Support prerelease .NET SDKs on .NET 6

[rainersigwald]

Issue Description

Steps to Reproduce

Have a prerelease .NET SDK installed (I got mine via VS internal preview builds) and opt into O# on .NET 6 per https://github.com/OmniSharp/omnisharp-vscode/releases/tag/v1.23.18-beta2.

Expected Behavior

Working

Actual Behavior

Starting OmniSharp server at 12/7/2021, 9:14:54 AM
    Target: s:\play\xunit_templ

[ERROR] Error: Unknown result output from 'dotnet --version'. Received 6.0.200-preview.21573.3

Logs

OmniSharp log

Starting OmniSharp server at 12/7/2021, 9:14:54 AM Target: s:\play\xunit_templ

[ERROR] Error: Unknown result output from 'dotnet --version'. Received 6.0.200-preview.21573.3

C# log

Post the output from Output-->C# here

Environment information

VSCode version: 1.62.3
C# Extension: 1.23.18

Dotnet Information

.NET SDK (reflecting any global.json): Version: 6.0.200-preview.21573.3 Commit: a2bb0dde82

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22000
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\6.0.200-preview.21573.3\

Host (useful for support):
Version: 6.0.0
Commit: 4822e3c3aa

.NET SDKs installed:
5.0.209 [C:\Program Files\dotnet\sdk]
5.0.403 [C:\Program Files\dotnet\sdk]
6.0.100-preview.6.21321.7 [C:\Program Files\dotnet\sdk]
6.0.100-preview.6.21328.10 [C:\Program Files\dotnet\sdk]
6.0.100-rc.1.21410.3 [C:\Program Files\dotnet\sdk]
6.0.100 [C:\Program Files\dotnet\sdk]
6.0.200-preview.21573.3 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.0-rc.1.21409.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.21 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.0-rc.1.21406.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.21 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.12 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.0-rc.1.21407.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

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

Visual Studio Code Extensions

Extension	Author	Version
azure-account	ms-vscode	0.9.11
azure-pipelines	ms-azure-devops	1.195.0
better-toml	bungcip	0.3.2
cmake	twxs	0.0.17
cmake-tools	ms-vscode	1.9.2
code-spell-checker	streetsidesoftware	2.0.13
codesnap	adpyke	1.3.4
codespaces	GitHub	1.4.1
copilot	GitHub	1.7.4016
cpptools	ms-vscode	1.7.1
csharp	ms-dotnettools	1.23.18
docs-article-templates	docsmsft	0.2.11
docs-authoring-pack	docsmsft	0.1.15
docs-build	docsmsft	0.3.8
docs-images	docsmsft	0.0.9
docs-linting	docsmsft	0.0.8
docs-markdown	docsmsft	0.2.100
docs-metadata	docsmsft	1.0.7
docs-preview	docsmsft	0.3.57
docs-scaffolding	docsmsft	0.0.12
docs-yaml	docsmsft	0.2.7
EditorConfig	EditorConfig	0.16.4
errorlens	usernamehw	3.4.1
gc-excelviewer	GrapeCity	4.0.45
gistfs	vsls-contrib	0.3.0
github-vscode-theme	GitHub	5.1.0
gitlens	eamodio	11.7.0
hexeditor	ms-vscode	1.8.2
LinkCheckMD	blackmist	0.3.1
markdown-all-in-one	yzhang	3.4.0
mono-debug	ms-vscode	0.16.2
powershell	ms-vscode	2021.10.2
remote-containers	ms-vscode-remote	0.205.2
remote-wsl	ms-vscode-remote	0.58.5
remotehub	GitHub	0.18.0
rewrap	stkb	1.15.4
richcodenav	MS-RichCodeNav	0.1.351
rust	rust-lang	0.7.8
vscode-commons	redhat	0.0.6
vscode-docker	ms-azuretools	1.18.0
vscode-github-actions	cschleiden	0.21.3
vscode-graphviz	joaompinto	0.0.6
vscode-markdownlint	DavidAnson	0.45.0
vscode-pull-request-github	GitHub	0.32.0
vscode-xml	redhat	0.18.1
vscode-yaml	redhat	1.2.2
whatsprintisit	lszomoru	0.0.1

[filipw]

This is a bug in the extension because it does not parse the version correctly, but even if it did, we would not be able to use MSBuild from a newer SDK than that for which OmniSharp was built.

The highest version OmniSharp can support is 6.0.100 because it aligns with its own NuGet dependencies it needs to load into process. A higher SDK would require higher NuGet versions and we'd crash. This is a known current limitation.

I guess the best we can do in short term is to limit the SDK search to 6.0.100 specifically. It would still not support higher SDKs than those for which OmniSharp would built, but it would at least not crash if you have newer + 6.0.100 installed at the same time.