Directly create virtual project when dotnet run-api is missing for now

src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/FileBasedPrograms/FileBasedProgramsProjectSystem.cs

@@ -135,8 +135,10 @@ public async ValueTask TryRemoveMiscellaneousDocumentAsync(DocumentUri uri, bool
         var content = await _projectXmlProvider.GetVirtualProjectContentAsync(documentPath, cancellationToken);
         if (content is not var (virtualProjectContent, diagnostics))
         {
-            // 'GetVirtualProjectContentAsync' will log errors when it fails
-            return null;
+            // https://github.com/dotnet/roslyn/issues/78618: falling back to this until dotnet run-api is more widely available
+            _logger.LogInformation($"Failed to obtain virtual project for '{documentPath}' using dotnet run-api. Falling back to directly creating the virtual project.");
+            virtualProjectContent = VirtualProjectXmlProvider.MakeVirtualProjectContent_DirectFallback(documentPath);
+            diagnostics = [];
         }
 
         foreach (var diagnostic in diagnostics)

src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/FileBasedPrograms/VirtualProjectXmlProvider.cs

@@ -106,4 +106,106 @@ internal static bool IsFileBasedProgram(string documentFilePath, SourceText text
         var isFileBasedProgram = root.GetLeadingTrivia().Any(SyntaxKind.IgnoredDirectiveTrivia) || root.ChildNodes().Any(node => node.IsKind(SyntaxKind.GlobalStatement));
         return isFileBasedProgram;
     }
+
+    #region Temporary copy of subset of dotnet run-api behavior for fallback: https://github.com/dotnet/roslyn/issues/78618
+    // See https://github.com/dotnet/sdk/blob/b5dbc69cc28676ac6ea615654c8016a11b75e747/src/Cli/Microsoft.DotNet.Cli.Utils/Sha256Hasher.cs#L10
+    private static class Sha256Hasher
+    {
+        public static string Hash(string text)
+        {
+            byte[] bytes = Encoding.UTF8.GetBytes(text);
+            byte[] hash = SHA256.HashData(bytes);
+#if NET9_0_OR_GREATER
+            return Convert.ToHexStringLower(hash);
+#else
+            return Convert.ToHexString(hash).ToLowerInvariant();
+#endif
+        }
+
+        public static string HashWithNormalizedCasing(string text)
+        {
+            return Hash(text.ToUpperInvariant());
+        }
+    }
+
+    // See https://github.com/dotnet/sdk/blob/5a4292947487a9d34f4256c1d17fb3dc26859174/src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs#L449
+    internal static string GetArtifactsPath(string entryPointFileFullPath)
+    {
+        // We want a location where permissions are expected to be restricted to the current user.
+        string directory = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
+            ? Path.GetTempPath()
+            : Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
+
+        // Include entry point file name so the directory name is not completely opaque.
+        string fileName = Path.GetFileNameWithoutExtension(entryPointFileFullPath);
+        string hash = Sha256Hasher.HashWithNormalizedCasing(entryPointFileFullPath);
+        string directoryName = $"{fileName}-{hash}";
+
+        return Path.Join(directory, "dotnet", "runfile", directoryName);
+    }
+    #endregion
+
+    // https://github.com/dotnet/roslyn/issues/78618: falling back to this until dotnet run-api is more widely available
+    internal static string MakeVirtualProjectContent_DirectFallback(string documentFilePath)
+    {
+        Contract.ThrowIfFalse(PathUtilities.IsAbsolute(documentFilePath));
+        var artifactsPath = GetArtifactsPath(documentFilePath);
+
+        var targetFramework = Environment.GetEnvironmentVariable("DOTNET_RUN_FILE_TFM") ?? "net10.0";
+
+        var virtualProjectXml = $"""
+            <Project>
+              <PropertyGroup>
+                <IncludeProjectNameInArtifactsPaths>false</IncludeProjectNameInArtifactsPaths>
+                <ArtifactsPath>{SecurityElement.Escape(artifactsPath)}</ArtifactsPath>
+              </PropertyGroup>
+              <!-- We need to explicitly import Sdk props/targets so we can override the targets below. -->
+              <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
+              <PropertyGroup>
+                <OutputType>Exe</OutputType>
+                <TargetFramework>{SecurityElement.Escape(targetFramework)}</TargetFramework>
+                <ImplicitUsings>enable</ImplicitUsings>
+                <Nullable>enable</Nullable>
+              </PropertyGroup>
+              <PropertyGroup>
+                <EnableDefaultItems>false</EnableDefaultItems>
+              </PropertyGroup>
+              <PropertyGroup>
+                <LangVersion>preview</LangVersion>
+              </PropertyGroup>
+              <PropertyGroup>
+                <Features>$(Features);FileBasedProgram</Features>
+              </PropertyGroup>
+              <ItemGroup>
+                <Compile Include="{SecurityElement.Escape(documentFilePath)}" />
+              </ItemGroup>
+              <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
+              <!--
+                Override targets which don't work with project files that are not present on disk.
+                See https://github.com/NuGet/Home/issues/14148.
+              -->
+              <Target Name="_FilterRestoreGraphProjectInputItems"
+                      DependsOnTargets="_LoadRestoreGraphEntryPoints"
+                      Returns="@(FilteredRestoreGraphProjectInputItems)">
+                <ItemGroup>
+                  <FilteredRestoreGraphProjectInputItems Include="@(RestoreGraphProjectInputItems)" />
+                </ItemGroup>
+              </Target>
+              <Target Name="_GetAllRestoreProjectPathItems"
+                      DependsOnTargets="_FilterRestoreGraphProjectInputItems"
+                      Returns="@(_RestoreProjectPathItems)">
+                <ItemGroup>
+                  <_RestoreProjectPathItems Include="@(FilteredRestoreGraphProjectInputItems)" />
+                </ItemGroup>
+              </Target>
+              <Target Name="_GenerateRestoreGraph"
+                      DependsOnTargets="_FilterRestoreGraphProjectInputItems;_GetAllRestoreProjectPathItems;_GenerateRestoreGraphProjectEntry;_GenerateProjectRestoreGraph"
+                      Returns="@(_RestoreGraphEntry)">
+                <!-- Output from dependency _GenerateRestoreGraphProjectEntry and _GenerateProjectRestoreGraph -->
+              </Target>
+            </Project>
+            """;
+
+        return virtualProjectXml;
+    }
 }