msbuild-loader-netframework

name

msbuild-loader-netframework

description

How MSBuildLocator loads MSBuild assemblies and discovers installs on .NET Framework (the net46 target / #if NET46 / non-NETCOREAPP branches in src/MSBuildLocator). Covers the AppDomain.AssemblyResolve handler, search-path probing, and Developer Console + Visual Studio Setup (COM) discovery. Use when editing or reviewing net46 loader, registration, or VS-discovery code, or diagnosing assembly-resolution behavior on .NET Framework hosts.

.NET Framework (net46) MSBuild loader

Scope: src/MSBuildLocator/MSBuildLocator.cs #if NET46 and Framework (#else of NETCOREAPP) branches. FEATURE_VISUALSTUDIOSETUP is defined only when TargetFramework == net46 in Microsoft.Build.Locator.csproj. For build/test and cross-cutting conventions, see AGENTS.md.

Assembly-resolution handler

• s_registeredHandler is a static ResolveEventHandler; IsRegistered is s_registeredHandler != null.
• RegisterMSBuildPathsInternally stores the handler in the static field before subscribing to AppDomain.CurrentDomain.AssemblyResolve; the event subscription keeps the delegate alive, while the field tracks registration state.
• AssemblyResolve can fire repeatedly for the same assembly; results are cached in loadedAssemblies keyed by AssemblyName.FullName.
• Resolution is explicitly not thread-safe; every cache lookup/load runs under lock (loadedAssemblies).
• Handler path: parse eventArgs.Name with new AssemblyName(eventArgs.Name); for each registered search path, if <msbuildPath>\<Name>.dll exists, return Assembly.LoadFrom(targetAssembly).
• Search paths come from RegisterMSBuildPath(...), or from RegisterInstance(...) as instance.MSBuildPath plus the VS NuGet path when it exists.

Discovery sources (net46 only)

• Developer command prompt: GetDevConsoleInstance() reads VSINSTALLDIR, then parses VSCMD_VER (trimming any suffix after -), then falls back to VisualStudioVersion; yields DiscoveryType.DeveloperConsole.
• Visual Studio Setup COM API: under FEATURE_VISUALSTUDIOSETUP, VisualStudioLocationHelper.GetInstances() enumerates VS 2017+ setup instances with Microsoft.Component.MSBuild; yields DiscoveryType.VisualStudioSetup.
• DiscoveryType.DotNetSdk exists in the enum but belongs to the Core path; net46 GetInstances(...) does not call SDK discovery.

What net46 does NOT do

• No AssemblyLoadContext, hostfxr, or .NET SDK discovery — those are #if NETCOREAPP paths (see the msbuild-loader-netcore skill).

Register-before-load contract

• CanRegister is false once any strong-named Microsoft.Build* assembly in s_msBuildAssemblies is loaded in the current AppDomain.
• JIT caveat: a method that references Microsoft.Build types can trip the contract when JIT-compiled, even if that reference never executes. Keep locator calls isolated before any such reference.