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Unable to add SecuBSMPx.dll library in visual studio?

I am working with SecuGen fingerprint libraries (SecuBSPMx.dll and SecuBSPMx.NET)
in my project, SecuBSPMx.NET is added in assembly reference but SecuBSPMx.dll not added in Visual Studio. I have tried in many versions of Visual studio and separate laptops but error is same. Below is an error message picture is attached with this email.

I'm not sure if I understand you, but I supposed SecuBSPMx.NET depends on SecuBSPMx.dll. I had a situation almost like this recently. What worked for me, was copying the dependency dlls to /bin/debug folder where the runtime dll's are located.
This can also be achieved by placing the dll in the project folder, and setting the property "copy to output" to "copy always" or "copy if newer.
Also I had to physically remove the reference to the dll in my project and add it again

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

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?

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 .

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

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.

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

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.

Why won't the debugger update?

Ive had this problem multiple times and it ruins my projects, I make some changes, like say I have a button in the top left corner of the form and move it to the top right corner, then I press debug but nothing happens to the form, it doesn't change the button is still in the top left-hand corner instead of the top right, and it also doesn't except any new code, its like it saved the project right there and won't move on. Does any one know why or had this problem before?
Please, Help!!!!
Additional Details:
Compiler: Microsoft Visual C# 2008 Express Edition
I once fixed this problem by rebuilding the solution, but its never worked again.

Hey, I got it fixed, I right clicked Form1 in the solution explorer, clicked properties and change the Build Action to none then back to Compile and it seems to be working.

In the Visual Studio settings under Projects and Solutions->Build and Run change the option On Run, when projects are out of date: from Never Build to Always Build
Tools > Options > Projects and Solutions > Build and Run > On Run, when projects are out of date: "Always Build"

In addition to #SwDevMan81's answer with whom I agree, I would say that setting both the output and reference paths may help avoid such behaviour for class libraries. For instance, your application references a class library that you are currently writing, and you perform some changes to this referenced library, but the changes don't show.
What happens is that the compiler will copy localy (to the project's output directory) thereferenced DLL and as long as it is there, it won't get updated. You may verify it by clicking right on the referenced assembly, then clicking Properties. Look at the Filepath property. If you see it doesn't match your actual filepath, then you will have to make sure to set the reference path accordingly in the project properties, then removing then removing the actual reference to add it where the actual build is, that is, where your class library output folder is set. So, whenever you regenerate your class library, your application gets the update automatically. Here's an example:
Application Project references : The ClassLibrary1.dll assembly.
Once you will generate your application, the ClassLibrary1.dll file will be copied to your application output directory. Let's suppose C:\Open\Projects\ApplicationProject1\Debug\bin. So, this directory will now contain the ClassLibrary1.dll file.
You rewrite a method to behave completely differently;
You regenerate the ClassLibrary1 assembly;
You rerun your application (remember that the file already exists!);
Ends up wondering why the changes didn't take effect? That is because your application referenced the cached assembly within its Debug\bin folder.
To workaround:
Remove the assembly reference from your application project;
Go to the project's properties and click the Reference Path tab;
Browse to your ClassLibrary1 output folder, then open it;
Your Reference Path property is now set for this library, then re-add the ClassLibrary1 assembly to your application project;
Run once, stop running, and see if the Path property of your referenced assembly is still the same as the one in the project's Reference Path property;
You're done (if everything worked fine).
C:\Open\Projects\ClassLibrary1\Debug\bin\ClassLibrary1.dll
In the end, this might be the cause of your problem if your GUI Forms are part of a class library, and the solution I described should work.

SwDevMan81 and Tanner's answers didn't work for me.
However,
Build > Rebuild Solution
... did!

Had The Same issue. Build > Rebuild Solution also worked for me

Class Library Project File not compiling into .dll or debugging

in my solution:
i have a class library project that compiles into a dll.
i have a web project.
(i have multiple solutions with different web projects but the same class library)
one of the files in the class project (utilities.cs) - all of a sudden won't compile into the .dll
i had made a change to this file, but the change wouldn't show on the website. so i put a breakpoint in the .cs file, and tried running it... breakpoint didn't get hit! eventually i put a breakpoint in the .aspx file that called the function. then i stepped the debugger one line on, so it would step into the .cs file, except i got the error:
"This source file has changed. It no longer matches the version of the file used to build the application being debugged."
so the code in utilities.cs is not being updated into the dll. which is weird, because code in my other .cs files IS being updated (i tried changing a few lines) and IS able to catch the debug.
any ideas how to fix this? or even what the cause of the problem is??

Try:
Right click on Solution in Solution Explorer -> Properties -> Configuration Properties -> Make sure Build checkbox is checked for your class library.

You need to find out why the DLL will not build:
Look at the output window for any errors
Check the Errors window
Fix the errors and try building again.
The dll that you are trying to debug is from an older version of your code that did compile OK and the debugger pickes that up and is trying to debug against that - it is complaining because the code and the dll no longer match.
Try doing a Build -> Clean on the solution and then rebuilding to see the errors.

When You adding new references to Web project You must choose "Projects" tab:
Projects lists all reusable components created from local projects.
Add or Remove References in Visual
Studio
Add a Reference to a Visual Studio
Project in a Web Site
Always rebuild all projects in Release and Debug mode.

I did exactly what #nightcoder said but a simple change was required. Go to Configuration Properties(Right click on Solution in Solution Explorer -> Properties -> Configuration Properties) and select "All Configurations" in Configuration option.
This created the .dll for me

This happened to me after switching between branches in a project which have significant differences between them, I tried every solution from the web but non worked.
Finally, I had to delete my local project files and clone the project from the GitHub repo again, after that when I started the project, the project build worked without errors, and dll was generated.

Add codebase as reference instead of copy Visual Studio

This may be a ridiculous question for you C# pros but here I go. I'm a Flash developer getting started in Silverlight and I'm trying to figure out how to create a "codebase" (a reusable set of classes) for animation. I'd like to store it in a single location and reuse it across a bunch of different projects. Normally in Flash I would add a "project path" reference and then start using the code. My question is, how do I add a folder to visual studio so that I can "use" those classes in my project. I tried "Add > Existing Item" but that copied the files into my project directory.

The easiest way would to create a new ClassLibrary project and build it. This will output a .dll file in a folder you can specify in the project settings menus, which you reference from every project that needs it.
Also, you can copy this .dll into the /bin/ folder of your project - this will do the same thing for this specific project, but when you start the next one you can change some details in the codebase library without breaking the first project.

The solution described by Tomas (adding a reference to a dll binary) is the correct solution to this problem; better than referencing the source code and compiling it into each project.
But just for extra information, if you ever do need to add a source code file to your Visual Studio project without having it make a copy of the file you can use the following steps:
Right click on your project in Solution Explorer and select Add -> Existing Item.
Navigate to the location of the source code file and select it.
On the "Add" button in the dialog window there is a drop down arrow. Click this and select "Add as Link".
This will allow you to use this source code file in your project without having VS make a copy of the file.

In Solution Explorer, right-click on the project node and click Add Reference.
In the Add Reference dialog box, select the tab indicating the type of component you want to reference. (for instance for a class library a dll)
Select the components you want to reference, then click OK.