Load hostfxr library from dotnet root so it can be found from NativeAOT

Author: dsplaisted

src/Resolvers/Microsoft.DotNet.NativeWrapper/NETBundlesNativeWrapper.cs

@@ -5,15 +5,55 @@ namespace Microsoft.DotNet.NativeWrapper
 {
     public class NETBundlesNativeWrapper : INETBundleProvider
     {
+        // lpFileName passed to LoadLibraryEx must be a full path.
+        private const int LOAD_WITH_ALTERED_SEARCH_PATH = 0x8;
+
+        [DllImport("kernel32.dll", CharSet = CharSet.Unicode, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
+        private static extern IntPtr LoadLibraryExW(string lpFileName, IntPtr hFile, int dwFlags);
+
         public NetEnvironmentInfo GetDotnetEnvironmentInfo(string dotnetExeDirectory)
         {
+            PreloadHostFxrLibrary(dotnetExeDirectory);
+
             var info = new NetEnvironmentInfo();
             IntPtr reserved = IntPtr.Zero;
             IntPtr resultContext = IntPtr.Zero;
 
-            int errorCode = Interop.hostfxr_get_dotnet_environment_info(dotnetExeDirectory, reserved, info.Initialize, resultContext);
+            //  If directory doesn't exist, we may not be able to load hostfxr, so treat as if there are no installed frameworks/SDKs.
+            if (Directory.Exists(dotnetExeDirectory))
+            {
+                int errorCode = Interop.hostfxr_get_dotnet_environment_info(dotnetExeDirectory, reserved, info.Initialize, resultContext);
+            }
 
             return info;
         }
+
+        private void PreloadHostFxrLibrary(string dotnetExeDirectory)
+        {
+            string? hostFxrPath = FindHostFxrLibrary(dotnetExeDirectory);
+            if (hostFxrPath != null)
+            {
+                LoadLibraryExW(hostFxrPath, IntPtr.Zero, LOAD_WITH_ALTERED_SEARCH_PATH);
+            }
+        }
+
+        private static string? FindHostFxrLibrary(string installRoot)
+        {
+            string hostFxrDirectory = Path.Combine(installRoot, "host", "fxr");
+            if (!Directory.Exists(hostFxrDirectory))
+            {
+                return null;
+            }
+
+            string libraryName = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
+                ? "hostfxr.dll"
+                : RuntimeInformation.IsOSPlatform(OSPlatform.OSX)
+                    ? "libhostfxr.dylib"
+                    : "libhostfxr.so";
+
+            return Directory.EnumerateFiles(hostFxrDirectory, libraryName, SearchOption.AllDirectories)
+                .OrderByDescending(File.GetLastWriteTimeUtc)
+                .FirstOrDefault();
+        }
     }
 }