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How to fix Project after sitting it as Startup with Visual Studio

When I try to run my first Selenium script in Visual Studio it gives me the message "a project with an out put type of class library cannot be started delicately".
I already set the project as my Startup project.

You cannot use class libraries independently you need to include in the different project by giving their reference after building them. This might help Creating Class Libraries and Adding References to Assemblies

I don't know about the specifics of running a selenium script but this error comes when the output type of your project is not an executable.
If you right click on your project in VS, go to properties, in tab Application, you have a field called Output Type. In your project it is probably set to class library which cannot be executed. You need to change this to an executable type.

adding reference to class library

I have a class library lets call it UtilityLibrary.
I have a console application. So I right clicked on my solution and added an existing project (UtilityLibrary). I noticed that I can change the code of UtilityLibrary from within my console application.
The issue is if I had added UtilityLibrary to another application and the code had been changed it could causes issues. I was trying to avoid adding a dll reference so thought I would add a reference to my project however I am worried about the code being edited.
Have I added the reference to my project incorrectly?

If you where to check the Debug or Release folder if your console application you should see a UtlityLibrary.dll file, which would still mean that you are, in the end, adding a DLL reference (visual studio will do it for you).
Since you do not want to allow external entities to change your application, simply add a DLL reference instead.
Just to clarify, you can only change the source of one DLL from another application becuase visual studio can find the source files and makes them available to you.

No. You have done nothing wrong. And you can't just change the code of the compiled assembly. You are utilizing a feature from Visual Studio. Since you've build on this machine, and the PDB's are included, it can tell the assembly originated from that source code on your machine. So it allows you to edit it.
These edits don't come in the compiled assembly immediately, you have to compile it again in order to be in the assembly. (Note that you can extract the source code from any .NET assembly by using Refactor for example)

Adding references for code moved into a class library

From a solution, which I made in Visual Studio 2013, I want to transfer a method to a DLL (class library project).
When I paste the method in the DLL project, it doesn't recognize parts of the code and it's showing this error`:
"are you missing a using directive or an assembly reference?"
Because of that, the DLL can't be built. I am assuming, that a certain reference to the solution is required, but I am not sure how to proceed.
I know how to add a reference from a solution to a DLL, but I'm not sure how it's done the other way around or even if it's possible.

You can't add reference to EXE from class library (assuming EXE uses that class library) as it will introduce circular reference.
Usually you need to refactor all dependencies so class library either have them all or allow application to inject dependencies from EXE/other clients. In later case class library needs to define base classes/interfaces to allow such injection.

Yes, you need to restore the same references that the original project uses, if they are used in the code you want to move.
If you need to do this by hand (i.e. without tools like ReSharper):
Move the code to the new assembly.
For each namespace or type giving the error, find it in the Object Browser.
Locate the assembly containing that namespace and type, and add a reference to that assembly in your new project.
You may also have to add a Project Reference to the original project.

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.

Type or namespace cannot be found, when reference does exist

I have a wpf Application in which I am trying to reference a class library i have created.
I have added a reference to the .dll
And i have added the using statement to my file, and the intellisense actually sees the new namespace.
Then in my code I am able to create new objects of classes in my added .dll just fine. the intellisense sees all the methods ect..no problem, no errors.
when I try to build my wpf application, all the sudden I get the type or namespace cannot be found error on my added dll.
Then i get errors whenever i am trying to create objects from that .dll.
I don't get what is happening.. why does it work before I build, but when I build it decides it doesn't know where that .dll is i am referencing?
Also i have gone to that class library i am trying to add, and mades sure it builds with no errors.

The most common cause of this is that your .DLL targets the full .NET Framework, but the WPF Application targets the Client Profile.
For example, if your library targets .NET 3.5, make sure your WPF Application targets the full .NET 3.5 or 4.0 framework, not the client profile.

A few thoughts on this matter that you can try:
Check that a lower version is not calling a higher version assembly (e.g. .NET 3.5 project is not calling a .NET 4.0 assembly).
Clean -> Build (or Rebuild)
Manually delete bin/obj folders of both caller and calling projects. This forces everything to be built - the 'hard ' way. This may sound redundant, but has worked for me a couple of times.
Restart VS - sometimes, there is just no explanation.
Reboot - when nothing else works, give it a break and try again.

You can Copy any .dll files from other project,
two .dll files.
eg: A.dll,A.pdb
and Rename this two files to your project's name.
Then you can rebuild this project.

If you're getting this without a DLL - just a code file check it's set to compile. In Visual Studio 2013 I keep adding code files and it sets the Build Action (found under Advanced section of Properties of the code file to "Content" rather than "Compile". Change this.

Sometimes an explicit naming of the assembly which contains the namespace is required.
xmlns:myns="clr-namespace:MyNamespace;assembly=MyAssembly"
I don't know why it is sometimes required.