The type or namespace <blah> does not exist

Ok, I have had this one a million times before and it's been answered 1 million +1 times before.

And yet, once again. I have 3 projects, A, B, and C, each a DLL. Each project is .Net 4.0 (not the client build, full 4.0). Project C references A and B. They are referenced as projects, and the output is set to copy locally.

In C, I have two using statements in my .cs file:

using A;
using B;

When I compile, I get the complaint that is cannot find B. A is fine. B depends on A.

What the heck should I do? I've removed and re-added, closed VS2010, re-opened it, looked at the .csproj file. And I just cannot get it. Again, for the millionth time.

Someone please slap enough sense into me that I learn the source of this once and for all!

And yes, this is probably answered somewhere in StackOverflow, but not in any of the top answers I've checked so far. The terms are just too generic to be of use, too many questions where the answer is "duh, add a reference". I'm past that point.

Here are the errors I get. There are 3 kinds, but from past experience, the last one is the true one.

Error   130 'AWI.WWG.EXPMRI.MriUpload.Data.MriUpload' does not contain a definition for 'Database' and no extension method 'Database' accepting a first argument of type 'AWI.WWG.EXPMRI.MriUpload.Data.MriUpload' could be found (are you missing a using directive or an assembly reference?) 

Error   114 'object' does not contain a definition for <blah>

Error   59  The type or nam开发者_开发技巧espace name '<blah>' could not be found (are you missing a using directive or an assembly reference?)    

Aha I looked at the warnings, not just the errors, and here is what I see:

Warning 69  The referenced project '..\..\..\..\..\..\..\Partners\integration\framework\connectors\Partners.Connectors.Base\Partners.Connectors.Base\Partners.Connectors.Base.2010.csproj' does not exist.  AWI.WWG.EXPMRI.MriUpload.Objects

That .csproj file is the "B" in this case. Even though I remove and re-add the project reference I get this. But it feels like I'm getting closer!

Hmm, I just found another DLL, call it "D", which "A" references. When I add it to the project, I start to get the complaint:

----------------
The  Add Reference Dialog could not be shown due to the error:

The specified path, file name, or both are too long.  The fully qualified file name must be less than 260 characters, and the directory name must be less than 248 characters.
----------------

Could this be related, or just another distraction?

Ok, I found the issue, though I do not understand it.

When I add the reference through the IDE, it adds this to the csproj file of "C":

<ProjectReference Include="..\..\..\..\..\..\..\Partners\integration\framework\connectors\Partners.Connectors.Base\Partners.Connectors.Base\Partners.Connectors.Base.2010.csproj">

This does not compile, it WARNS that it cannot find the referenced project, then all those ERRORs happen. But then I change the ProjectReference to the following:

<ProjectReference Include="C:\...\Partners.Connectors.Base.2010.csproj">

... and it works just fine. Note that neither of those paths are anything close to 256 characters. The fully qualified one is only 135 characters. But perhaps the IDE is doing some silly decoration of the path.

The solution has to do with the file path limits in Windows, and they way the IDE translates relative paths into full ones, as explained in this blog.

The immediate solution is to edit the csproj file manually to use the absolute path. Until the reference is re-added, the absolute path will be valid. One day I may shorten my folders, but it's not top priority at the moment.

If you suspect you have this issue, look at the Warning messages from the compiler. I often have these turned off myself, only looking at errors. But the warning about "the referenced project does not exist" was the clue that solved this for me.

In case the other link disappears, here is the link to the MS article. http://support.microsoft.com/kb/2516078

It is worth noting that this same error manifests for a variety of issues such as client-framework-targeting issues, and is logged as a warning when a reference fails to load. Presumably the reference error is only a warning because if the reference is not actually needed it doesn't matter.

I would make sure that your project has included the references to the assemblies.

I would check that the build order matches your dependencies

Finally, if everything is setup properly, you should see the following Build Order:

Doesn't look like this is your problem, but for completeness, I should add that another thing to check (if your project targets the .NET Framework 3.5 or above) is that the Target Framework for both projects match. If you are linking something that targets the Client Profile from a full version of the Framework, you will also get a 'not found' error:

Go to warning section and resolve all warning and you are done...

The warning section will tell you what all internal dlls dependencies are needed for the project you are referencing to.

I know this isn't the answer to your issue, but the error is quite similar when you are trying to reference a project with a higher .net version than the one you're using. IE: you can't reference something with .net 4.5 from .net 3.5

Basically, this sounds like a missing reference.

Some sanity checks I can think of are:

1. Are you sure that the project that generates the error is C?
2. Are you sure you are did not make a spelling mistake in the namespace B in your using?
3. Can there have been some compilation error in B before compiling C? (That may cause the compiler to fail finding the namespace in B).
4. Do you have any other compilation error or warning?

Edit

Another suggestion: is the class in the B assembly defined as public?

I got this when updating a project that we normally use via NuGet. I thought if I simply copied the updated built dll over to the packages folder I could test it without having to setup NuGet on my machine, but it wasn't that simple because my app was still looking for the old version number. Hope that helps someone out there.

After many hours of frustration, I discovered the following process to resolve this issue with a VS2017 solution:

Insure that all reference assemblies have been recognized and have current properties.

If assemblies do not show proper reference, right click the entry
and view properties.  This action often resets the reference. This
action must be completed for each project in the solution.

After resolving all references, if the error continues, delete the
following:

        -The Obj folder

        -The Bin folder

        -Reference to the offending assembly

        -Clean and Rebuild the solution.  Errors should occur.

        -Re-reference the needed assembly.

The editor should no longer show the namespace error and build should succeed.

Create clean project and test minimal sets of assemblies you use in your project. This way you will be sure if there is something bad in your solution or if newly created project has same symptoms. If so, then maybe VS, .net etc is corrupted or something.

I started getting this error suddenly while trying to solve another problem

I solved this by going to Solution=>properties=>project dependencies and all the dependencies were off for the two projects I was getting a namespace error for. I checked the check boxes and rebuilt the solution with no errors.

I solved this using global::[namespace][type I want to use] in C# 6.0

With VS2017, this issue came up for me when the project in my solution was unloaded.

In my case, I have to check where the "WorkFlow"¹ was implemented. Hence, I compare the framework version of the projects/class libraries that uses this "WorkFlow". After check that all projects/class libraries uses the same framework, I have to search ".WorkFlow" in the project/class library that was causing the builing error.

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Workflow.Targets(121,5): error : The type or namespace name 'WorkFlow' no exists in the namespace 'Proyect_to_build' (are you missing a using directive or an assembly reference?)

It turns out that the .dll that contains "WorkFlow" was missing in the "Reference" folder. Once added the .dll, the project/class library compiled successfully.

Again, in my case, I wasn't using this .dll and I only need compile the project/class library for enable breakpoints in a certain part of the program (where "WorkFlow" is not involved at all), but well, after add it (the .dll with the "WorkFlow" source code), it compiled.

¹ "WorkFlow" comes from a legacy code using custom code for WorkFlows.