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CS0246 error notification on VS, but no error is shown in Unity

I need to build a custom inspector for my project, so after some research I manage to implement a simple class for testing. But a strange behaviour is bugging me, my custom inspector class is showing the CS0246 error when I use any of my classes, while in Unity, no error is shown. The project runs normally and my script does modifies the inspector as expected, but I would like to solve this situation and remove this error notification from VS. Other scripts doesn't show any errors, this situation is happening just with this custom inspector script.
This is an image showing what I described above:
EDIT - This is my ChampionshipInternalCreator class as resquested:
EDIT 2 - This is the alt+enter suggestions:
"ChampionshipInternalCreatorCustomInspector" script is at: Assets\Scripts\Internal data management\Data creation and testing\Editor
"ChampionshipInternalCreator" script is at:
Assets\Scripts\Internal data management\Data creation and testing\Championship creation
As far as I know, it should be working normally as documentation states:
I also notice that this script doesn't make VS point errors the way a normal script does. For example, if I produce an error, VS doesn't point the error but Unity does.

Thanks to #SeLeCtRa help through chat, the problem were solved.
For some reason the reference to the Assembly-CSharp were lost, which makes an small alert icon appear in the solution explorer, as you can see here.
Firstly, I had to delete the problematic Assembly-CSharp reference, then, with Analyzer option selected, navigate to the Project tab, then click on Add reference. This will open the Reference Manager window, where you should select the Projects tab and mark the Assembly-CSharp.
In my case though, this option wasn't available, so I had to click the Browse button and navigate to my project folder, there, if you search for "Assembly-CSharp", 2 files with that name will be returned. The correct one is at YourProjectFolder\obj\Debug directory. After mark this file, the problem is solved.

Did not find new element in the hierarchy for item

I am trying to create a simple page for a Xamarin app but am completely unable to proceed with the most basic of steps. I check out the branch from the project repo, and attempt to add a new file. When I do so (Right clicking on folder, Add-> New Item->Content Page) I get this error:
It does actually create the .xaml file but not the .xaml.cs file to go with it. I first tried to manually create the cs file and associate it but ran into increasingly mysterious errors. Today I created them outside of VS before booting it up and both files are now present but with the dotted icon instead of a regular one and the cs file is not nested beneath the xaml file.
If I try to open my projitems file inside VS to make the association I am given the error "Specified method is not supported". If I choose to 'Open With', no screen ever pops up allowing me to choose. After this I closed down VS and edited the projitems file in notepad again and linked the xaml and cs file. This looked as though it worked but when I add it to the main TabbedPage with a simple Children.Add(new MyReviews()); it is not recognized -> The type or namespace name 'MyReviews' could not be found.
These error happen if I make my own branch based on the master or try to add a new file to the master directly. No one else in the project is having these errors.
The only thing I really did differently from the rest is try to get the emulator working for android which apparently has a lot of compatibility issues and requires older versions of the xamarin plugins. I was not able to do this and ended up uninstalling and reinstalling everything twice since I didn't touch the installer software the first time and everything stuck.
Is anyone able to help me debug these issues or know why I am encountering so many at every turn?

I've been in the exact same situation. As it turned out, the rest of my team had added Universal Windows Platform (UWP) support to their VS2017 install, and I hadn't. Running the Visual Studio Installer and adding UWP fixed this issue completely.
Of course, you might not need or want to include UWP support in any of your Xamarin apps (I didn't). It seems most likely that it's just one or two support assemblies that are mssing or wrong without the UWP support, but I haven't found the time or energy to figure out any more details.

Unable to Implement CalendarContract w/ Xamarin.Android

I have been wrestling with the Android Calendar for a couple of days now and I'm at my wit's end.
Following the instructions found here:
http://docs.xamarin.com/guides/android/user_interface/calendar/
I have begun a new ICS project and added permissions to read and write to the calendar. According to the instructions linked above I should now be able to "interact with calendar data by using the CalendarContract class." However, when I input the first line of example code:
var calendarsUri =CalendarContract.Calendars.ContentUri;
I get an error saying that "The name 'CalendarContract' does not exist in the current context."
I have download sample code for the CalendarDemo located here:
http://docs.xamarin.com/samples/CalendarDemo/
I can open the project, manipulate the code, deploy it to my test device with the modifications that I have made and it works perfectly. The problem is that I cannot seem to get CalendarContract to "exist in my context" if I create a project from scratch. Even adding in the CalendarListActivity.cs into a new project won't work. This leads me to believe that there is something I have to set in Xamarin Studio, not via code and not indicated in their tutorial, that I have yet to find.
What setting or reference do I need to add into a new or existing project in order to use this feature? I've looked everywhere I can think of and cannot find the elusive switch, checkbox, reference or whatever is required. I've burned three days trying to figure this out and I have lost my patience.

I was able to reproduce the error you're having. I created an ICS project in Xamarin Studio and got the same message. I then loaded the solution in Visual Studio and noticed that the CalendarDemo project has a reference to Mono.Android v4.0, but the one created in Xamarin Studio only referenced Mono.Android v2.2 even though I specified Ice Cream Sandwich. That is why you're not able to build.
Changing the Target version in Project Options didn't seem to do anything. A workaround is to add
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
to the .csproj file. See the CalendarDemo project as an example.
BTW: This is on Xamarin Studio v4.2.2 (build 2). I don't normally use Xamarin Studio, so I have never had to deal with this. I'm not sure if it's a bug in XS, but it certainly isn't expected behavior.
Just as a test, I created another project in Xamarin Studio, but this time I just selected the standard Android Application project. Surprisingly, it referenced Mono.Android 4.3.0. So apparently there's a bug in the Android Ice Cream Sandwich project template.
I recommend using the standard Android Application project going forward.

If you are using Xamarin Studio, right-click on CalendarContact and select Resolve. It should give you two options, first will add "using Android.Provider;" to your using statements and the second will append the code to "Android.Provider.CalendarContract.Calendars.ContentUri"
...or just add manually the using statement. :)

You should import this library
> using Android.Provider;
> using Java.Util;

Went from a plain silverlight app to XNA app, SharedGraphicsDeviceManager.Current is null

I've had this Silverlight application where in the end needed some more rendering power, so I've tried to convert the project to a Silverlight/XNA project. It builds and runs, but when I navigate to GamePage.xaml where the XNA is it says:
"SharedGraphicsDeviceManager.Current is null in the OnNavigatedTo function."
It is really weird, as I just copied source from a blank project and created the correct support libs, and referenced the same libs. Did I forget something somewhere? Is SharedGraphicsDeviceManager.Current initiated somewhere?

Getting C# to recognize a dll outside of Visual Studio

Several years before I started working at this job another developer who is no longer here wrote an application in classic ASP using HTML, vbscript and javascript. This is fine but the problem is that 2 pages were written in C# with an HTML file and a code behind file. There was no solution files for these two pages. They may have been originally created in Visual Studio but they don't exist in it now.
That is important because there is a lot of things that Visual Studio just does for you without even thinking.
My problem is that in these two C# pages I need to get them to reference a DLL. This is a simple task when using Visual Studio. You just add a reference to the project and life is good. But outside of VS nothing seems to work.
I tried putting the dll in the same folder as the pages and then I tried the following:
Using myDLL;
myDLL dll = new myDLL();
myDLL dll = myDLL();
I found some code online that said to create an internal static class and use [DLLImport()] but that didn't work either. It couldn't find the dll or the Entry Point for the dll. I am currently researching how to create an entry point, just in case this is the method to make everything work.
Outside of having to rewrite these pages in vbscript (which I don't have the time to do) I am at a loss.
Has anyone ran into this problem before? Is there something that I can put in the web.Config? Or is this just impossible and I am hosed.
BTW this is all running under the 2.0 .net framework.

If you drop the DLL you want the code to reference into the bin folder of the website, then open the web.config and locate the following section configuration -> system.web -> compilation -> assemblies.
You need to add the display name of the assembly to that list - so that the compiler will reference that assembly during it's late-bound build process.
Now you should be able to use the stuff that's in it on those pages.
If you don't the know the display name of the assembly (typically yourassembly, version=*.*.*.*, Culture=neutral, PublicKeyToken=null for culture-invariant, non-strong-named assemblies) you can open it in a tool like ILSpy (there are others, it's just become my favourite) and it tells you when you select it in it's UI:
sorry for the poor highlighting - jerky hand following far too much coffee
If all the code in that assembly is in a single namespace, also, you can also add a default using to all the .cs or .aspx code in the project by adding that namespace to configuration -> system.web -> pages -> namespaces - making it simpler to use that code in the pages.

I created a VS Solution/Project for my app. I compiled and published it to the web server. When I published it I had it copy all project files.
I ran it and it crashed because it could not find my dll.
I tried adding the lines that Andras mentioned above and it seemed like it was getting me closer but it only changed the errors I was getting.
Then I went into IIS on the web server. I expanded the folder listing under Web Site. I right clicked on the folder that contained my app and made that folder into an application folder.
After I did that everything just worked. So then I thought I would see what happened if I backed out all of the additional code I added to my C# app and the Web.Config file. It still worked. All I needed to do was to make the folder an application folder in IIS and put a Using statement in my C# app and life is wonderful again.
Thanks for all the comments and suggestion. Andras thanks for the link to ILSpy. That is a cool little tool.
Take care,
Robert

I agree with Jon, it sounds like you should try creating a new project for these files. It's always better to leave code better off than you found it. If a new project is not an option for some reason, you should indicate this in your question.