For your third question, yes, if you upgrade your frontend/cms/commerce site, you will have to upgrade CM. But that can be done after you complete upgrading your frontend site. 

Your second question, I've had the same issue few times with Visual Studio 2017 and the cure was to clean the solution, close visual studio (all instances), open the solution and then the update-epidatabase worked just fine from package manager console.

As an alternate you could add the attribute updateDatabaseSchema="true" to episerver.framework to work out the database schema update issue:

<episerver.framework updateDatabaseSchema="true">

LinkItemCollection is still valid property (check Alloy sample for example, front page has property which is using LinkItemCollection) and LinkItem are the items in the collection (LinkItem cannot be used as a property).

After the database update, do you still get the error about the LinkItem? And the solution worked ok before the update?

Thank you Antti, its works for me. 

After that I am facing anothere issue. 

--------------

 CS1519: Invalid token ',' in class, struct, or interface member declaration

Line 44:     }
Line 45:     
Line 46:     public virtual EPiServer.Personalization.GuiSettings, EPiServer EditTreeSettings {
Line 47:         get {
Line 48:             return ((EPiServer.Personalization.GuiSettings, EPiServer)(this.GetPropertyValue("EditTreeSettings")));

public virtual EPiServer.Personalization.GuiSettings, EPiServer EditTreeSettings

Should be:

public virtual EPiServer.Personalization.GuiSettings EditTreeSettings

You shouldn't specify the dll a class is located in when writing C# code.

I'm pretty sure this is a JIT compilation error, the code is quoted is actually from the framework assembly.

I'd suggest to remove bin folder, clean cached ASP.NET files, build the solution and try again. 

also check C# compiler version you have and might be good to look at target framework

That makes senses. CMS 11 requires .NET 4.6.1. Make sure to install that version or newer on your machine, and your web.config has been updated to targetFramework="4.6.1" (targetFramework="4.5.2" works magically for the sample Commerce site (Quicksilver), but I guess that was because 4.6.1 and 4.5.2 are highly compatible.

@Dipak, Valdis and Quan already mentioned some hints about the framework and compiler version.

So make sure your computer has .Net framework 4.6.1 in minimum installed and your test and production environment when you deployment there next time. I personally have updated many solutions many moons ago to use 4.7.1 framework version ;) (what ever 4.6.1+ framewrok version you choose to use, you need to make sure all of your environments have it and install it there if not yet installed).

Have a look at the Episerver 11 braking changes here in the documentation. Also note that ASP.NET MVC 5.2.3 is also required but the nuget dependencies should have updated that one.

I'm not sure how familiar you are with the framework version configuration so here goes just in case:

<system.web>
    <httpRuntime targetFramework="4.6.1" requestValidationMode="2.0" />
    <compilation targetFramework="4.6.1" />
	<!-- other stuff removed -->
</system.web>

So you set the version in those two elements (naturally the value in both would be 4.7.1 if you choose to go with the currently latest framework version).

Quans memory serves well ".NET 4.5.2, 4.6 and 4.6.1 are compatible, in-place updates on top of .NET 4, .NET 4.5, and .NET 4.5.1. This means that applications built to target any of these previous .NET 4.x versions will continue running on .NET 4.5.2 without change. No recompiling of apps is necessary." - Source

This is a good page for checking the changes between framewrok updates. For example many moons ago the change to Uri.EscapeDataString functionality by framework upgrade got me off-quard :D

In the web.config file change is required for EditTreeSettings config item

 <add name="EditTreeSettings" type="EPiServer.Personalization.GuiSettings, EPiServer.Cms.AspNet" />

@Dipak, yes that is correct and should have been done automatically by updates.

Thank you everyone ... !