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How to know what assembly to add C#

I've ran into this issue a couple times and I'm wondering if anyone has a better solution than trial and error or searching stack overflow.
Lets say we are using some .net class Foo
Foo resides in the Bar.Baz namespace
The following statement
using Bar.Baz;
is not sufficient to compile the program, we are missing an assembly reference. So add a reference to System.Bar.Baz It still doesn't work so after searching the internet I find that I actually have to add a reference to Some.Other.dll and now it compiles.
My question is how do I know what namespace maps to what reference when the usual one doesn't work?
Most recent problem was
The type or namespace name 'DbContext' could not be found Instead of adding a reference to System.Data.Entity I had to install through Nuget.

If it is a .NET framework function, you can just search it on MSDN, and it will tell you in which assembly the class/function exists.
You can also use ReSharper which is a very nice plugin to Visual Studio, and it can help you add assemblies automatically.

If you're using Visual Studio 2013 or higher, one easy way to discover which namespace a class belongs to is using the Peek definition feature. You can easily find it in the right-click context menu.
In the screen below, I used it with KeyValuePair:
Also, take a look at the documentation.

Weird: C# Type or Namespace name could not be found - Builds successfully

I have a weird error showing up in my project when it is open in the VS2012 IDE. Everywhere where I make use of another referenced project it suddenly says "Type or Namespace name could not be found". And by "says", I mean it has the text underlined in red with the error when I hover over it. The intellisense doesn't work for that code. BUT (and here's the weird part), the errors do not show up in the error console and the project builds and runs fine.
I can even debug and step through the code and it works perfectly fine. So at runtime the project is referenced fine but at design time the IDE can't find it. This worked for the past 2 weeks, and only then suddenly went a little bonkers. It's really annoying because I am rubbish at coding without intellisense!
Has anybody ever seen anything like this or have any suggestions?

I had this. I referenced assemblies whose "Target Framework" were set to ".Net Framework 4" in the "Application" secion of the projects properties. I changed this to ".Net Framework 4.5" not just in the referenced assembly's project but also the project I was building and it worked. Give this a try.

I have found this is a known problem with VS2012. Check to see how you are building, 64 bit or 32 bit. It won't work with 64 bit but it will with 32 bit. It will say things are missing and design will not work, however the program will run fine. I have heard the new VS update that hasn't been released yet will fix it.

Manually delete all the references to the libraries of the other projects and re-add. Intellisense rebuilds whatever it needs at that point and doing this has helped me in the past.

Another source of this problem is a solution with multiple projects containing code for the same namespace. The compiler can handle this. Intellisense won’t.

Related to a couple other answers here, I had a "Data" project using a "Data" namespace. Built fine, but just started recently showing errors from intellisense (even though it continued building fine.) (VS 2015.)
I fixed this by changing my "Data" project and namespace to "MyCompany.Data".
Oddly, the problem didn't seem to show up until recently, but making the change did fix it. Presumably there was a conflict in namespace with another project or reference, which can build fine, but intellisense can't handle.
You can change this in the Application tab of Project properties. You can also open up an EDMX diagram, right click, choose model browser, choose the second collapsable item in the model browser tab, hit properties, and there you'll find the Namespace option for generated Entity Framework entities and contexts. (Similiarly, if you modify an Entity Framework Model's namespace, you might also need to change the related connection to match [in Web.Config for ASP.Net and MVC.])

Make sure that there is no class with the name same as Project default namespace.

make sure that you don't reference .net framework 4.5 projects in .net 4 projects!

Examine your .proj files in a text editor and make sure the paths to your references are correct.

This can also happen if you set the Build Action to "None" on the referenced file and forget about it.

I had the same problem, where it would build and run fine, it just would always show that error and I couldn't use intellisense with the class.
I actually used the automatic method of creating the class in a new file to resolve the issue, then just copied the code over from the real class. I deleted the old file, renamed the new file, and now it works.

Namespace errors when converting a project to VS 2010

So I'm a fledgeling programmer that's been assigned to get to know an application that's been in development (in VS2005, then in 2008) for quite some time. Over the life of the project the naming conventions therein have become somewhat convoluted. For the sake of example, here is a piece of the naming structure.
_MegaCorp.sln
Application_Install
Megacorp
DbResources.csproj
Forms
Megacorp.Forms.Concept.csproj
Properties
Resources
Megacorp
Forms
Concept
xyz.cs
It's maybe not the best way to organize information, but it works fine in VS2008. Updating to VS2010 shouldn't necessitate an overhaul of the existing code, right?
What's happening is this: when xyz calls for DbResources, visual studio is looking in Megacorp.Forms.Concept.Megacorp, not in the parent Megacorp folder. All the proper references and using statements are in order: as I said, everything works fine in VS2008. What's happening here?
Error 826 The type or namespace name 'xxx' does not exist in the namespace 'MegaCorp.Forms.Concept.Megacorp.Forms.Concept' (are you missing an assembly reference?) C:\x\y\c-sharp\Megacorp\Forms\Concept\ConstructEditor.Designer.cs 99 63 MegaCorp.Forms.Concept
UPDATE: If I add global:: before each use of a class in xyz.cs, this resolves the problem. However, there are about 1,000,000 lines of code, and over 400 errors reported by the compiler. I'm sure that if I combed through each of the 400 errors, pasting global:: before each one, more errors would arise. There must be another way to do this, because again, the solution comprises a very functional and in-use application compiled in VS2008.
Thanks for your help!

I had similar issue and found out that it is related to Target Framework. Check if all the added references are of solutions for same Target Framework.

Class Library Intellisense not showing up after adding the dll to the references

In C#, I made a ClassLibrary that has one Namespace and one Class.
I saved it and build it.
in other Projects, when i use it, I add it to my references by browsing to the .dll location.
But The Problem is that its name is not showing up in the Intellisense.
i.e when I: using ... my dll doesn't show ..
I'm Importing the library to a ConsoleApp.
both of the App and the library target Framework is .NET Framework 4.0
and I made their Assembly Version 4.0.0.0 so they're the exact same.
is there a setting or something that I'm missing ?
how can i make it pop up ?
I'm using VS2010 Professional
Thank you for your help

Maybe this be usefull, I was having a similar issue, I have a Web project, add the reference to a Class Library by selecting the project, but if I made a change on the class library, I can´t see that change on the intellisense of the Web Project, after try many things, I see that in the recently added reference, the value of the option "Local Copy" was set "True", then I change it to false and everething works!

I had a similar issue but in my case it was a property on the class. If you go to the file properties and look for a Build Action. Somehow mine was set to Content it had to be set to Compile.
I am using Visual Studio 2013. I hope this helps someone else.

Is the namespace for your assembly different than the namespace for your currently open project? I've had times when the current project and an assembly share the same namespace path the intellisense can mess up.
In general, Visual Studio is pretty good about intellisense generation, especially for C#. But sometimes there are some interesting conditions regarding ambiguities, and especially mixing project types where it just doesn't quite work.
Placing your content in the same namespace makes me wonder if you've actually fixed the problem (it may just be autocompleting the namespace in the currently loaded project rather than the assembly), but if it allows you to continue working, then go with it!

Right click on project on which you add reference of your dll/project select menu project dependancies and select/MarkCheckBox for reference project/dll. then it will work fine.

If the class library project had its name changed after creation, then intellisense may fail finding it due to directory issues, I believe.
I created my class with the generic "ClassLibrary1" or whatever, and then later changed the default namespace, class name, and project name inside of VS2017. I closed VS2017 and changed the directory name to match my default namespace, and then re-associated the project file in VS2017, and then re-added the reference in my main project file.
All seems to be fixed now.

the type or namespace name could not be found (Reference present)

After I build a few of my references, which are present in the project and accessible through intellisense before the build is performed.
Unfortunately none of my projects are set to target Client Profile, as answered in this thread.
The type or namespace name could not be found
This seems to be the most common cause, does anyone know what else other than this can cause the same issue?
Edit:
The Types that aren't found are all from the referenced DLLs. I have the correct using directives for these and they are present in the resource folder, however they become underlined red after a build (I did try cleaning to).
My initial thought was the framework (the project is on v3.5), but I checked all my resources working or otherwise all where on v2 bar a few (working ones) on v4 which I removed temporarily to make sure we're causing issues.
Unable to access the project from my current location but will post any requested specifics as soon as I can.
edit:
Resolved the issue by deleting all of the custom class libraries... found and rebuilt each of their projects then added the new DLLs back into the project. Stored libraries weren't very organised, so I probably have some confliction between old/new versions.

Have you read the warning in the error list? You might need to install nuget package used by the referenced projects
for me the error warning was
All projects referencing "project file" must install nuget package Microsoft.Bcl.Build.

Are the projects being built correctly ?
Sometimes, you need to manually generate them (right click on them, generate), before being able to run a full build.
I dont exactly know why, but this might be the way to go for you.

~A few simple things to try before getting into more complicated water...
Try clicking:
Build->Clean Solution
Build->Build Solution
If this doesn't work, check your using statements are all there and correct, check your references all still exist.

I had a similar issue today, and I thought I would mention it here in case it helps someone else out. In my case, I have a VB.net Class Library which is referenced by a C# class library. The solution would build fine, but in the IDE, as soon as I started editing a file in the C# project, I would get errors about not finding a reference to the VB.Net library. It turns out that the VB.Net class library had a reference to System.web (lowercase w). I'm not sure how that came about -- but the solution was to delete that reference and replace it with a reference to System.Web (uppercase W).

Make sure all the projects in your solution have the same "Target framework" value