Allow choosing the Target Framework.

[loligans]

Issue Description

As a user who has to target multiple Target Framework versions, I would like to be able to choose the specific framework Omnisharp should use. This functionality is homologous to this Visual Studio feature:

This will allow macro's like #if NETSTANDARD and #if NETFRAMEWORK to work correctly and provide intellisense in those areas.

Environment information

VSCode version: 1.44.2
C# Extension: 1.21.17

Dotnet Information

.NET Core SDK (reflecting any global.json): Version: 3.1.201 Commit: b1768b4ae7

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.201\

Host (useful for support):
Version: 3.1.3
Commit: 4a9f85e9f8

.NET Core SDKs installed:
2.1.504 [C:\Program Files\dotnet\sdk]
2.1.508 [C:\Program Files\dotnet\sdk]
2.1.602 [C:\Program Files\dotnet\sdk]
2.1.700 [C:\Program Files\dotnet\sdk]
2.1.801 [C:\Program Files\dotnet\sdk]
2.2.202 [C:\Program Files\dotnet\sdk]
2.2.300 [C:\Program Files\dotnet\sdk]
2.2.401 [C:\Program Files\dotnet\sdk]
3.0.100-preview6-012264 [C:\Program Files\dotnet\sdk]
3.1.101 [C:\Program Files\dotnet\sdk]
3.1.201 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0-preview6.19307.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.0-preview6-27804-01 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0-preview6-27804-01 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

Extension	Author	Version
azure-account	ms-vscode	0.8.9
csharp	ms-dotnettools	1.21.17
debugger-for-chrome	msjsdiag	4.12.6
debugger-for-edge	msjsdiag	1.0.14
docomment	k--kato	0.1.9
dotnet-test-explorer	formulahendry	0.7.1
ecdc	mitchdenny	1.3.0
EditorConfig	EditorConfig	0.14.5
gitlens	eamodio	10.2.1
markdown-all-in-one	yzhang	2.8.0
msbuild-project-tools	tintoy	0.3.5
ng-template	Angular	0.901.4
partial-diff	ryu1kn	1.4.1
terraform	mauve	1.4.0
uridecode	sryze	0.3.5
vscode-icons	vscode-icons-team	10.1.1
vscode-solution-explorer	fernandoescolar	0.3.9
vscode-typescript-tslint-plugin	ms-vscode	1.2.3
vscode-versionlens	pflannery	0.26.0
vscode-xml	redhat	0.11.0
vscode-yaml	redhat	0.7.2
vscodeintellicode	VisualStudioExptTeam	1.2.6
vsliveshare	ms-vsliveshare	1.0.2104

[pellared]

Related issue: #1783

[pellared]

When trying to use VS Code for the development of https://github.com/open-telemetry/opentelemetry-dotnet
I get a ton of errors like:

Assembly 'OpenTelemetry.Api' with identity 'OpenTelemetry.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=7bd6737fe5b67e3c' uses 'System.Diagnostics.DiagnosticSource, Version=5.0.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' which has a higher version than referenced assembly 'System.Diagnostics.DiagnosticSource' with identity 'System.Diagnostics.DiagnosticSource, Version=5.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' [OpenTelemetry.Tests]

I guess that this is because the assemblies inside the System.Diagnostics.DiagnosticSource NuGet package have different versions for different targets. net45 and net46 have version 5.0.0.1 while the others (net5, netstandard1.1 and netstandard1.3) have 5.0.0.0.
My understanding is that VSCode C# extension uses net452 for OpenTelemetry.Api and netcoreapp2.1 for OpenTelemetry.Tests as TargetFrameworks (because of #1783).

Having the ability to choose a platform could help I guess. It should basically help in any project which targets multiple frameworks.

[pellared]

1. Is it a feature that would be good to have?
2. Is it something where a contribution is welcome?
3. If so, then could you give any advice on how to proceed to implement this feature.

[microestc]

wish to support it

[dnchattan]

this would be very helpful for projects which utilize code generators. In my case I'm building for net472/net5/net6 and am using protobuf to generate grpc contracts. I primarily build and work on the .net5/6 versions of the code and don't generally build the net472 flavor in my inner loop, however omnisharp insists on using the generated sources from that platform target for all intellisense checking.

Ironically, this is to support a building another code generator, which is why it's targeting net472 in the first place (msbuild.exe still runs in this version, and requires any custom build tasks to be built for it).

[yamachu]

Sorry to be an advertisement, but I am using this extension to deal with the problem.
https://marketplace.visualstudio.com/items?itemName=yamachu.targetframeworksswitcher

When developing projects such as MAUI, it is difficult to develop without being able to select a target framework...
So I would like to see a selection feature implemented.

[Sour-Codes]

In full support of this thread. It's the 1+ VS has over Code. Can't imagine how much time I lost over the years to reorder target framework first, restart Omnisharp, and wait for the service to boot up.