this case I can give you a good advice: "There is already something out there that'll fit your needs.".

Since the dataset was not very big (less than 20 records), I was able to this as below without a hit to performance
workflowItems.ForEach(x => x.Equipment = barcodeEquipmentList
.Where(e => e.Process.Id == x.Equipment.Process.Id)
.FirstOrDefault());

Linq extracting objects

I have a JSON "multi-level" response that I need to deserialize and from the deserialized classes structure I need to extract all the objects of a certain class.
Below the code I'm using, at the end I find that my result is empty, not populated.
// given these two classes:
[DataContract]
public class ThingsList
{
[DataMember(Name = "status")]
public string Status { get; set; }
[DataMember(Name = "since")]
public double Since { get; set; }
[DataMember(Name = "list")]
public Dictionary<string, ThingsListItem> Items { get; set; }
public DateTime SinceDate { get { return UnixTime.ToDateTime(Since); } }
}
[DataContract]
public class ThingsListItem
{
[DataMember(Name = "url")]
public string Url { get; set; }
[DataMember(Name = "title")]
public string Title { get; set; }
}
// I can deserialize my json to this structure with:
ThingsList results = JsonConvert.DeserializeObject<ThingsList>(e.Result);
// now I need to "extract" only the ThingsListItem objects, and I'm trying this:
var theList = from item in results.Items.OfType<ThingsListItem>()
select new
{
Title = item.Title,
Url = item.Url
};
// but "theList" is not populated.
The points here are (I believe):
- I try to use results.Items.OfType() in order to extract only the ThingsListItem objects, that in the "upper" class are declared in the
public Dictionary Items { get; set; }
row.
Any idea? Tell if it's not clear...
Thanks
Andrea

EDIT: updated my response for clarity.
Since your Dictionary values are of type ThingsListItem you can access them directly by using the Dictionary's Values property. There is no need to use OfType to check their type and extract them. Simply use:
var items = results.Items.Values;
The Values property would return an ICollection<ThingsListItem>. You can then iterate over the results with a foreach. LINQ does not have to be used.
While the Values property described above should be sufficient, I will point out a few issues with your original LINQ query attempt.
1) The following query is probably what you were after. Again, the Dictionary's Values property is key (no pun intended) to accessing the items:
var theList = from item in results.Items.Values
select new
{
Title = item.Title,
Url = item.Url
};
2) Why are you using new? That will return an IEnumerable of anonymous types. You already have a defined class, so why project into a new anonymous type? You should retain the underlying ThingsListItem items by selecting the item directly to get an IEnumerable<ThingsListItem>:
var theList = from item in results.Items.Values
select item;
foreach (var item in theList)
{
Console.WriteLine("Title: {0}, Url: {1}", item.Title, item.Url);
}
You would usually project into a new anonymous type to define a type with data properties you are interested in. Generally you would use them immediately after the query, whereas a selection into an existing class could be used immediately or passed around to other methods that are expecting that type.
Hopefully this has cleared up some questions for you and you have a better idea of using LINQ and when to use the new keyword. To reiterate, for your purposes it seems the Values property should suffice. Using LINQ to select the item is redundant when there are other immediate means to do so.