Develop Reference

Old C# projects not running in Microsoft visual studio 2017

I last downloaded and used Microsoft visual studio 2017 for a C# class back in the spring. I have since opened and loaded an old project today and I get this when running it:
“the current .NET SDK does not support targeting .NET Core 2.0. Either target .NET Core 1.1 or lower, or use a version of the .NET SDK that supports .NET Core 2.0."
Any ideas as to why this is happening? When I open and run my c++ projects from the summer everything runs fine.
Thank you!

Ok, looks like you will have to go to this Microsoft page and choose and install .NET Core SDK you need:
.NET Core 2.0 downloads
Also it's good thing if you install updates in Visual Studio 2017 - check for the notifications flag in upper right corner of VS 2017.

Targeted .NET framework for the project depends on Visual Studio version?

I have a funny situation where the C# project targets version of .NET framework depending on Visual Studio version rather than depending on selected .NET target version in Application tab of project's properties.
I had VS2013 premium, update 4, installed and the given project targets .NET 3.5.
Afterwards I've installed VS2015 Enterprise edition and opened solution with the given project with VS2015. Resharper suggested use of nameof() operator (which is .NET 6.0 feature) and this compiled, but when I open given solution in VS2013 it doesn't compile anymore.
Here's the screenshot of Application tab for the given project:
Here's the code snippet from the project when I open it in VS2015:
Which compiles successfully.
And here's the code snippet from the project when I open it in VS20103:
Which doesn't compile.
Any clues about this?
UPDATE:
To rephrase my question. Why does the given project (that targets .NET 3.5) compile at all in VS2015 even though I've used .NET 6.0 feature (nameof() operator)?

This is normal behaviour. If you try to compile something .NET 4.0 specific in VS 2005, you will get the same result. Using a certain VS version you can only develop using up to a certain .NET version.
If I remember correctly:
VS 2005 - .NET 2.0
VS 2008 - .NET 3.5
VS 2010 - .NET 4.0
VS 2013 - .NET 4.5
Update:
The .NET version is indeed not relevant in this case, as the nameof operator is a feature of the Roslyn compiler. Nothing more, nothing less. The VS 2013 definitely uses a different one (at least a compiler which does not cover up with C# 6), therefore it does not compile.

.Net framework 4.5 Installed , still getting the error Project is Incompatiable

I have installed .NET framework 4.0 on my system , but when I open my project on VS 2010 , still getting the message ,
This Project is incompatiable with the current version of Visual Studio
Any Idea? I don not want to change the target framework to v4.0 in .csproject.

The error you are getting is not about an incompatible .NET framework version, it is about an incompatible Visual Studio version!
Visual Studio project versions are not linked to .NET versions. A Visual Studio 2008 project can target .NET 2.0 up to 3.5, but still you can't open a VS 2008 project with Visual Studio 2005, even though Visual Studio 2005 can create .NET 2.0 projects, too.
Having the latest .NET framework on your machine does not guarantee that you can open projects with the latest Visual Studio version in an older Visual Studio.
You can simply not downgrade Visual Studio projects easily. There are posts describing how to to it by modifying the project files, etc., but in general you can only upgrade projects using the wizard that's automatically opened by Visual Studio.

As I know you can't target .NET 4.5 with VS2010 the same way you couldn't target .NET 4.0 with VS2008. You will need to install VS2012 if you want to target .NET 4.5.
EDIT
found this answer but haven't tried it by myself

Multi-targeting .NET Framework 4 and Visual Studio 2012

I have installed Visual Studio 2012 Professional on my machine. I don't have Visual Studio 2010 installed, but I want to keep developing my applications using .NET Framework 4, but I don't have this option on my Visual Studio 2012. Look at the image:
I don't know what can I do to fix this problem, because I click on "More Frameworks..." link and try to install the Multi-Targeting for .NET 4.0.3 (installed on my machine), but it does not work. It says that this version of framework is still installed. What can I do to fix this problem?
Obs: I have lots of projects in C# and VB.NET with .NET 4.0, and I need to keep these projects with new projects on .NET 4.0.

I solved the problem doing these tasks:
Uninstall the Visual Studio 2012
Uninstall any reference for the .NET Framework 4.0 or 4.5
Clean the register with a tool like Glary Utilities and restart the computer
Install the .NET Framework 4.0 and install Microsoft Multi-Targeting for .NET Framework 4.
Install Visual Studio 2012 again

You fixed your problem, but it's likely that .NET Framework 4 Targeting Pack (not to be confused with the .NET Framework itself) failed to install for some reason. Did you get a warning at the end of the Visual Studio installation? Either way, in the future, a repair install of Visual Studio 2012 should have fixed the problem (Add or Remove Programs -> Microsoft Visual Studio [Edition] 2012 -> Change -> Repair).

I think that your issue is probably some mishap with the installation of .NET Framework. I installed Visual Studio 2012 Beta and have all kind of troubles with compatibility between framework versions. The only remedy was reinstalling Visual Studio, including .NET Framework.
Check out How to: Target a Version of the .NET Framework (MSDN).

After creating new a project, go to its properties (select your project in Solution Explorer then right click and select properties) and select the target framework!

What is the difference between *which version of the framework is running* and *which version of the runtime is running*?

In this post, #kbrimington mentions that there is a subtle difference between which version of the framework is running and which version of the runtime is running.
What is the difference exactly?

Here is a quick breakdown that might give you the information you want. It can be confusing, but it is worth your time to understand the difference between all three and how they differ and interrelate.
CLR .NET C#
---- ---- ----
1.0 1.0 1.0
1.0 1.1 1.0
2.0 2.0 2.0
2.0 3.0 3.0
2.0 3.5 3.0
4.0 4.0 4.0
4.0 4.5 5.0

The .NET framework is the framework that describes the available classes. So the version of the framework tells you which classes are available.
The runtime (CLR) is the part that executes the code, it is a virtual machine that translates your compiled application (MSIL) to a byte code that can be executed on the host OS.

When developing in Visual Studio 2008 or 2010, you can choose which version of the .Net framework to use. This is called multi-targeting, as explained here and here:
The multi-targeting feature of Visual Studio lets you specify the specific version or profile of the .NET Framework that is required for your application. The key benefit of multi-targeting is that you can use the current version of Visual Studio to create and develop projects that target an earlier version of the .NET Framework. For example, you can continue to develop projects that were created in Visual Studio 2005 and Visual Studio 2008 without adding new .NET Framework dependencies. Multi-targeting helps guarantee that applications use only functionality that is available in the specified .NET Framework version. In addition, multi-targeting lets you continue to deploy older applications without requiring you to add a new .NET Framework version in your deployment package.
An app compiled on .Net 3.5 in Visual Studio 2008 will use a different version of the runtime from the same app compiled on .Net 3.5 in Visual Studio 2010.
This means that, when you are supporting or enhancing apps written in older versions of .Net or Visual Studio, you can still do that development in a newer version of Visual Studio, taking advantage of the newer features.

What matters is whether the .NET version installs side-by-side or update the existing version. Verions 1.0, 1.1, 2.0 and 4.0 are side-by-side versions. 3.0, 3.5, 3.5SP1 and the upcoming 4.5 are updates for the existing 2.0 and 4.0 versions. And use the same CLR version, the runtime version you are asking about. The updates add extra assemblies. You cannot otherwise have, say, both 2.0 and 3.0 installed at the same time, the 3.0 update replaces 2.0. The only way to get the update version is to read the registry, as shown in the question you linked.