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Visual Studio Online build fails - Local works

I upgraded to VS2015, and MVC was updated to 5.0.
I am able to build successfully on my local PC, but when I checkin to Visual Studio Online, and my CI build builds, it fails:
"The type or namespace name 'Mvc' does not exist in the namespace 'System.Web' (are you missing an assembly reference?)"
I've used nuget manager to remove, and re-install MVC on my project - same results.
Looking at my references in the project, all looks OK.
I also notice in the log before the error happens:
Installing 'Microsoft.AspNet.Razor 3.2.3'.
Installing 'Microsoft.AspNet.Mvc.FixedDisplayModes 5.0.0'.
Successfully installed 'Microsoft.AspNet.Mvc.FixedDisplayModes 5.0.0'.
Successfully installed 'Microsoft.AspNet.Razor 3.2.3'.
All packages listed in packages.config are already installed.
But it seems when it builds, it doesn't know what MVC is. How can I resolve this?

Late to the game here, but I was having a very similar issue: Error CS0246: The type or namespace name 'blank' could not be found (are you missing a using directive or an assembly reference?).
This was building locally but not in VSTS. My solution was to delete the reference under the project's references, and then just add it back in again. The reference was to a namespace in a local project to the solution. For some reason it was being found locally but not found when checked into and built on VSTS.
Deleting the reference and then re-adding it worked for me.

Check the version number mentioned in all web.config files. I mean also those in the view folders; that they match the version 5.2.3.0

Scan all your references and ensure, that all of them related to MVC have Copy Local property set true. Not only Mvc one.

I fixed this by removing all the folders within my Packages folder, leaving only repositories.config. This forces VSOnline to restore all the packages on each build.

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

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.

The type or namespace name could not be found [duplicate]

This question already has answers here:
Getting "type or namespace name could not be found" but everything seems ok?
(44 answers)
Closed 8 years ago.
I have a C# solution with several projects in Visual Studio 2010.
One is a test project (I'll call it "PrjTest"), the other is a Windows Forms Application project (I'll call it "PrjForm"). There is also a third project referenced by PrjForm, which it is able to reference and use successfully.
PrjForm references PrjTest, and PrjForm has a class with a using statement:
using PrjTest;
Reference has been correctly added
using statement is correctly in place
Spelling is correct
PrjTest builds successfully
PrjForm almost builds, but breaks on the using PrjTest; line with the error:
The type or namespace name 'PrjTest' could not be found (are you missing a using directive or an assembly reference?)
I've tried the following to resolve this:
Removed Resharper (since Resharper had no trouble recognizing the referenced project, I thought it might be worth a shot)
Removed and re-added the reference and using statement
Recreated PrjForm from scratch
PrjForm currently resides inside the PrjTest folder, I tried moving it to an outside folder
Loaded the solution on a different computer with a fresh copy of VS 2010
I have done my homework and spent far too long looking for an answer online, none of the solutions has helped yet.
What else could I try?

See this question.
Turns out this was a client profiling issue.
PrjForm was set to ".Net Framework 4 Client Profile"
I changed it to ".Net Framework 4", and now I have a successful build.
Thanks everyone!
I guess it figures that after all that time spent searching online, I find the solution minutes after posting, I guess the trick is knowing the right question to ask..

In my case I had:
Referenced DLL : .NET 4.5
Project : .NET 4.0
Because of the above mismatch, the 4.0 project couldn't see inside the namespace of the 4.5 .DLL. I recompiled the .DLL to target .NET 4.0 and I was fine.

PrjForm was set to ".Net Framework 4 Client Profile" I changed it to ".Net Framework 4", and now I have a successful build.
This worked for me too. Thanks a lot. I was trying an RDF example for dotNet where in I downloaded kit from dotnetrdf.
NET4 Client Profile:
Always target NET4 Client Profile for all your client desktop applications (including Windows Forms and WPF apps).
NET4 Full framework:
Target NET4 Full only if the features or assemblies that your app need are not included in the Client Profile. This includes:
If you are building Server apps, Such as:
ASP.Net apps
Server-side ASMX based web services
If you use legacy client scenarios, Such as:
o Use System.Data.OracleClient.dll which is deprecated in NET4 and not included in the Client Profile.
Use legacy Windows Workflow
Foundation 3.0 or 3.5 (WF3.0 , WF3.5)
If you targeting developer scenarios and need tool such as MSBuild or need access to design assemblies such as System.Design.dll

Another thing that can cause this error is having NuGet packages that have been built with a newer version of .NET.
The original error:
frmTestPlanSelector.cs(11,7): error CS0246: The type or namespace name 'DatabaseManager'
could not be found (are you missing a using directive or an assembly reference?)
Further up in the log I found this:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3275: The primary reference "[redacted]\DatabaseManager\bin\Release\DatabaseManager.dll" could not be resolved because it has an indirect dependency on the assembly "System.Data.SQLite, Version=1.0.94.0, Culture=neutral, PublicKeyToken=db937bc2d44ff139" which was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0".
The solution was to re-install the NuGet packages:
http://docs.nuget.org/docs/workflows/reinstalling-packages

I solved mine because the other project was coded with .NET 4.5 and the other one was coded 4.0

The using statement refers to a namespace, not a project.
Make sure that you have the appropriately named namespace in your referenced project:
namespace PrjTest
{
public class Foo
{
// etc...
}
}
Read more about namespaces on MSDN:
Using Namespaces

I encountered this issue it turned out to be.
Project B references Project A.
Project A compiled as A.dll (assembly name = A).
Project B compiled as A.dll (assembly name A).
Visual Studio 2010 wasn't catching this. Resharper was okay, but wouldn't compile. WinForms designer gave misleading error message saying likely resulting from incompatbile platform targets.
The solution, after a painful day, was to make sure assemblies don't have same name.

It is also possible, that the referenced projects targets .NET 4.0, while the Console App Project targets .NET 4.0 Client Library.
While it might not have been related to this particular case, I think someone else can find this information useful.

The compiled dll should have public Class.

I had the same issue. The target frameworks were fine for me. Still it was not working.
I installed VS2010 sp1, and did a "Rebuild" on the PrjTest. Then it started working for me.

Other problem that might be causing such behavior are build configurations.
I had two projects with configurations set to be built to specific folders.
Like Debug and Any CPU and in second it was Debug and x86.
What I did I went to Solution->Context menu->Properties->Configuration properties->Configuration and I set all my projects to use same configurations Debug and x86 and also checked Build tick mark.
Then projects started to build correctly and were able to see namespaces.

Changing the framework to
.NET Framework 4 Client Profile
did the job for me.

For COM/ActiveX references, VS 2012 will show this error right on using statement. Which is quite funny, since it's saying that may be you are missing a using statement.
To solve this: register the actual COM/ActiveX dll even if it's in the neighbor project, and add a reference through COM channel, not project channel. It will add Interop.ProjectName instead of ProjectName as a reference and this solves this strange bug.

If your project (PrjTest) does not expose any public types within the PrjTest namespace, it will cause that error.
Does the project (PrjTest) include any classes or types in the "PrjTest" namespace which are public?

just changed Application's target framework to ".Net Framework 4".
And error got Disappeared.
good luck;
:D

check your Project Properties, your Reference Paths should be empty like this:
Regards