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Adding CsWinRT nuget breaks using Windows.System namespace

I've created a .NET 5.0 project, one of the dependencies I have is on this API:
AnalyticsInfo.VersionInfo.DeviceFamily
After installing Microsoft.Windows.SDK.Contracts, I'm able to use this API. Then, I needed to install the Microsoft.Windows.CsWinRT package to resolve this error:
Error NETSDK1130 Referencing a Windows Metadata component directly when targeting
.NETCoreApp,Version=v5.0 is not supported.
Use the C#/WinRT projection tool (https://aka.ms/cswinrt) or a provided projection for this target.
After installing this, I no longer have access to the Windows.System.Profile namespace to call the AnalyticsInfo API:
Error CS0234 The type or namespace name 'System' does not exist in the namespace 'Windows' (are you missing an assembly reference?)

With .NET 5, built-in support for WinRT APIs in .NET is removed (because it's Windows specific) , so we can't use Microsoft.Windows.SDK.Contracts any more.
The solution as explained here Built-in support for WinRT is removed from .NET is to
Remove references to the Microsoft.Windows.SDK.Contracts package.
Instead, specify the version of the Windows APIs that you want to
access via the TargetFramework property of the project. For example:
<TargetFramework>net5.0-windows10.0.19041</TargetFramework>
Note with that in place, there's no need to manually add a reference to C#/WinRT (Microsoft.Windows.CsWinRT) it should be done automatically and shown as "Microsoft.Windows.SDK.NET.Ref" in the list of Frameworks Dependencies.

Reference manager is empty in vs 2017 .net core class library project

I'm trying to add reference to System.Configuration.dll assembly.
I'm using .Net Core, as it can be seen in my .csproj file:
<TargetFramework>netcoreapp2.2</TargetFramework>
(Output type of my project is set to Class Library.)
To do this, in solution explorer, I right-click on Dependencies of my project and then click on Add Reference.... Problem is that the opened Reference Manager is empty and there is no assembly I can add a reference to.
However, without referencing that assembly I'm able to write the following line without any build error:
using System.Configuration;
First Question: How is it possible that the above line gets successfully built, without me referencing that assembly?
If I try to use ConfigurationManager class to read data from app.config file (As told here ), I get an error complaining that:
CS0103 The name 'ConfigurationManager' does not exist in the current context
Question two: what can I do to solve that error? what's wrong with my code or project?
My case is not the same as this question and this is not a duplicate of that question.

Add Reference is for your local references. You need to use Nuget Package Manager in order to include references.
most importantly .Net Core does not use .Net Framework packages where ConfigurationManager is .Net Framework package.
You need to use .Net Core or .Net Standard libraries with .Net Core

HttpUtility not recognised in .Net 4.5

I Developed a WinForm application in with the target framework set to .net 4.0, now I wish to add to a project that has it's target framework set to .net 4.5. After I added the 4.0 WinForm application to my 4.5 project I keep getting the an error on my HttpUtility object.
data += "&batch_data=" + HttpUtility.UrlEncode(batch, System.Text.Encoding.GetEncoding("ISO-8859-1"));
"The name 'HttpUtility' does not exist in the current context"
I did include the System.Web namespace where the HttpUtility is located.
Visual Studio Error:
CS0234 The type or namespace name 'HttpUtility' does not exist in the namespace 'System.Web' (are you missing an assembly reference?)

The problem is somewhere else.
As you can see in MSDN the HttpUtility class is present in System.Web in .NET Framework 4.5.
You're probably targeting the Client Profile: target the full framework in Project Properties. Otherwise:
either you did not add the right using statement using System.Web;
or you did not add the reference to System.Web.dll in the project.

WebUtility
You also have another possibility: Use the WebUtility class.
The WebUtility class is recommended by Microsoft itself and should be used outside of web applications.
Like the HttpUtility class it also provides you with the possibility to encode and decode URLs.
This way you don't have the problems with importing the library into your project or setting some specific profiles.
From the Documentation (Source)
The HttpUtility class is used internally by the HttpServerUtility class, whose methods and properties are exposed through the intrinsic ASP.NET Server object. Additionally, the HttpUtility class contains encoding and decoding utility methods that are not accessible from the Server.
To encode or decode values outside of a web application, use the WebUtility class.

The HttpUtility class exists from .NET 1.1, so I think it is not possible for regular projects to 'not see it', as long as you have included a reference to System.Web.
You might be using a PCL (Portable Class Library), which uses a stripped down version of the framework that is supported on the platforms you selected, like Windows Store apps, Windows Phone, Silverlight, etc.

I hope this link will help you.
http://msdn.microsoft.com/en-us/library/system.web.httputility(v=vs.110).aspx
Dot net framework 4.5 support HttpUtility as it is under System.Web namespace.
Also adding a System.Web reference, without System.Web.Extensions reference into your project. If it doesn't work remove the existing and add new reference of System.Web into project. Also check which framework it is targeting it should be .NET Framework 4 or 4.5 without Client.

I encountered this issue in .net 4.5.2 (using VS2019). I did check that I was using full framework and I also tried explicitly declaring System.Web in a using statement, though VS complains that the using clause is not needed.
System.Web.Utility appears to have been replaced by System.Net.Webutility

Can't compile c# project because of some tuple error when referencing an external dll

I have an error, when I try to build my .net 4, c# project. Everything works great, but when I add an external reference to a given DLL, it stops working, it can't build, throws this type of some errors:
Error 36 The type 'System.Tuple' exists in both 'C:\Program Files\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\mscorlib.dll' and 'C:\Projects\Project1\ExternalRefernces\SharpSNMP\SharpSnmpLib.dll' C:\Projects\Project1\CheckerStore.cs 17 21
Note, I did not do anything with the new library, just added as a reference. Any ideas?

You can solve this problem by specifying an external alias. Select the SharpSNMP reference in your project. In the properties window change Aliases from global to say SharpSNMP. In your code type this
extern alias SharpSNMP;
...
System.Tuple<T1,T2> sysTulpe;
SharpSNMP::System.Tuple<T1,T2> sharpTulpe;
or
extern alias SharpSNMP;
using SharpSystem = SharpSNMP::System;
...
System.Tuple<T1,T2> sysTulpe;
SharpSystem.Tuple<T1,T2> sharpTulpe;
See Aliases: overcoming name conflicts part 2: extern alias

What you can do is either change the target version to 3.5 or make some changes in the SharpSNMPLib. The source can be fetched from here or here.
The changes you need to make is specifically moving the System.Tuple type somewhere else.
Edit:
I belive you have added a reference to a precomplied DLL. A DLL that is NOT compiled for framework version 4. What you need to do is download the source code (see links above) and compile the project with target version 4.
Why you need to do this is because there are conditional build parameters depending on the framework target version. The SharpSNMPLib System.Tuple is used for version <= 3.5 and the framework System.Tuple is used for version >= 4.
Edit:
Reproduced your problem using framework System.Tuple and SharpSNMPLib.dll.
Successfully built SharpSNMPLib targeted on version 4.
Successfully built application using framework System.Tuple and the new SharpSNMPLib.dll.

Simply go to your CheckerStore.cs file, line 1721 (if I'm right). Find the Tuple class, and reference it using it's fully qualified name.

The library for some reason re-implements some system types. Likely reason is to make code to be source level compatible when using older versions of the framework.
Most likely there is a version of this SharpSNMP library that works with 4.0 framework. Check if you already have correct on in your source tree. Check with creators of the library what versions of the assembly you need to use with given framework version and what is recommended way of doing it.

C# using System.Linq error

Why might "using System.Linq" cause the following error?
The type or namespace name 'Linq' does
not exist in the namespace 'System'

Reference System.Core
And then there are others that merge this namespace too - but that's the primary one on .Net 3.5 and above.
If you're project is currently .Net 2.0, say, and you're using the right version of VS (2005 and above) - you can simply right-click on the proejct properties; and change the 'Target Framework Version' to 3.5. System.Core will then become available.
If you don't see that in the options - then I guess you're using an older VS

The most probable reason is that you are using wrong version of .NET Framework.

Try to add System.Core assembly to your project

You'll get this error if you don't have "System.Core.dll" referenced (the assembly which contains the core LINQ APIs).

System.Linq is available in .Net 3.5 and above version.

Maybe you're targeting an older framework, Linq came in with 3.5 IIRC.

You are using lower version of .NET Framework than 3.5 to compile the source code or you don't have added the System.Core assembly to your project.

Manually type using System.Linq in the starting of the project, you will not be able to find this namespace in add reference dialogue box.
If you are still getting error then try to Add Reference System.Core.
If you are getting an error that it has been already referred then you can unload your project and then edit your csproject file, manually copy reference to System tag and paste and change the name to System.Core and reload the project.

In my case the only thing that worked was:
Adding a new Razor item (e.g. MVC 5 View Page)
That automatically pulls in some NuGet packages
The package that makes System.Linq available to Razor Views IntelliSense seems to be Microsoft.AspNet.WebPages.