Develop Reference

The type or namespace name 'Core' does not exist in the namespace

I encounter this kind of error when I debug a project
I already have EntityFramework.dll, System.Core.dll, System.Data.Entity but still Core is not referenced.
Can someone help me?
I'm having error in this Line
using System.Data.Entity.Core.Objects;
This is the error:
The type or namespace name 'Core' does not exist in the namespace 'System.Data.Entity' (are you missing an assembly reference?)

Enrique, I had this problem crop up when I started opening a project on my desktop that I had normally been opening on my VM. Masoud's answer is not helpful... I don't think he read your original post.
What I did to clear this compile error up was to open NuGet package manager and install the update that was available for EntityFramework. I had version 6.1.1 installed in the project and I upgraded to 6.1.3. I should not have had the error to start with, but upgrading the version appeared to jiggle whatever it was loose because I started compiling fine after that.

Sometimes, when you add references from other sources than nuget manager (example: Local source) you need to verify the HintPath of that reference before moving your application in another environment

Reference to Microsoft.SharePoint.dll

I have a server that Shrepoint installed on it.
I want to test this code:
SPUtility.GetLocalizedString Method
When I add reference to Microsoft.SharePoint.dll I can resolve SPSite and ... .The problem is When I want to build the project it does not recognize using Microsoft.SharePoint any more:
and I get this error:
Error 13 The type or namespace name 'SharePoint' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)
where is the problem? I add reference to Microsoft.SharePoint.dll but in build time it does not works

Microsoft.SharePoint.dll from SharePoint 2010 use Framework 3.5
but
Microsoft.SharePoint.dll from SharePoint 2013 use Framework 4.0
try it.

Change Visual Studio project solution properties platform target to x64 and the target framework to .NET Framework 3.5.

Try to use this namespace:
using Microsoft.SharePoint.Utilities
In the link you gave, it says that the method resides in Namespace Microsoft.SharePoint.Utilities instead of Microsoft.SharePoint. This is just a guess maybe work, haven't tested it,

Missing reference

In project i have reference to Microsoft.Office.Word.Server
and in code i have method that takes argument of type ConversionJob, but when i try to compile it i have an errors
The type or namespace name 'Office' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)
The type or namespace name 'ConversionJob' could not be found (are you missing a using directive or an assembly reference?)
but when a press f12 on ConversionJob it tooks me to the class defenition
using Microsoft.Office.Word.Server.Service;
using Microsoft.SharePoint;
using System;
namespace Microsoft.Office.Word.Server.Conversions
{
public class ConversionJob
{
public ConversionJob(WordServiceApplicationProxy serviceApplicationProxy, ConversionJobSettings settings);
public ConversionJob(WordServiceApplicationProxy serviceApplicationProxy);
...
what is wrong?
target framework is .Net framework 3.5
Solution
Ive solve the problem. I look to the output and there was message:
Microsoft.Common.targets(1360,9): warning MSB3268: The primary reference "<assembly reference>" could not be resolved because it has an indirect dependency on the framework assembly "<assembly" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v3.5". To resolve this problem, either remove the reference "<assembly>" or retarget your application to a framework version which contains "<assembly>"
i try to find solution in google and - > http://www.sanderstechnology.com/2011/warning-msb3268-you-are-about-to-experience-pain/10646/#.TtYrXGP20Z8

Check the target framework of your application, might be you´re building for the client profile but the referenced assembly requires the full framework.

Try changing the "Target Framework" to be ".NET Framework 4" instead of ".NET Framework 4 Client Profile".
Project Properties -> Application Tab -> Target Framework

You likely need to delete (MAKE SURE TO BACK IT UP) the 12.x.x.x .dll in your GAC folder, and make sure you are referencing the 14.x.x.x in your solution.

If you look at the project references, is it finding the assembly? The icon will look different if it can't resolve it.

1.Try to delete and then manualy add the reference (sometimes it helps - VS losting needed dll but not show it with another sign if not clean)
if 1. ok then backup and try to remove from GAC folder (to understatnd from where it calls assembly)

Probably you have a namespace or class with the name Office somewhere in your code. The compiler uses your namespace instead Office.
In other words I think you have a name collision with your project or any of the references you have in it.
Please investigate for overlapping namespaces / classes.

How to diagnose an issue with csproj references?

I have an issue with a class being used in another project.
Visual studio is able to add a using statement using Resolve (Ctrl+.), but:
The type or namespace name 'SomeClass' could not be found (are you missing a using directive or an assembly reference?
Resolving this again fully qualifies the type but the namespace in the type path is highlighted as not being found.
The namespace appears also in intellisense and the target class is visible in the Object Browser.
Removing unused namespaces removes the added using statement.
I've tried:
Rebuild.
Confirm project dependency in solution options.
Delete reference and re-add.
Confirm class accessibility (despite namespace not being found).
Checked build mode (Debug/Release).
Restart Visual Studio.
Cleared bin folders in both projects.
Restart machine.
What is the next step to diagnosing this baffling issue?

This is likely to be a .Net framework versioning issue. The class being referenced was probably built using a higher version of the framework.
Try checking your framework versions all align in properties, build settings. A project cannot reference an assembly of a higher framework version.

Last time I had this problem my class was not marked public. Just an idea.

This issue was caused by similar namespaces e.g. Company.Product.Project and Product.Project with:
namespace Company.Product.Project
{
using Product.Project;
The class was in Product.Project, but the compiler was searching for Company.Product.Project.
See: Should 'using' statements be inside or outside the namespace? for more details.

Cannot find using System.Data.Linq

I'm using C#, EF 4 in asp.net 4 and VS 2010.
I'm trying to load namespace System.Data.Linq with this code using System.Data.Linq
and I receive this error:
Error 2 The type or namespace name 'Linq' does not exist in the namespace 'System.Data' (are you missing an assembly reference?)
Any idea what I'm doing wrong.

Right click your solution/project. Click Add Reference and search for System.Data.Linq and add the reference there and it should compile.

Just to confirm that Adding the reference to the project didn't work for me because it was already selected.
However, selecting "Copy Local, True" in the Properties pane for the reference made it start working.

That namespace is LINQ-to-SQL, so you'll also need to add a reference to System.Data.Linq.dll; it won't be added by default just by adding Entity Framework.

Try re-change target framework for your project.
Go to Proporties > Application > Target Framework change to another than used now, and next change it back.

VS Installer
The above answers didn't work for me, my problem was that I needed to add LINQ to SQL tools in Visual Studio.

For VS 2022 and project on .NET 6.0 --- Project \ Referencies \ search \ MindBox.Data.Linq, everywhere "OK", install (based on personnel expirience)
Also, may be, need install -- Referencies \MS.System.Linq.Queryable (or MS.Queryable, not remember)
After all action LinqToSQL will be work in project.