I have a struct like this:
struct Test
{
string name;
string family;
public Test...
}
in my code I have a list of this struct:
List<Test> testList=new List<Test>();
I have a linq expression like this:
var list =testList.Select(n=>n.Name);
but how can I filter this selection by Family of the testList? something like this:
var list=testList.Select(n=>n.Name).Where(f=>f.Family=="");
this Where clause just apply on selected Names which are a list of string
Any ideas how to do that?
Just put the Where before the Select:
var list=testList.Where(f=>f.Family=="").Select(n=>n.Name);
In Linq you need to apply the filter before projecting (unless the filter applies to the results of the projection rather than the original collection).
Filter using Where before you select just one property using Select. That way, you still get the full object:
testList.Where(t => t.Family == "").Select(t => t.Name)
After all, Select will take the object and then only pass on whatever you return in the lambda. In this case, you only return a string, so you throw all the other information from the Test object away. And as such, the information you want to filter on isn’t available anymore.
If you switch that around, you can filter on the test object, and then return only that one string.
Normally when I am checking for an empty value like that, I'll use string.IsNullOrEmpty(), just in case.
testList.Where(f=> string.IsNullOrEmpty(f.Family)).Select(n=>n.Name);
You should first apply where clause and the select your desired data:
var list=testList.Where(f=>f.Family=="").Select(n=>n.Name);
So what I was trying was to fetch only those columns from table which has to be updated and I tried it as below:
var user = (from u in context.tbl_user where u.e_Id == model.Id select
new { u.first_name, u.last_name, u.role, u.email, u.account_locked })
.FirstOrDefault();
But when I tried to assign new value to the fetched data as below
user.first_name = model.FirstName;
I saw below error getting displayed
Property or indexer 'anonymous type: string first_name, string
last_name, string role, string email, bool account_locked.first_name'
cannot be assigned to -- it is read only
But when I retrieved all the values from table without filtering as below it worked fine.
var user = (from u in context.tbl_user where u.e_Id == model.Id select u).FirstOrDefault();
Why it doesn't work for first query. I've read in many sites that it is good to retrieve only required properties from database in terms of performance and security. But I am really not able to understand what's wrong with the first approach I opted. Any explanations are much appreciated.
Update
Are there any other ways to fetch only required column and update them and store them back?
Anonymous Types properties are read-only so you can not change them.
Stop doing micro-optimizing or premature-optimization on your code. Try to write code that performs correctly, then if you face a performance problem later then profile your application and see where is the problem. If you have a piece of code which have performance problem due to finding the shortest and longest string then start to optimize this part.
We should forget about small efficiencies, say about 97% of the time:
premature optimization is the root of all evil. Yet we should not pass
up our opportunities in that critical 3% - Donald Knuth
If you want to just fetch specific columns you can create a custom class and fill the properties in your query like others have mentioned.
As others said anonymous type is read only, for achieving what you want, you will have to create a type for it with properties that are required:
public class User
{
public string FirstName {get;set;}
public string LastName {get;set;}
.....................
.....................
}
and then, you have to use it in your linq query:
var user = (from u in context.tbl_user
where u.e_Id == model.Id
select new User
{
FirstName = u.first_name,
LastName = u.last_name,
......................,
.....................
}).FirstOrDefault();
Anonymous types are read-only by design.
In order to retrieve something that you can edit, you have to create a class and select your entities into that class, as done here: How to use LINQ to select into an object?
Or here:
var user = (from u in context.tbl_user where u.e_Id == model.Id select
new User_Mini { u.first_name, u.last_name, u.role, u.email, u.account_locked })
.FirstOrDefault();
Note: you won't be able to call context.SubmitChnages() when editing this new object. You could do something like this though: LINQ to SQL: how to update the only field without retrieving whole entity
This will allow you to update only certain parts of the object.
string rep = "Joe Shmoe"
ObjectSet<StoreData> storeData = edmContext.StoreData;
ObjectSet<CallData> callData = edmContext.CallData;
IEnumerable<string> repStoreData = storeData.Where(r => r.RepName == rep).Select(s => s.Location);
IEnumerable<CallData> repCallData = Here is where I want to filter down the callData collection down to just the records that have a location that is contained in the repStoreData collection
I've tried using some form of Join and Any but don't really understand the arguments those are asking for.
This was my best attempt and it is a no go.
... = callData.Join(d => d.LOCATION.Any(repStoreData));
Well you don't have to use a join. You could just use:
callData.Where(d => repStoreData.Contains(d.LOCATION))
That's assuming d.LOCATION is a single string.
However, you probably don't want to do that with your current declaration of repStoreData as IEnumerable<string> - LINQ won't be able to turn that into a query to be executed at the database.
If you're able to declare repStoreData as IQueryable<string>, however, that would be more likely to work well. I don't know whether that will work with ObjectSet<T>, but I'd hope so.
I'm using entity framework to connect with the database. I've one little problem:
I've one table which have one varbinary(MAX) column(with filestream).
I'm using SQL request to manage the "Data" part, but EF for the rest(metadata of the file).
I've one code which has to get all files id, filename, guid, modification date, ... of a file. This doesn't need at all the "Data" field.
Is there a way to retrieve a List but without this column filled?
Something like
context.Files.Where(f=>f.xyz).Exclude(f=>f.Data).ToList();
??
I know I can create anonymous objects, but I need to transmit the result to a method, so no anonymous methods. And I don't want to put this in a list of anonymous type, and then create a list of my non-anonymous type(File).
The goal is to avoid this:
using(RsSolutionsEntities context = new RsSolutionsEntities())
{
var file = context.Files
.Where(f => f.Id == idFile)
.Select(f => new {
f.Id, f.MimeType, f.Size, f.FileName, f.DataType,
f.DateModification, f.FileId
}).FirstOrDefault();
return new File() {
DataType = file.DataType, DateModification = file.DateModification,
FileId = file.FileId, FileName = file.FileName, Id = file.Id,
MimeType = file.MimeType, Size = file.Size
};
}
(I'm using here the anonymous type because otherwise you will get a NotSupportedException: The entity or complex type 'ProjectName.File' cannot be constructed in a LINQ to Entities query.)
(e.g. this code throw the previous exception:
File file2 = context.Files.Where(f => f.Id == idFile)
.Select(f => new File() {Id = f.Id, DataType = f.DataType}).FirstOrDefault();
and "File" is the type I get with a context.Files.ToList(). This is the good class:
using File = MyProjectNamespace.Common.Data.DataModel.File;
File is a known class of my EF datacontext:
public ObjectSet<File> Files
{
get { return _files ?? (_files = CreateObjectSet<File>("Files")); }
}
private ObjectSet<File> _files;
Is there a way to retrieve a List but without this column filled?
Not without projection which you want to avoid. If the column is mapped it is natural part of your entity. Entity without this column is not complete - it is different data set = projection.
I'm using here the anonymous type because otherwise you will get a
NotSupportedException: The entity or complex type 'ProjectName.File'
cannot be constructed in a LINQ to Entities query.
As exception says you cannot project to mapped entity. I mentioned reason above - projection make different data set and EF don't like "partial entities".
Error 16 Error 3023: Problem in mapping fragments starting at line
2717:Column Files.Data in table Files must be mapped: It has no
default value and is not nullable.
It is not enough to delete property from designer. You must open EDMX as XML and delete column from SSDL as well which will make your model very fragile (each update from database will put your column back). If you don't want to map the column you should use database view without the column and map the view instead of the table but you will not be able to insert data.
As a workaround to all your problems use table splitting and separate the problematic binary column to another entity with 1 : 1 relation to your main File entity.
I'd do something like this:
var result = from thing in dbContext.Things
select new Thing {
PropertyA = thing.PropertyA,
Another = thing.Another
// and so on, skipping the VarBinary(MAX) property
};
Where Thing is your entity that EF knows how to materialize. The resulting SQL statement shouldn't include the large column in its result set, since it's not needed in the query.
EDIT: From your edits, you get the error NotSupportedException: The entity or complex type 'ProjectName.File' cannot be constructed in a LINQ to Entities query. because you haven't mapped that class as an entity. You can't include objects in LINQ to Entities queries that EF doesn't know about and expect it to generate appropriate SQL statements.
You can map another type that excludes the VarBinary(MAX) column in its definition or use the code above.
you can do this:
var files = dbContext.Database.SqlQuery<File>("select FileId, DataType, MimeType from Files");
or this:
var files = objectContext.ExecuteStoreQuery<File>("select FileId, DataType, MimeType from Files");
depending on your version of EF
I had this requirement because I have a Document entity which has a Content field with the content of the file, i.e. some 100MB in size, and I have a Search function that I wanted to return the rest of the columns.
I chose to use projection:
IQueryable<Document> results = dbContext.Documents.Include(o => o.UploadedBy).Select(o => new {
Content = (string)null,
ContentType = o.ContentType,
DocumentTypeId = o.DocumentTypeId,
FileName = o.FileName,
Id = o.Id,
// etc. even with related entities here like:
UploadedBy = o.UploadedBy
});
Then my WebApi controller passes this results object to a common Pagination function, which applies a .Skip, .Take and a .ToList.
This means that when the query gets executed, it doesn't access the Content column, so the 100MB data is not being touched, and the query is as fast as you'd want/expect it to be.
Next, I cast it back to my DTO class, which in this case is pretty much exactly the same as the entity class, so this might not be a step you need to implement, but it's follows my typical WebApi coding pattern, so:
var dtos = paginated.Select(o => new DocumentDTO
{
Content = o.Content,
ContentType = o.ContentType,
DocumentTypeId = o.DocumentTypeId,
FileName = o.FileName,
Id = o.Id,
UploadedBy = o.UploadedBy == null ? null : ModelFactory.Create(o.UploadedBy)
});
Then I return the DTO list:
return Ok(dtos);
So it uses projection, which might not fit the original poster's requirements, but if you're using DTO classes, you're converting anyway. You could just as easily do the following to return them as your actual entities:
var dtos = paginated.Select(o => new Document
{
Content = o.Content,
ContentType = o.ContentType,
DocumentTypeId = o.DocumentTypeId,
//...
Just a few extra steps but this is working nicely for me.
For EF Core 2
I implemented a solution like this:
var files = context.Files.AsNoTracking()
.IgnoreProperty(f => f.Report)
.ToList();
The base idea is to turn for example this query:
SELECT [f].[Id], [f].[Report], [f].[CreationDate]
FROM [File] AS [f]
into this:
SELECT [f].[Id], '' as [Report], [f].[CreationDate]
FROM [File] AS [f]
you can see the full source code in here:
https://github.com/aspnet/EntityFrameworkCore/issues/1387#issuecomment-495630292
I'd like to share my attempts to workaround this problem in case somebody else is in the same situation.
I started with what Jeremy Danyow suggested, which to me is the less painful option.
// You need to include all fields in the query, just make null the ones you don't want.
var results = context.Database.SqlQuery<myEntity>("SELECT Field1, Field2, Field3, HugeField4 = NULL, Field5 FROM TableName");
In my case, I needed a IQueryable<> result object so I added AsQueryable() at the end. This of course let me add calls to .Where, .Take, and the other commands we all know, and they worked fine. But there's a caveat:
The normal code (basically context.myEntity.AsQueryable()) returned a System.Data.Entity.DbSet<Data.DataModel.myEntity>, while this approach returned System.Linq.EnumerableQuery<Data.DataModel.myEntity>.
Apparently this means that my custom query gets executed "as is" as soon as needed and the filtering I added later is done afterwards and not in the database.
Therefore I tried to mimic Entity Framework's object by using the exact query EF creates, even with those [Extent1] aliases, but it didn't work. When analyzing the resulting object, its query ended like
FROM [dbo].[TableName] AS [Extent1].Where(c => ...
instead of the expected
FROM [dbo].[TableName] AS [Extent1] WHERE ([Extent1]...
Anyway, this works, and as long as the table is not huge, this method will be fast enough. Otherwise you have no option than to manually add the conditions by concatenating strings, like classic dynamic SQL. A very basic example in case you don't know what I'm talking about:
string query = "SELECT Field1, Field2, Field3, HugeField4 = NULL, Field5 FROM TableName";
if (parameterId.HasValue)
query += " WHERE Field1 = " + parameterId.Value.ToString();
var results = context.Database.SqlQuery<myEntity>(query);
In case your method sometimes needs this field, you can add a bool parameter and then do something like this:
IQueryable<myEntity> results;
if (excludeBigData)
results = context.Database.SqlQuery<myEntity>("SELECT Field1, Field2, Field3, HugeField4 = NULL, Field5 FROM TableName").AsQueryable();
else
results = context.myEntity.AsQueryable();
If anyone manages to make the Linq extensions work properly like if it was the original EF object, please comment so I can update the answer.
I'm using here the anonymous type because otherwise you will get a
NotSupportedException: The entity or complex type 'ProjectName.File'
cannot be constructed in a LINQ to Entities query.
var file = context.Files
.Where(f => f.Id == idFile)
.FirstOrDefault() // You need to exeucte the query if you want to reuse the type
.Select(f => new {
f.Id, f.MimeType, f.Size, f.FileName, f.DataType,
f.DateModification, f.FileId
}).FirstOrDefault();
And also its not a bad practice to de-normalize the table into further, i.e one with metadata and one with payload to avoid projection. Projection would work, the only issue is, need to edit any time a new column is added to the table.
I tried this:
From the edmx diagram (EF 6), I clicked the column I wanted to hide from EF and on their properties you can set their getter and setter to private. That way, for me it works.
I return some data which includes a User reference, so I wanted to hide the Password field even though it's encrypted and salted, I just didn't want it on my json, and I didn't want to do a:
Select(col => new {})
because that's a pain to create and maintain, especially for big tables with a lot of relationships.
The downside with this method is that if you ever regenerate your model, you would need to modify their getter and setter again.
Using Entity Framework Power Tools you can do the following in efpt.config.json:
"Tables": [
{
"ExcludedColumns": [
"FileData"
],
"Name": "[dbo].[Attachment]",
"ObjectType": 0
}
]
I'm filling a drop-down list using the following:
var columnNames = db.Mapping.MappingSource.GetModel(typeof(StaffDirectoryDataContext))
.GetMetaType(typeof(Person)).DataMembers;
I'm then converting that to a List<String> to populate a drop down list.
I then want to be able to get a set of results based on the user's selection. For example, if they select "First name" from the drop down list and type "Bob" into the text box I want to run a LINQ query where first name = bob.
I'm probably being thick but I can't find a way! Pseudo code would be...
var q = from x in dc.Persons
where x.[selected column name] == [textbox value]
select x;
Can anybody help? Essentially I have the column name as a String value, and I can't figure out how to tell the LINQ query that that's the column to filter on!
Could do this in ADO.NET with my eyes closed, but determined to use LINQ all the way!!
Thanks in advance.
David Buchanan has posted a solution for this problem using reflection :
msdn forum
I'm not sure you can do this dynamically, but you can do it conditionally. Something like this:
switch(selected column name)
{
case "student_no":
q = q.where(p=>p.StudentNo == value);
break;
case "student_id":
q = q.where(p=>p.StudentId == value);
break;
}
You can iterate through your columns and keep building the wheres. The SQL won't be executed as long as none of the calls force the IQueryable to execute.
I think expression trees are the right way to do this, but I don't know them very well so I'm going to give you the alternate way I would have done this if I didn't feel like learning expression tree building..
public interface IFilter { IEnumerable RetreiveFilter(string filterValue); }
public class FirstNameFilter : IFilter
{
private const string FILTER_TYPE_NAME = "First Name";
public IEnumerable RetreiveFilter(string filterValue)
{
return _myData.Where(person => person.FirstName = filtervalue);
}
public override string ToString()
{
return FILTER_TYPE_NAME;
}
}
Create a class like this for each filter type, and then fill your dropdown with these filters, and when they type info into the filter text, it will execute against the ((IFilter)filterDropDown.SelectedItem).RetreiverFilter(filterTextBox.Text);