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Using VS2017 and VS2015 C# projects in the same location, side-by-side

Is anyone out there using Visual Studio 2015 and 2017 .CSPROJ files alongside each other in the same folder? Are there any other incompatibilites, besides the solution and project files that I should worry about?
Basically what I want to do is allow our developers to continue to use VS2015 and those projects -- just for a while -- while all still working on exact same set of same code files. I don't want to move/change anything else.
I've already created all the new project files and solution file. But 'm wondering if I can just put them in the same physical disk folder alongside each other and have everyone go merrily about their development tasks with either VS2015 or VS2017.
The duplication of work synchronizing the project files themselves is not a concern. I just want to make sure there aren't any OTHER format differences that might cause some sort of build/run problem for me.
Is anyone else out out there doing this? Is this workable?

There is not any incompatibility, but you should check when you are creating a new project in VS2017 the kind of project, you should have two things in mind, first, when you are creating .net projects, you have to keep using the old .net framework instead of .net core, and second (only if you are going to create desktop/libraries/console projects), you need to use the Common option instead the new option (check on the dropdown to select the .net framework version that allows you to select versions lower than 4.5), cause if not, then you have the incorrect kind of project selected and this kind of project doesn't work in VS2015 as they change the structure of the .csproj file. Hope this little explanation helps you.
Update
What I wanted to say with this explanation is that every projects that comes from VS2015 or 2017 if they have the same structure (same old kind of .net framework), then you can do what you want to do, but if for some reason you need to make a .net core, then this will not work.
Cheers.

Just be aware that VS2017 supports C# language version features that 2015 does not. If anyone uses those features you will not be able to build the projects in VS 2015.
You can enforce language version in the .csproj files but this is done per BuildConfiguration and can be tedious to configure for larger solutions with many projects.
To make the changes
Right click the Project and select Properties
Then Build Then
Advanced (Bottom Right corner) General > Language Version
Change to C#6.0
Overall it is possible but you may need to retain VS2015 if you have any SSIS or SSRS packages as the tooling is still not available for general release (Oct 17) and is still in preview.

You should be able to run Visual Studio 2015 and Visual Studio 2017 side by side without any issues. Our application ran on both Visual Studio 2015 and Visual Studio 2017 before every developer migrated to VS 2017.
You do need to keep in mind few things though:
Visual Studio 2017 by supports C# 7.0. Make sure that you use same language version in both the IDEs.
VS 2017 also supports stuff like editor.config. I would suggest to avoid using it (though I feel it should not be an issue)
If you are using on .NET Core project without csproj. Do not think you would be able to upgrade it.

visual studio 2012 c# go to definition always returns metadata

ide: vs 2012 v11.0.60610.01 update 3
project: web site, mix of vb and c#
problem: vb navigates to definition, c# navigates to metadata
complaint: complete pain in the ass when managing a 30k code-line project.
solution: you be the first to figure this out, it's beyond my level of patience.
i find metadata to be 110% useless.
is there ANY way to get c# to navigate to the actual definition like vb does?
thanks

If it's navigating to the metadata, rather than the definition, it probably has to do with how your assemblies were added. If you add a reference as a DLL, it will always show metadata, even if that project is included in your solution; you have to add a reference by project to have access to the source.

"i find metadata to be 110% useless. is there ANY way to get c# to navigate to the actual definition like vb does?"
Resharper's go to definition/go to implementation options work exactly as you'd expect - worth trying it to see if it fixes your issue.
This question might be of help if you choose not to try resharper

I realize this is an old thread, but it seemed to be one of the few that focused on the role of mixed languages (C# and VB.NET) as a contributing factor to this.
I ran a few tests across VS.NET 2010, 2012, 2013, and 2015. My empirical results (not validated by any official Microsoft documentation) is that all 4 support Find All References of VB.NET members consumed by C# (albeit, split into apparently separate result groups), as long as project references were added. However, VS.NET 2010, 2012 and 2013 end up in the "from metadata" view for Go To Definition from the C# code to a VB.NET member. Where as VS.NET 2015 appears to have this cross-language Go To Definition navigation working correctly.
I also confirmed that using Resharper's support for External Sources does allow Go To Definition to work under VS.NET 2012, and that almost motivated me to purchase licenses for Resharper for our current project that has mixed C# and VB.NET projects, but now my plan is just to expedite adopting VS.NET 2015.

Cannot see Oracle Data Provider for .NET in Visual Studio 2012 RC

I am porting a web application to Entity Framework 4.5. My main goal is to support both MSSQL and Oracle databases and be able to tell the ORM which data source to target programmatically (in the Global.asax). The SQL and Oracle schemas are obviously functionally identical; this is an approach we already use in production with another ORM we want to throw away.
NOTE: The machine we are testing it on has Windows 8 Release Preview Build 8400 64 bit, Visual Studio Professional 2012 RC 11.0.50706.0 and .NET framework 4.5.50501. It is not a standard development machine (the others have XP and 7 w/Visual Studio 2010 and .NET 4), but a new one we are using to test these new technologies. I have no idea if any of this new technologies may be the problem, haven't tested it on a "standard" dev machine yet.
Short version
I'll explain the situation in detail right below, but the main questions are:
In the "Add new ADO.NET Entity Data Model" wizard, I cannot see the ODP.NET data provider I installed (see below) and thus I cannot create a model from an Oracle database. Why? Furthermore, in the "About Visual Studio" pop-up I don't see listed Oracle Developer Tools for Visual Studio. My fear is that the Oracle Developer Tools aren't working somehow, but the Oracle Universal Installer said everything was correctly installed (and I rebooted several times).
What is the best practise to achieve this requirement?
Long version
First, we created the Entity Data Model from a SQL Server database. So far so good.
Then, we installed ODAC 11.2 Release 4 and Oracle Developer Tools for Visual Studio (11.2.0.3.0) from http://www.oracle.com/technetwork/topics/dotnet/utilsoft-086879.html
I have managed to initialize our model, SIAEntities (which inherites from ObjectContext), with:
System.Data.EntityClient.EntityConnectionStringBuilder builder = new System.Data.EntityClient.EntityConnectionStringBuilder();
builder.Metadata = "res://*/SIA.Models.SIAModelOracle.ssdl|res://*/Models.SIAModel.csdl|res://*/Models.SIAModel.msl";
builder.ProviderConnectionString = "data source=***;persist security info=True;user id=***;password=***";
builder.Provider = "Oracle.DataAccess.Client";
SIAEntities db2 = new SIAEntities(builder.ConnectionString);
where SIA.Models.SIAModelOracle.ssdl is a custom written file (embedded resource) which overrides the SSDL part of the .edmx and has the following modifications:
Provider attribute of the root Schema node set to Oracle.DataAccess.Client
All data type names converted to their Oracle equivalent (datetime > date, nvarchar > varchar2, image > blob, etc)
An oracle.dataaccess.client node added in the web.config which maps bool to number(1,0) and tinyint to number(3,0)
The error arrived when executing the first query (a simple SELECT): I tracked it as being due to the double quotes in the query that gets executed against the db, forcing it to be case-sensitive.
By manually editing the custom .ssdl file and uppercasing everything I can let EF "see" the table and recognize a column after the other, but I didn't push this solution too far (thus stopping before getting to read the whole table) because a) it didn't seem too clean b) I have difficulties understanding the links between the .ssdl and the .csdl/.msl parts so I wasn't sure where to uppercase and where not to!
Another strategy I was contemplating to avoid this manual work could be to try and generate a separate model from the Oracle schema and wrap the two ObjectContexts with another class that handles the choice between either of them, but as question #2 says, I cannot do that.

Edit: Oracle has now released a version (11.2.0.3.20) of Oracle Developer Tools for Visual Studio that works with Visual Studio 2012 and .NET 4.5. You'll need this version (or higher) of these tools for Visual Studio 2012 Entity Designer support and other Visual Studio 2012 integration. Get it here:
http://www.oracle.com/technetwork/topics/dotnet/utilsoft-086879.html
Edit #2: Visual Studio 2013 is supported beginning with version 12.1.0.1.2
FYI - I created a video to highlight some of the other features of these tools:
http://www.youtube.com/watch?v=68tlel4iJdM
Christian Shay
Oracle

Oracle has released ODP.NET with VS2012 and .NET 4.5 support. I don't see much from them in the way of documentation for this release.

Undefined Web.config error in VS 2008

I'm working on a web app using VS 2008, .Net 3.5 and C#. Most of the projects in the solution are either classic asp.net pages with some MVC 1 in the mix, the rest is shared libraries. The solution is one that is some 5 years old and has gone through a variety of developers working on it and clearly has some performance and architectural issues.
Previously, I've been working on the project using VS 2008 on a Win XP machine, but have just transitioned over to a new box using Win 7 Ultimate. To do so, I've installed VS 2008, asp.net 3.5. To support future work on the solution I've also installed VS 2010 and asp.net 4.0.
Opening the solution on the new box with VS 2008 works fine, and it builds without error. However, when I attempt to run it with the debugger, I get the following message:
"There is an error in web.config. Please correct before proceeding. (You might rename the current web.config and add a new one.)"
I think it's clear that there is some sort of environmental issue regarding web.config on the new machine, but the error message is not "helpful". Adding a new web.config is not an option as the existing one is quite long and involved (too much to post here).
I'm hoping someone has a suggestion or two about where I might look for missing elements or changed configurations that might produce such an error message. Lacking that, I'll revisit this post and provide the web.config in the hope that will elicit further help.

So I don't know if you ever found an answer to this, but I ran into it in VS2013 today. Oddly, it would work if I had the web.config open in an editor window.
For others who may see this error I was able to clear it in VS2013 by deleting the \bin and \obj directories.
After that, it worked fine again...

To maybe give you a better idea of what the web.config problem is, I would follow the suggestion of renaming the current one and adding a new one. You can use a diff tool (e.g. WinMerge) to find out what the differences are between your existing config file and the "clean-slate" new one.
You can copy the non-issue sections from the old one to the new one, like the connection strings and the application settings, forms authentication modes, etc. You might have something wrong with the compiler section or something a little more specific to the .NET framework.
Did you convert the project at all from targeting .NET 2.0 to .NET 3.5? There's a big web.config section that gets added to 3.5 projects that wasn't required for 2.0 projects regarding the compiler, as it uses a separate compiler for .aspx inline-code than was used for 2.0.

iis 7.5 (on windows 7) is a pretty different animal than iis 6 (on windows xp). there are some configuration incompatibilities. one thing that comes to mind is a little different schema for the attributes on the elements for http handlers and modules. there are other things that are different. usually on the error page, it will tell you what it doesn't like. i imagine it was there and maybe you didn't notice it. or maybe you were hitting the page remotely and it didn't have all the detail. i suggest using a browser on the local machine and take a closer look - pretty sure it will tell you what is making it puke.

I ran into this ambiguous error for the first time today in VS2019. I had added a new Entity Model (.edmx) which required adding a new connection string to a separate project's web.config. After adding it, I immediately started getting the error on startup. I noticed there were several config transforms for the various environments. After adding the missing transformed connection strings to each of them that error has gone away.

Can I edit a Visual Studio 2008 project in 2010 while keeping it a 2008 only project?

So on my computer I only have Visual Studio 2010 Express. I need to edit a Visual Studio 2008 project, however it needs to be kept in C# 3.5.
Before I open it in 2010 and accidentally upgrade it, will it be kept in 2008 or C#3.5 format?
I realize I wont be able to use .Net 2010 features, so thats no problem. I just want to make sure if I save my 2008 project using 2010 I will still be able to open it in 2008 and it will work fine.

The target framework of the project is independent of the solution format. That is, you can convert your project to Visual Studio 2010 and still target the 3.5 Framework. But, once it's converted to 2010, you will not be able to open it in 2008.

You cannot open a project in VS2010 without it being upgraded. The Framework version can be controlled independently, so you can still target 3.5.
The changes to the project/solution files to use 2010 (assuming you don't use any newer features) are fairly minimal, so there may be mileage in just not committing the project/solution files (you can probably achieve this by dropping an ignore file for your source control provider in your local folder, including adding the ignore file to itself, so that doesn't get committed).
If you're mostly editing code (and not often changing the projects in the solution, or files in the projects) you could also have two copies of the files, one for each version, until you can commit to 2010 everywhere (and with Express for free, you might be able to do this quickly?).

No, VS2010 will convert the VS2008 project file. After it is converted, VS2008 can no longer open it. There are ways to hack around this, the simple solution is to just use VS2010 consistently. Next best thing is to only check-in the source code changes. Next best thing is to give the 2010 project a different name and maintain them both, at least for now.

You'd be hard pressed to accidentally upgrade it. When you open a VS2008 project in VS2010, it automatically starts the upgrade wizard, and you have to decide whether to let it do the upgrade, including making a decision whether to let it take a backup before it does the upgrade.
You can't open a VS2008 project in VS2010 and work on it without upgrading it to VS2010 format first. And once you've allowed VS2010 to upgrade it, you won't be able to open it in VS2008 any more.
Careful though - don't confuse the version of the framework you want to target, with the version of VS that you're using.
You can target framework 3.5 with VS2010.
There are hacks that let you open VS2010-upgraded projects in VS2008, but it's not a great plan to go down that road (guess how I know).

When you are going to convert it with compatible to 2010 it will ask for the backup...
so... later you can retrieve your file back ...

I might be wrong but I'm almost sure that project files don't differ between 2008 and 2010 or at least they are compatible. Solution files are a different matter and old solutions do indeed need to be converted. Usually you have few solutions for 2008 and 2010 versions of VS, but they contain the same projects, so the projects don't need to be converted.

I was on a seminar where they took this (short of classic?) accident as a sample for versioning. They get a 2008 project from git and start it in vs 2010.
Did the upgrade process and then "ops, i can't revert the upgrade". They commited the vs2010 converted files to a branch in GIT. When they "came home again", merged the changes in project together with 2008 solution files.
This may not come in touch to you, but the sense is:
All files is intact but the solution/project files. So just keep them safe and you can continue use the 2008 version after done your editing in 2010.
Though, without any kind of manual handling of solution/project files - answer (on the question in your subject) is No!

In short, no. Once it's upgraded, it stays that way. There is no official way to downgrade it either. The main driver behind this fact is because of the compiler. Even if you stay in .NET Framework 3.5, you can still use C# 4.0 features (like optional / named parameters) since 2010 will always use the 4.0 compiler.
Imagine a situation where it stayed in a 2008 format but you used a C# 4.0 language feature. Since VS 2008 always uses the C# 3.0 compiler, whenever you open it in 2008, it will no longer compile. This is a good reason to avoid work-arounds that have been posted for downgrading - you get mixed compiler features that can break things pretty easily.