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How do I build multiple "targets" or "flavors" of an app using Visual Studio?

I need to build three completely different flavors or targets for my C#/XAML UWP app. Each of them will get placed in the Windows Store using a different name and will have different branding on it. Essentially, I produce three apps that have very similar features but need to be three different apps for business reasons. There is almost zero difference in the source code for these variations.
In Visual Studio, I created three folders and each contains the icons, manifest, certificate files, etc., for the target apps. I use a pre-build event to detect the configuration that I selected for the build and copy the appropriate files to the root of the project. Note that instead of "release" and "debug" builds, I have "release1", "release2", etc. and my configuration folders use matching names.
This all worked great with Visual Studio pre-2019! When I first set things up this way, it worked and was the easiest way I could find.
Visual Studio 2019 validates the element in the project file and my build process no longer works. If I make this empty, something during the build process sets it. I think it gets set when I create app packages for the store and is otherwise untouched.
What is the better way to build multiple targets or flavors of the app? Alternatively, how can I avoid this problem (and it's a problem since that value doesn't actually help me anywhere!)
So far, I have the way I do it now and I've also considered creating separate projects for each flavor with each project using all of the code from a shared project. In fact, I have a shared project that contains code that was once shared with the Windows Phone app that was part of the same solution. I just don't know if that multiple-project solution will work or not and it's hard to justify to the boss the day or two it might take to change the project structure.

Solution Output Directory

The project that I'm currently working on is being developed by multiple teams where each team is responsible for different part of the project. They all have set up their own C# projects and solutions with configuration settings specific to their own needs. However, now we need to create another, global solution, which will combine and build all projects into the same output directory.
The problem that I have encountered though, is that I have found only one way to make all projects build into the same output directory - I need to modify configurations for all of them. That is what we would like to avoid. We would prefer that all these projects had no knowledge about this "global" solution. Each team must retain possibility to work just with their own sub-solution.
One possible workaround is to create a special configuration for all projects just for this "global" solution, but that could create extra problems since now you have to constantly sync this configuration settings with the regular one, used by that specific team. Last thing we want to do is to spend hours trying to figure out why something doesn't work when building under global solution just because of some check box that developers have checked in their configuration, but forgot to do so in the global configuration.
So, to simplify, we need some sort of output directory setting or post build event that would only be present when building from that global, all-inclusive solution. Is there any way to achieve this without changing something in projects configurations?
Update 1
Some extra details I guess I need to mention:
We need this global solution to be as close as possible to what the end user gets when he installs our application, since we intend to use it for debugging of the entire application when we need to figure out which part of the application isn't working before sending this bug to the team working on that part.
This means that when building under global solution, the output directory hierarchy should be the same as it would be in Program Files after installation, so that if, for example, we have Program Files/MyApplication/Addins folder which contains all the addins developed by different teams, we need the global solution to copy the binaries from addins projects and place them in the output directory accordingly.
The thing is, the team developing an addin doesn't necessary know that it is an addin and that it should be placed in that folder, so they cannot change their relative output directory to be build/bin/Debug/Addins.

The key here is that team is responsible for a deliverable. That deliverable is a collection of binaries. So the "global" solution ... or "product that uses the deliverables from teams" is interested in ensuring that all of the 'current deliverables' work together. That is, that you have a deliverable from the collaborative effort.
So this begs a few questions. Do the team deliver what they consider to be a 'release'. This may be automatic in the build system. If it builds and all tests pass then publish it.
What you are looking for is a team publishing or promoting a release. The source code is how you got there, the binaries are the result. Each team controls what binaries it considers to be a release (this may be automated by the build system).
Not exactly what you asked, but I hope it is the answer that leads to the right questions to give good results.

One very simple way would be to create the solution. Include all the projects and add a project (or more) to handle the global solution build tasks. The projects in the global solution should then have a reference to the projects they need and then let Visual Studio handle how to get the binaries from each project. They will (under normal circumstances) be copied to the output folder of the build project. So the project added specifically for the global build tasks would have a copy of all the referenced projects
Another way would be to create a global MSBuild script that references the rest of the build scripts. Each project is on it's own a MSBuild script
EDIT
From the comments it would seem that there are two categories of projects. One that needs building and one that does not.
For those that need building reference them as projects in the aggregating project for those that do not require building add them either as references or add the dll as resources.
Using the later change the property of the Build action to None and copy to output directory to Copy if newer
In both cases you now have all dll's in the output directory you can then have a post build action on the aggregating project moving the dlls that should be in a specific folder (ie not in the output folder)

Have a look at the practice of Continuous Integration and the usage of a Build Server with scripted builds. This is an indispensable instrument when developing different parts of an application as a team, and your problems are a great illustration of the reason why.

You've not mentioned if you use a Version Control system. I've found in practise that each developer maintains his/hers/their teams configuration and builds locally on there machine, since you don't check *.suo or *.user files most of the personal configuration only affects the individual team member.
On a completely seperate machine check-out the same code from all repositories and compile the project on the build machine (this can be completely automated). This maintains your build servers independance.
Don't worry about it being a "Solution". You can easily build multiple solutions one after the other.
Since the output path is relative (and probably "bin\Debug") it'll get built wherever you check it out to. If you want all the binaries in the same output folder you could tweak the output path on every configuration to match. Something like "....\bin\Debug" (obviously this affects where the projects get built to on the local machines but it might not matter). That way multiple projects would get built the same target output.
You could also include a seperate setup build on the build server which isn't on each developers local machine to package up the final product.

Ensure required install actions for a dll are executed without duplicating code

I have a c# solution with two regular projects and a setup project. One of the regular projects is an executable, while the other is a dll, that I also use in other solutions. The dll project relies on there being a certain event log source, that it can log to, and since the program is intended to be run by users that are not allowed to create log sources, this source must be created at installation.
I have done this by creating an installer class for my executable project, creating the log source in the installer, and included that installer in my custom actions in the setup project. This works, but now I have to create a similar installer for every other project, that also uses that dll.
The best solution would be, if I could write an installer for the dll, and then choose the dll for the custom actions in the setup project. This way I would only have to state the log creation requirement once. However, I am not able to select the dll project output for the custom actions in the setup project.
Another good solution would be, if I could somehow specify that the installer for the executable should be transitive, such that it would also perform install actions for any projects that the executable project depended on, but I don't know how to specify that requirement.
So what can I do to avoid duplicating installation code between different projects?

You should be able to add an installer class to your DLL then register the DLL for execution of custom actions in a setup project. If you have tried this and encountered problems, could you please be more specific about which version of Visual Studio and which type of setup project you are using?

I just have a MyApplication.Installation assembly where I put a custom action that creates the event source. All my setup projects reference this assembly and invoke its custom action.

How about this? You create a simple batch file or a powershell script to create the log file that you want to create.You could make an installer for the dll file(or even the entire solution it doesn't matter.) You can then invoke the batch file that you just wrote from the installer.[Refer here] . This way, you are not duplicating the creation logic for a dependent files/resources; and you can use the same batch file for multiple setup projects basically(provided they use the same resources.)
I hope this answers your question.
One step further, what environment are your clients on? Are they still on Win XP(SP2 or before)? If that is the case, you have to do something similar to what you already have in mind right now. However, if that is not the case, if your clients are on Win 7, You could use nuget to publish your bins(Refer here). I admit that this is still looked at as a source code sharing solution. But I believe that the approach can be extended to publishing binaries too.

Is it possible to compile a console application into a single .dll file?

There are 5 console apps working off each other's outputs and are separately installed on my computer (C# 4.0, I am the author) . Management would like to distribute this suite of apps to other users, but aren't thrilled about asking non-tech users to install/configure 5 separate applications. Is there any way I can compile each program down into a .dll and reference them through a single master application?

Q. The main issue seems to be that you don't want 5 separate installation steps?
A. Make an installer for the suite :) Use any MSI builder (WiX, Visual Studio setup projects, InstallShield, and many others; Heck, you could even do an XCOPY deployment in most cases)
Q. How do I directly invoke these programs from within a single process?
A. Options:
Load the assemblies in your AppDomain.
Use a separate AppDomain in case of name(space) conflicts or version conflicts
Q. How do I optionally 'hide' the presence of the external console apps from view
A. Look at ilmerge to possibly combine the 'external' assemblies so they aren't visible anymore. If you can't use ilmerge (conflicts, or e.g. WPF apps) you might embed them as resources and load them on demand as per 1. and 2. above
Update: https://libz.codeplex.com/ is a nice looking project that makes this easy (haven't tried it myself)

Just because each of them is a separate .exe file doesn't mean you can't treat them as one application. And they don't have to be installed or configured separately either.
But a much better solution would be to rewrite each of the applications, so that they expose classes or interfaces that can be used without actually running the application. This way, communication between the parts is going to be much easier.
In .Net, the only difference between .exe and .dll is that you can run .exe directly. But you can treat both as libraries, so you can use functionality from one .exe in another .exe. Another step might be separating the core of each application into a .dll and make the .exes just deal with input and output. With this, the combined application wouldn't have all the code that it doesn't need from the other ones.

Its possible if every assembly is using different class names. Just include the whole source code when you compile the final version in one project.

Go to Project's properties, Application and change OutputType from Console to Class Library.
EDIT
Would like to express my doubts on architectual desicion like this, correct me if I'm wrong in my thinking:
Having different EXE applications standalone, I presume, you have different Applications that works standalone.
What advantage you gain by converting them in DLL's and puting them together in one master app ? Why do not just use this EXEs with one master app and launch them ?
In this way you leave as is it already working + you add a layer (master app) so for final user all this seems like one single app.

That is possible - several options:
you put the functionality of each console app into a separate class within the same project and have one "master console app" provide their functionalities
you put the functionality of each console app into a separate class each in different project with DLL as target, then you reference those DLLs as needed from your "master console app"
Note: IF you go the DLL route you could embed the DLLs into the console EXE using the technique from http://blogs.msdn.com/b/microsoft_press/archive/2010/02/03/jeffrey-richter-excerpt-2-from-clr-via-c-third-edition.aspx
With both option (all in one EXE or EXE + embedded DLLs) you can just make an XCOPY deployment if there are no other dependencies...

How do you package external libraries in your .Net projects?

A lot of my projects contain the Castle/NHibernate/Rhino-Tools stack. What's confusing about this is that Castle depends on some NHibernate libraries, NHibernate depends on some Castle libraries, and Rhino-Tools depends on both.
I've built all three projects on my machine, but I feel that copying the NHibernate/Castle libraries is a bit redundant since I built Rhino-Tools using the resulting libraries from my NHibernate and Castle builds.
Right now, I include all projects in seperate folders in my /thirdparty/libs folder in my project tree. Should I simply just have /thirdparty/libs/rhino-tools in my project and use the Castle/NHibernate libs from there? That would seem to make logical sense in not duplicating files, but I also like having each project in it's own distinct folder.
What are your views on this?

This is one of the problems that we're trying to tackle in the Refix open source project on CodePlex.
The idea is that Refix will parse all the projects in your solution, and before your project compiles, copy the necessary binaries from a single local repository on your machine into a folder within the solution tree and point the projects at them. This way, there's no need to commit the binaries. Your local Refix repository will pull binaries from a remote one (we're setting one up at repo.refixcentral.com), and you can set up an intermediate one for your team/department/company that can hold any additional software not held centrally.
It will also try to resolve conflicting version numbers - Visual Studio can be too forgiving of mismatched component version numbers, leading to solutions that compile but fall over at run time when they fail to load a dependency because two different versions would be needed.
So to answer the question "how do you package external libraries in your .Net projects", our vision is that you don't - you just include a Refix step in your build script, and let it worry about it for you.

I use a folder for each, which seems to be the convention.
Does it really make a difference if you're copying them?
What if you want to switch one out? Let's say you go with a new O/R mapper. It will be much easier to just delete the NHibernate folder than to selectively delete DLLs in your Rhino-Tools folder.
Take this to it's logical conclusion and you won't have any folder organization in your lib folder since everything uses log4net :)

Add additional probing paths to your app.config files to locate the dependency dlls. This way your can get away with having just one copy of everything you want. Though there are some quirks to using this feature (you must create the folder structure in a certain way). Look here for more details on the tag.

I will definetly recommend having a thirdparty or vendor folder in each of your project trees. If you find it annoying to have 32 copies of the rhino-tools package, you can have a single copy of it in your code repository, and do external references to it in your project tree.
Lets say you are using SVN, you can make a repository called "thirdparty libs" and in this have versioned copies of the libs. You then make an external property on your "thirdparty"-folder in your project tree which then in turn automaticly will do a check out of your centralized thirdparty libs. This way you for instance only have to update in one place if a security or a bugfix comes out, but each project is still in command of choosing which thirdparty libs, and which versions to use.
About the deps internally in thirdparty libs, i wouldn't mind those. The first time you compile your project, and some of the libs arent copied to your bin-folder because of implicit dependencies you can add an external attribute into your bin-folder, which will then automaticly check out the missing libs. That way you still only have to update your thirdparty libs in one place.