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

Question

I downloaded a C# project and I wish to debug the project to see how an algorithm implementation works.

The project has come in a Folder, inside this folder there are -

.sln file and
a folder which has source files and a .csproj file.

I installed Visual Studio and opened the .sln file present in the main folder. I built the project successfully, but when I try to debug the project I get this message:

A project with an Output type of Class Library cannot be started directly In order to debug this project, add an executable project to this solution which references the library project. Set the executable project as the startup project.

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

What should I do to get round this hiccup?

score 211 · Accepted Answer · edited Jun 19 '17 at 18:13

211

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.

edited Jun 19 '17 at 18:13

charwayne

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answered Jul 29 '10 at 13:58

Eugene Cheverda

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And don‘t forget to set the newly created project as startup project (Right click on the new project↦Set as StartUp Project) – bugybunny Oct 05 '18 at 07:28
@bugybunny you comment is whats missing in the answer above! – whisk Apr 23 '20 at 20:15
I'm baffled! I created a new solution containing a class. I then added a Unit Test Project to the same solution, added a Windows Form to the test project, added references to the class in the unit test. I then right-clicked and set the test project as the startup project. The solution properties say that the unit test project is the single startup project. When I try to debug, I still see the same error message that the class can't be started directly. What am I missing? (VS 2017). – Robert Cody Feb 27 '21 at 22:16
Ah -- I found my error. I had to set the application type for the Unit Test Project to a Windows Forms on the Application tab for the unit test project. – Robert Cody Feb 27 '21 at 22:20
I've also created a new Console App project in my solution to stop this error from showing, and it does stop it, but I want to use the DLL from the Class Library in another project/CMS and uploaded it there but what I did isn't available...the DLL doesn't seem to have been built at all. – Fab Jan 09 '23 at 19:40

score 138 · Answer 2 · edited Apr 16 '14 at 14:41

138

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.

edited Apr 16 '14 at 14:41

Halil

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answered Nov 04 '11 at 04:43

Suhail Pawane

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This only works if the project's Output Type is not Class Library though right? – Devon Apr 30 '21 at 19:26
right-click on the Project Solution > select properties > A window pops up(solution property page), now you can see common Properties – pedram Oct 19 '21 at 22:51

score 89 · Answer 3 · edited Sep 18 '18 at 15:51

89

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

edited Sep 18 '18 at 15:51

kenorb

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answered Dec 11 '15 at 15:09

Priyank Desai

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This worked for me also in VS 2015. It had to build me a new web.config, and then it was up and working again. – Eric D. Johnson Feb 04 '16 at 18:45
Worked for me in Visual Studio 2019 as well. – user0474975 Sep 11 '19 at 15:21
right-click on the Project Solution > select properties > A window pops up(solution property page), now you can see common Properties – pedram Oct 19 '21 at 22:51
that times when right click menu wont take entire screen height to display :) – Alexander Selishchev Nov 17 '21 at 07:45
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This worked for me as well. After upgrading from VS2019 to VS2022, suddenly got this error. Thanks. – Yeshurun Kubi Jul 19 '22 at 20:21

score 22 · Answer 4 · edited Oct 21 '12 at 13:29

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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.

edited Oct 21 '12 at 13:29

Stephan

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answered May 21 '12 at 10:22

Ashish Agrawal

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score 12 · Answer 5 · answered Jul 29 '10 at 13:47

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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.

answered Jul 29 '10 at 13:47

James Curran

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Hey James, Okay I have added a new project in the same solution. A new .cs file is created which has a main function. Now what should I reference to. To the dll you mean? When I right click on the references tab, I get a Add Reference window. – HaggarTheHorrible Jul 29 '10 at 13:55
Navigate to the Projects Tab. There will be the set of assemblies defined in your solution available for referencing. – Eugene Cheverda Jul 29 '10 at 13:59
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Yes, exactly. Don't forget to add specific usings to have ability of using classes of that library. – Eugene Cheverda Jul 29 '10 at 14:08

score 11 · Answer 6 · edited Apr 21 '17 at 03:57

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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

edited Apr 21 '17 at 03:57

HDJEMAI

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answered Apr 21 '17 at 03:29

Chamara

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score 6 · Answer 7 · edited Jun 02 '16 at 06:20

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.

score 6 · Answer 8 · answered Jan 17 '18 at 03:42

    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)

score 5 · Answer 9 · answered Jul 29 '10 at 13:47

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.

score 2 · Answer 10 · edited Mar 22 '21 at 16:23

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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 .

edited Mar 22 '21 at 16:23

Alexxus

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answered Aug 09 '17 at 12:09

Saikat Banik

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score 2 · Answer 11 · answered Sep 13 '22 at 01:52

Accepted answer works if your solution has a project that compiles to an exe. If your solution does not have any projects that compile to an exe, then you have to use 'Start external program'.

VS2019 instructions:

right click -> properties on the main solution
debug, start external program, and add command line arguments

VS2022 instructions:

right click -> properties on the main solution
scroll down to Debug
Debug > General > Open debug launch profiles UI
left click the 'new' icon in the top left, select 'executable'
fill it out as per VS2019 (pick the exe and add command line arguments)
when clicking the start button, first select the profile you made

score 1 · Answer 12 · answered May 03 '17 at 06:22

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.

score 0 · Answer 13 · answered Jul 24 '18 at 01:06

0

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.

answered Jul 24 '18 at 01:06

user2063329

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score 0 · Answer 14 · answered Dec 17 '21 at 08:47

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.

score 0 · Answer 15 · answered Aug 01 '22 at 14:53

If the question involves an Azure project, make sure you have the "Azure development" tool set installed, or when you go to run a solution you may get this same error.

Tools > Get Tools and Features... > Tick the box next to Azure development > Click install

score 0 · Answer 16 · answered Nov 10 '22 at 15:25

None of the answers provided above helped me resolve this error, this is what resolved the issue for me.

Right click on the solution and select "Properties", which is in my case "Sintctech.Data".
Select the section called "Application".
Check what you have selected as your output type. If it is "Windows Application", change it to "Console Appication".

Rebuild and the problem should be fixed.

score 0 · Answer 17 · answered Mar 08 '23 at 20:24

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Right Click and choose properties on Library. Output Type select console application.

answered Mar 08 '23 at 20:24

Umit KOC

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

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