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LINQ to SQL, 'Linq' does not exist in the namespace 'System.Data'

I tried to connect my project to a database, using LINQ to SQL, following a few online guides. I created a simple database Library, added new LINQ to SQL Classes item in the project, named SimpleLibraryDatabase, connected to the server and dragged the required tables. So far no problems, but when i tried to build the solution, it failed. Source of the problem appears to be in the auto-generated file SimpleLibraryDatabase.designer.cs, where it gives me over 70 errors, most of them saying:
The type or namespace name 'Linq' does not exist in the namespace 'System.Data' (are you missing an assembly reference?)
What is also weird for me, in this generated context class, there is no constructor, that takes 0 arguments, unlike in all the tutorials I've watched.
This is my first time playing with databases in C#.

LINQ to SQL is a component of .NET Framework version 3.5, It's just available on .NET Framework 3.5 ~ 4.8 and not accessible to classes in the .Net core.
LINQ to SQL is a component of .NET Framework version 3.5 that provides a run-time infrastructure for managing relational data as objects.
Microsoft

Entity Framework - Multiple CLR Types

I am getting the following error when executing one one of my LINQ queries
The mapping of CLR type to EDM type is ambiguous because multiple CLR
types match the EDM type 'Product'. Previously found CLR type
'TF.MyProject.DAL.Product, newly found CLR type
'TF.MyProject.DTO.Product'.
The Entity Framework generated class and the DTO object both are under their own namespace. This has worked previously and below is what I have tried so far...
Regenerated the EDMX Model
Removed and Created a new EDMX Model
Run Custom Tool to generate the Template files
Ensure all projects within the solution are using the same Entity Framework version.
Entity Framework version: 6.1.3
Below is how my current project is set up
MyProject
/DAL/MyProjectModel.edmx
/DTO/Product.cs
Any idea or suggestions will be helpful. This has worked in my previous projects with version 6. Not sure if it is specific to a version within release 6..
Note: Moving the DAL to a project of its own resolves the issue

You have two classes with the same name Product. Entity Framework uses Class Names only, irrespective of namespaces or files where it is declared. The error message clearly says you have two Product classes, one in TF.MyProject.DAL namespace and another one in TF.MyProject.DTO.
Try renaming on of these Product classes to some other name and try again.

My entity framework 6 class library works from one project but not from the other

I have created a class library (targeted to .NET 4) containing all my entity framework models/context. I am referencing that class library from one project (targeted to .NET 4.5.1) and it works great, then I am referencing the same class library from another project (targeted to .NET 4) and it gives me this error:
Schema specified is not valid. Errors:
Multiple types with the name 'places' exist in the EdmItemCollection in different namespaces. Convention based mapping requires unique names without regard to namespace in the EdmItemCollection.
And then many more of the same error for all my entities.

You seem to be hitting a type conflict. When resolving types EF does not use namespaces and if it sees two types with the same names it sometimes does not know which one to use. You can find more details in the bug EF team is tracking this issue: https://entityframework.codeplex.com/workitem/483. Also note that as of EF6 most scenarios are fixed when using Code First - see this work item for more details: https://entityframework.codeplex.com/workitem/911

Entity Framework 4 issue

Using .NET Web API (.NET 4, EF 4) and I'm getting some strange errors when debugging and really can't figure what is going on.
Say in the DocumentRepository I have this constructor:
public DocumentRepository(DocPortalContext db)
{
this._db = db;
}
If I debug and hover over _db and drill into the items in the popup window I'm coming across the following errors:
System.Data.Entity.Infrastructure.IObjectContextAdapter.ObjectContext = 'System.Data.Entity.DbContext' does not contain a definition for 'System' and no extension method 'System' accepting a first argument of type 'System.Data.Entity.DbContext' could be found (are you missing a using directive or an assembly reference?)
and
System.Collections.Generic.ICollection>.IsReadOnly = 'System.Collections.Generic.Dictionary' does not contain a definition for 'System' and no extension method 'System' accepting a first argument of type 'System.Collections.Generic.Dictionary
No exceptions are being caught.
If I put a break point on this line in a repository method - return _db.Documents.AsQueryable(); - and hover over Document I get the following error:
System.Linq.IQueryable.Provider = 'System.Data.Entity.Infrastructure.DbQuery' does not contain a definition for 'System' and no extension method 'System' accepting a first argument of type 'System.Data.Entity.Infrastructure.DbQuery' could be f...
Some help would be much appreciated.
Additional information:
Drilling into _db in the following way gives message quoted at the end:
base(System.Data.Entity.DbContext) -> Internal Context -> _appConfig -> and finally Default Connection Factory has the following error beside it "The function evaluated requires all threads to run."
More information as per my comment:
This isn't really related to the question, but it has me thinking there could be something wrong with my install of .NET or VS - I don't know enough about the pipework to make that call, maybe one of you can. Anyway, I can browse to a URI in my WebAPI project and get JSON returned in the browser. When I try to consume the URI in my Website project, I get this in my browser: Could not find file 'C:\Program Files (x86)\IIS Express\System.Net.Http.StreamContent'. which is also caught as an exception.

By default EF 4 generates proxy classes on the fly that inherit from your POCO classes or your model first classes. That is most likely why you get these errors on runtime. The odd things is that all required usings should be included.
Are you sure you reference System in your POCO classes (presuming you have POCO classes)?

Do all projects in your solution have the same .NET version setup in project's properties?
Do those versions agree with the version on the MSDN describing the missing classes?

Try registering your entity-framework dll and sql-server dll in global assembly cache using gacutil.exe from visual studio command prompt

I had come across this issue recently on a small sample application where we just included nuget packages and created some data layer objects. This is what we did and it solved the issue
Make sure temp folders for .net are clean . usually in asp.net temporary files in windows directory.
Make sure your visual studio is upto date and not requiring a restart due to nuget reinstall. Not sure if it solved or did help but i checked this as well and restarted vs.
Clean the projects and made sure that dlls are present. if you are using some webmatrix dll for asp.net simple membership make sure you have selected to copy it locally. This was one issue when we were seeding data for roles and members for mvc4 simple membership use but may not be the case for your project.
We also checked that in web projects the configuration for ef corresponds to proper version of ef.
Build the solution and run it after a vs 2012 restart and it worked fine. Now I dont know exactly which of these things helped in resolving this issue but doing all 4 worked in one case.
Just thought to share if it helps.

These look like issues relating to an incompatibility between your project's target .Net version and the build version of the assembly you're referencing.
In this instance, you should make sure that the assembly you are referencing is less than or equal to your project's target .Net Framework version.

I have seen this message numerous times. 9 times out of ten, this means that there is a likely problem with the App.config file. Usually, it is just missing. In other words, for every project folder you have referencing the Entity Model you need to place a copy of that App.config file within it. If they are already there, then delete the App.config file that partners the edmx file, and recompile the Entity Model separately to regenerate it. (DON'T DELETE THE EDMX FILE or THE DESIGNER CLASS) Then you have to replace all the App.config's again. Yes, it sucks!!! Sometimes, LinqPad is great for checking this issue too. You can reference the dll file in LinqPad and attempt to run queries giving you the same error message letting you know that the problem is not just Visual Studio.
Hope this helps...Good Luck!!!!

How are you implementing your repository? Seems to me that maybe the problem is that your application is running on a different thread than your repository and that might be the answer for the "The function evaluated requires all threads to run." message.

Move generated classes after Reverse engineer code first used in Entity Framework Power tools beta 2

I am using Entity Framework 5.0 with Entity Framework Powertools beta 2. We have a database that we are not allowed to change and so are using the Reverse Engineer Code First option to create the classes. The generated classes are placed in the Models folder below the mappings.
Is there a safe/best way to move these generated classes into their own project, leaving the DbContext-type class and the Mapping file classes in the Models folder?

There is no problem with moving the entities to whatever project and/or namespace you like. Just copy the files to where you want them. You will, however, likely want to go in and change the namespaces on every file (though it's not specifically required).
You do of course have to add a reference to your new assembly to your parent project.