.slnx file format is not ignored

[iserrato]

Describe the bug

In Visual Studio, when a user creates a new solution file using the .slnx solution file format, it shows up as content in a project when the .slnx and the project file are in the same folder. We believe this stems from Microsoft.NET.Sdk,DefaultItems.targets only listing the classic solution file format, .sln, and omitting the xml format, .slnx.

To Reproduce

1. Launch VS and go to Projects and Solutions > General
2. Set Default Solution File Format to .slnx
3. File > New > Project, select .NET Core Console project and checking "Place solution and project in the same directory"
4. Observe the display in SE
5. Double-clicking the .slnx file results in error.
   

Exceptions (if any)

N/A

Further technical details

VS version 17.11 Preview 3 and higher

.NET SDK:
Version: 9.0.203
Commit: dc7acfa
Workload version: 9.0.200-manifests.78a0f7ac
MSBuild version: 17.13.20+a4ef1e90f

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22631
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.203\

.NET workloads installed:
[wasm-tools-net8]
Installation Source: VS 17.14.36023.58
Manifest Version: 9.0.4/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.net8\9.0.4\WorkloadManifest.json
Install Type: Msi

[wasm-tools-net6]
Installation Source: VS 17.14.36023.58
Manifest Version: 9.0.4/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.net6\9.0.4\WorkloadManifest.json
Install Type: Msi

[wasm-tools]
Installation Source: VS 17.14.36023.58
Manifest Version: 9.0.4/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.current\9.0.4\WorkloadManifest.json
Install Type: Msi

[aspire]
Installation Source: VS 17.14.36023.58
Manifest Version: 8.2.2/8.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.2.2\WorkloadManifest.json
Install Type: Msi

[wasm-tools-net7]
Installation Source: VS 17.14.36023.58
Manifest Version: 9.0.4/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.workload.mono.toolchain.net7\9.0.4\WorkloadManifest.json
Install Type: Msi

Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.4
Architecture: x64
Commit: f57e6dc747

.NET SDKs installed:
8.0.206 [C:\Program Files\dotnet\sdk]
8.0.300 [C:\Program Files\dotnet\sdk]
8.0.408 [C:\Program Files\dotnet\sdk]
9.0.105 [C:\Program Files\dotnet\sdk]
9.0.201 [C:\Program Files\dotnet\sdk]
9.0.203 [C:\Program Files\dotnet\sdk]
9.0.300-preview.0.25177.5 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.14 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.14 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.14 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Q:\src\VS\global.json