Add a fallback path when launching the BuildHost

src/Workspaces/MSBuild/Core/MSBuild/BuildHostProcessManager.cs

@@ -30,6 +30,9 @@ internal sealed class BuildHostProcessManager : IAsyncDisposable
     private readonly SemaphoreSlim _gate = new(initialCount: 1);
     private readonly Dictionary<BuildHostProcessKind, BuildHostProcess> _processes = [];
 
+    private static string MSBuildWorkspaceDirectory => Path.GetDirectoryName(typeof(BuildHostProcessManager).Assembly.Location)!;
+    private static bool IsLoadedFromNuGetPackage => File.Exists(Path.Combine(MSBuildWorkspaceDirectory, "..", "..", "microsoft.codeanalysis.workspaces.msbuild.nuspec"));
+
     public BuildHostProcessManager(ImmutableDictionary<string, string>? globalMSBuildProperties = null, IBinLogPathProvider? binaryLogPathProvider = null, ILoggerFactory? loggerFactory = null)
     {
         _globalMSBuildProperties = globalMSBuildProperties ?? ImmutableDictionary<string, string>.Empty;
@@ -186,10 +189,7 @@ private ProcessStartInfo CreateDotNetCoreBuildHostStartInfo(string pipeName)
 
     internal static string GetNetCoreBuildHostPath()
     {
-        // The .NET Core build host is deployed as a content folder next to the application into the BuildHost-netcore path
-        var buildHostPath = Path.Combine(Path.GetDirectoryName(typeof(BuildHostProcessManager).Assembly.Location)!, "BuildHost-netcore", "Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.dll");
-        AssertBuildHostExists(buildHostPath);
-        return buildHostPath;
+        return GetBuildHostPath("BuildHost-netcore", "Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.dll");
     }
 
     private ProcessStartInfo CreateDotNetFrameworkBuildHostStartInfo(string pipeName)
@@ -221,16 +221,34 @@ private ProcessStartInfo CreateMonoBuildHostStartInfo(string pipeName)
 
     private static string GetDotNetFrameworkBuildHostPath()
     {
-        // The .NET Framework build host is deployed as a content folder next to the application into the BuildHost-net472 path
-        var netFrameworkBuildHost = Path.Combine(Path.GetDirectoryName(typeof(BuildHostProcessManager).Assembly.Location)!, "BuildHost-net472", "Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.exe");
-        AssertBuildHostExists(netFrameworkBuildHost);
-        return netFrameworkBuildHost;
+        return GetBuildHostPath("BuildHost-net472", "Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.exe");
     }
 
-    private static void AssertBuildHostExists(string buildHostPath)
+    private static string GetBuildHostPath(string contentFolderName, string assemblyName)
     {
+        // Possible BuildHost paths are relative to where the Workspaces.MSBuild assembly was loaded.
+        string buildHostPath;
+
+        if (IsLoadedFromNuGetPackage)
+        {
+            // When Workspaces.MSBuild is loaded from the NuGet package (as is the case in .NET Interactive, NCrunch, and possibly other use cases) 
+            // the Build host is deployed under the contentFiles folder.
+            // 
+            // Workspaces.MSBuild.dll Path - .nuget/packages/microsoft.codeanalysis.workspaces.msbuild/{version}/lib/{tfm}/Microsoft.CodeAnalysis.Workspaces.MSBuild.dll
+            // MSBuild.BuildHost.dll Path  - .nuget/packages/microsoft.codeanalysis.workspaces.msbuild/{version}/contentFiles/any/any/{contentFolderName}/{assemblyName}
+
+            buildHostPath = Path.GetFullPath(Path.Combine(MSBuildWorkspaceDirectory, "..", "..", "contentFiles", "any", "any", contentFolderName, assemblyName));
+        }
+        else
+        {
+            // When Workspaces.MSBuild is deployed as part of an application the build host is deployed as a content folder next to the application.
+            buildHostPath = Path.Combine(MSBuildWorkspaceDirectory, contentFolderName, assemblyName);
+        }
+
         if (!File.Exists(buildHostPath))
             throw new Exception(string.Format(WorkspaceMSBuildResources.The_build_host_could_not_be_found_at_0, buildHostPath));
+
+        return buildHostPath;
     }
 
     private void AppendBuildHostCommandLineArgumentsConfigureProcess(ProcessStartInfo processStartInfo, string pipeName)

src/Workspaces/MSBuild/Core/Microsoft.CodeAnalysis.Workspaces.MSBuild.csproj

@@ -97,6 +97,7 @@
     <ItemGroup Condition="'$(DotNetBuildSourceOnly)' != 'true'">
       <_NetFrameworkBuildHostProjectReference Include="..\..\..\Workspaces\MSBuild\BuildHost\Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj">
         <TargetFramework>net472</TargetFramework>
+        <!-- NOTE: Update the `BuildHostProcessManager.GetDotNetFrameworkBuildHostPath` method if ContentFolderName changes. -->
         <ContentFolderName>BuildHost-net472</ContentFolderName>
       </_NetFrameworkBuildHostProjectReference>
     </ItemGroup>
@@ -105,6 +106,7 @@
     <ItemGroup>
       <_NetFrameworkBuildHostProjectReference Include="..\..\..\Workspaces\MSBuild\BuildHost\Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj">
         <TargetFramework>$(NetRoslynBuildHostNetCoreVersion)</TargetFramework>
+        <!-- NOTE: Update the `BuildHostProcessManager.GetNetCoreBuildHostPath` method if ContentFolderName changes. -->
         <ContentFolderName>BuildHost-netcore</ContentFolderName>
       </_NetFrameworkBuildHostProjectReference>
     </ItemGroup>
@@ -118,6 +120,8 @@
       <!-- We're setting both TargetPath and PackagePath here, the former when this is used in a regular build to deploy and the latter for
            when this called during pack. By putting the PackagePath in any/any, we only have to deploy this once rather than per TFM.
            Similarly, we set CopyToOutputDirectory for the regular build and PackageCopyToPutput for the packaging process. -->
+
+      <!-- NOTE: Update the `BuildHostProcessManager.GetBuildHostPath` method if the PackagePath changes. -->
       <Content Include="%(NetFrameworkBuildHostAssets.Identity)"
                Condition="'%(NetFrameworkBuildHostAssets.TargetPath)' != '' and '%(NetFrameworkBuildHostAssets.Extension)' != '.xml'"
                TargetPath="$([System.IO.Path]::Combine('%(NetFrameworkBuildHostAssets.ContentFolderName)', '%(NetFrameworkBuildHostAssets.TargetPath)'))"