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Use of different using statements using #if conditionals with two UWP projects

I am currently working on a system with multiple target platforms.
In my solution are two UWP platforms that share a Shared-Project. Now I have to tell both of these UWP projects which using statements they are supposed to load.
I am thinking about using #if conditionals in the Shared-Project like this for example:
#if UPW_Project_Client
using some.namespace.client.a;
using some.namespace.client.b;
#endif
#if UPW_Project_Server
using some.namespace.server.a;
using some.namespace.server.b;
#endif
Yet I did not find a suitable solution and would appreciate any help to get this problem fixed.

I can't say if this is the best way to solve what you are trying to do, but you can do this the way you want to with conditional compilation symbols.
If you have this in your shared project
#if UWP_Project_Server
using some.namespace.client.a;
using some.namespace.client.b;
#endif
#if UWP_Project_Server
using some.namespace.server.a;
using some.namespace.server.b;
#endif
Then in your server project, right click -> properties, then go to the build tab. Make sure the configuration is set to All Configurations. In the "Conditional Compliation Symbols" fields, just append UWP_PROJECT_SERVER. Needs a semicolon per symbol too.
Do the same for the client project but with UWP_PROJECT_CLIENT.
You can toggle the context of the shared project when viewing the files. Use the combo box near the top of the editor.
Client context
Server context
This is just my test project, ProjectSpecificImplementation is defined in both my Client and Server UWP projects.

Preprocessor directive cannot be put in AND in C#

I need to put some code under a preprocessor directive.
such directives can be defined in different projects.
My situation is the following:
File Constants.cs (which is in project Proj1Dll.csproj)
#define DEV
... rest of the code
File Page1.cs (which is in project MainProj.csproj, which references Proj1Dll)
#define SHOW_BTN
...
#if (DEV && SHOW_BTN)
public static void Foo()
{
Debug.WriteLine("Both DEV and SHOW_BTN directives are defined");
}
#endif
Such configuration is not working, in the sense that method Foo() is not part of the compilation.
If I check instead #if (DEBUG && SHOW_BTN) everything is fine.
Is there a way to deal with such directives when they are defined in different projects?

Assuming you are using Visual Studio, you can go to the Project Properties, and under the Build tab, you will see 'Conditional compilation symbols'
Put your synbols in here and they will be recognised project wide.
In your example you would put just DEV if you wanted more than one, separate them with semicolon e.g. DEV;SHOW_BTN
If you want something to be solution wide and need only one symbol you can use the configuration manager to create a new configuration specifically for this build and call it what you like - this then gives you the same effect.
(It's why in your example DEBUG works)
Hope that helps.

Removing the code with preprocessor in .net

I have .net C# application. In the application have two set of code for different client.
We were thinking of removing the part of code through preprocessor. Diabling the part with config file parameter is not an option for us.
We want simple setup like:
#define DEBUG
//....
#if DEBUG
Console.WriteLine("Debug version");
#endif
The only issue is, our part of code is distributed into multiple files and multiple projects in the solution.
So we want to define globally the preprocessor “DEBUG” at one place. Preferably in project property or something.
What is the best option for us?

Look for "Conditional Compilation Symbols" on the "Build" page of the project property dialog. You can set it per-build configuration.

VS2008: Multiple build configs with defines don't work as expected

I have a base library to maintain in multiple versions. I do a SVN switch whenever I need to work on another version.
I don't have multiple versions of my test application solution, so I thought that for different versions I could do multiple solution / project configurations that define symbols for the version to be able to have version-specific code in my test.
Currently I have the following build configurations in the test application solution: Debug, Release, DebugV10, ReleaseV10, DebugV15, ReleaseV15. In the *V10 and *V15 configs, I created and selected corresponding *V10 and *V15 PROJECT configurations for the two projects that have version-specific test code (not for all projects, most run normal Debug / Release configuration in the solution -Vx configuration).
In those project configurations I entered the corresponding conditional compilation symbols (VERSION10 and VERSION15).
Now in my code in the project I go like
#if VERSION10
// do v1.0 stuff
#elif VERSION15
// do v1.5 stuff
#else
// do trunk stuff
#endif
But apparently VS doesn't recognize the symbols. Even a simple #if DEBUG does not work anymore, allthoug define DERBUG constant is checked in all Debug* project configurations.
Is this a known thing? What can I do about it?

The concept that you describe sounds fine.
That is, the code uses:
#if VERSION10 ... #endif
and the Project configuration defines VERSION10
and a Solution configuration is set up that is set to use the above Project configuration.
As long as these are all set up correctly, I'd expect it to work.
The fact that you say even #if DEBUG isn't working as expected suggests that something is very broken.
I'd suggest you try the simplest possible case, to ensure you undersatand how to set it up: Create a new minimal "hello world" application that just has simple code that allows you to tell apart the versions:
#if VERSION1
Console.WriteLine("Hello from version 1");
#else
Console.WriteLine("Hello from version 2");
#endif
Then create project configurations ("Debug Version 1", "Debug version 2") and see if you can set them up (one with VERSION1 defined, one without) to get the two outputs when built.
Then add Solution configurations that use the above Project configurations, and build them to check that they print the right things when executed.
Once you've done this you should have a working system (an understanding of how these elements relate to each other) that you can apply back to your more complex project. To apply them back, I suggest deleting most of the project/solution configurations and then rebuilding them from scratch now that you are sure you know what to do - often rebuilding these things from first principles works out better than trying to tweak an existing 'broken' setup back into life.

Conditional compilation in C# is different from C and C++. See here for info on the ConditionalAttribute class.
For example:
doDebugOutput(); // unconditionally call the optional code
Conditional["DEBUG"]
void DoDebugOutput()
{
// do expensive debug-only output here
}

How to set project wide #define in C#

I have several classes in a project which need to only be in certain builds of the application which are currently not ready for release or debug.
To prevent these classes from being used, I want to set around them this:
#if USE_MYCLASS
// Code here...
#endif
Unfortunately, I don't know how to setup a project-wide #define.
Is there functionality in Visual Studio to set project-wide definitions?
If there is, though I don't need it right now, is there a functionality to set solution-wide definitions?
If there is no functionality for such (seeing as C# does not have include files, I suppose it's possible), is there any method or plugin of doing this functionality without using the command line compiler and /D?

You can do that in the project properties, but not in source code.
Project Properties => Build => Conditional compilation symbols
You can specify whichever symbols you need (space delimited, but IIRC is is quite forgiving). Note that DEBUG and TRACE can also be toggled with a checkbox.
I have some projects with multiple "release" build configurations, with different symbols in each (for building 2.0 vs 3.0 vs 3.5 versions - see <DefineConstants> here)