"A project with an Output type of Class Library cannot be started directly"

The project you have downloaded compiles into a dll assembly and provide a set of classes with implemented functionality.

You should add to your solution a new project with Output Type of either Console Application or Windows Application (VS Add Project wizard will offer you different templates of Projects).

In the newly added project, you can implement logic to test your Class Library.

Output type of the project you can find and change by the following steps:

Right click on project in Solution Explorer -> Properties. In opened tab with properties select Application and there will be ComboBox marked with Output Type label.

Just right click on the Project Solution A window pops up. Expand the common Properties. Select Start Up Project

In there on right hand side Select radio button with Single Startup Project Select your Project in there and apply.

That's it. Now save and build your project. Run the project to see the output.

This was the solution that worked for me since I couldn't find 'Common Properties' option.

Select your topmost level project in Solution Explorer. Go to Project, and in contextual menu Set as StartUp Project.

See also: A project with an Output type of Class Library cannot be started directly

Just needs to go:

Solution Explorer-->Go to Properties --->change(Single Startup project) from.dll to .web

Then try to debug it.

Surely your problem will be solved.

The strange part is that I don't see a main function anywhere.

That is exactly your problem. The project merely creates a DLL. It has no executable to run.

You will need to add a second project, which is an executable which references the other project, and calls something in it.

1) Right Click on **Solution Explorer**
2) Go to the **Properties** 
3) Expand **Common Properties**
4) Select **Start Up Project**
5) click the radio button (**Single Start_up Project**)
6) select your Project name 
7) Then Debug Your project

    Right Click on "Solution Explorer" -> "Properties"
    Expand "Common Properties"
    Select "Start Up Project"
    click the radio button "Single Start_up Project"
    select your Project name from the drop down list.

If still not working after the above steps, then try this.

    Expand solutions explorer.
    Right click on project name -> "Properties"
    Go to "Application" tab
    Select "Output type" 
 From the drop down list select the appropriate type according to your application.
    "Windows application" or
    "Console application"

Then save (ctrl + S)

Try debugging (F5)

You'll need some kind of app (Console Apps are my favorite for debugging, but a WinForm will do) which uses your Class Library. Just add a new project (in the same solution) of a Console Application or Windows Forms Application, and add a reference to your current project. Once you've done that, make any calls you need, set your break points, and go to town.

I had a similar issue when trying to use the Experimental Instance of Visual Studio 2013. This was for a vsix project (Creating Snippets).

Solution was:

Right Click Project in Solution Explorer > Properties > Debug

Setting the Start Action to "Start external program" and using the following path:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\devenv.exe

This option was set to "Start project" which won't work for the application output type Class Library, which caused that same error.

Note: Yours may differ depending on how you installed VS.

Error solutions is that you have already open your project but by mistake you have selected another class library .. that's reason this error is showing ... so what u need to do you u just select u r project then right click on u r project

after right click u can see the list box and select the "Set as start up project " option .

Suppose you have multiple project in the solution. Select the project that you want to view in browser and select 'Set as StartUp Project'. In your multiple project soln which was the main, the visual studio was unable to identify. this was the main problem.

You can right click the Class Library project and from the drop-down choose Initialize Interactive C# which will load your project context and you can work it in the interactive session.

In my case, the cause was that one of my projects in the solution wasn't loaded. The reason it couldn't load properly was that the file path length of one of the files was too long. Upon deleting this long file, I could reload the project, and build the solution.