Support .NET Framework 4.6.1

pythonnet · Metadorius · Mar 26, 2026 · Mar 27, 2026 · Mar 27, 2026 · Mar 27, 2026
commit 63cf5a790ea39d2c618be54f549a0c62b3f277aa
diff --git a/setup.py b/setup.py
@@ -127,7 +127,7 @@ def finalize_options(self):
 dotnet_libs = [
     DotnetLib(
         "python-runtime",
-        "src/runtime/Python.Runtime.csproj",
+        "src/compat/Python.Runtime.Compat.csproj",
         output="pythonnet/runtime",
     )
 ]

diff --git a/src/compat/Python.Runtime.Compat.csproj b/src/compat/Python.Runtime.Compat.csproj
@@ -0,0 +1,13 @@
+<!-- A dummy project to force MSBuild to package .NET 4.6.1 requirements -->
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net461</TargetFramework>
+    <OutputType>Library</OutputType>
+    <!-- Don't copy the dummy assembly/pdb to output; we only need the resolved references -->
+    <CopyBuildOutputToOutputDirectory>false</CopyBuildOutputToOutputDirectory>
+    <CopyOutputSymbolsToOutputDirectory>false</CopyOutputSymbolsToOutputDirectory>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\runtime\Python.Runtime.csproj" />
+  </ItemGroup>
+</Project>
diff --git a/src/runtime/AppConfig.cs b/src/runtime/AppConfig.cs
@@ -0,0 +1,44 @@
+using System;
+using System.Linq;
+using System.Reflection;
+
+namespace Python.Runtime
+{
+    static class AppConfig
+    {
+        /// <summary>
+        /// Override the AppDomain's config file path and reset the
+        /// ConfigurationManager cache so the CLR re-reads binding redirects.
+        /// Uses reflection to avoid a compile-time dependency on System.Configuration.
+        /// </summary>
+        public static void SetConfigFile(string path)
+        {
+            AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", path);
+            ResetConfigMechanism();
+        }
+
+        private static void ResetConfigMechanism()
+        {
+            var configManager = Type.GetType(
+                "System.Configuration.ConfigurationManager, System.Configuration, "
+                + "Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
+
+            if (configManager == null)
+                return;
+
+            configManager
+                .GetField("s_initState", BindingFlags.NonPublic | BindingFlags.Static)
+                ?.SetValue(null, 0);
+
+            configManager
+                .GetField("s_configSystem", BindingFlags.NonPublic | BindingFlags.Static)
+                ?.SetValue(null, null);
+
+            configManager.Assembly
+                .GetTypes()
+                .FirstOrDefault(x => x.FullName == "System.Configuration.ClientConfigPaths")
+                ?.GetField("s_current", BindingFlags.NonPublic | BindingFlags.Static)
+                ?.SetValue(null, null);
+        }
+    }
+}
diff --git a/src/runtime/Loader.cs b/src/runtime/Loader.cs
@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Text;
 
 namespace Python.Runtime
@@ -12,6 +13,22 @@ public unsafe static int Initialize(IntPtr data, int size)
         {
             try
             {
+                // On .NET Framework, the host is python.exe which has no binding
+                // redirects for netstandard2.0 shims (e.g. RuntimeInformation
+                // Version=0.0.0.0 vs the 4.0.2.0 shim on disk). Binding redirects
+                // via config files can't be injected after AppDomain creation, so
+                // resolve assemblies from our runtime directory directly.
+                AppDomain.CurrentDomain.AssemblyResolve += (_, args) =>
+                {
+                    var name = new System.Reflection.AssemblyName(args.Name);
+                    var dir = Path.GetDirectoryName(typeof(Loader).Assembly.Location);
+                    var path = Path.Combine(dir, name.Name + ".dll");
+
+                    return File.Exists(path)
+                        ? System.Reflection.Assembly.LoadFrom(path)
+                        : null;
+                };
+
                 var dllPath = Encodings.UTF8.GetString((byte*)data.ToPointer(), size);
 
                 if (!string.IsNullOrEmpty(dllPath))