diff --git a/Source/DafnyCore/Backends/CSharp/CsharpCodeGenerator.cs b/Source/DafnyCore/Backends/CSharp/CsharpCodeGenerator.cs
index 770aeb9a096..8f1af398528 100644
--- a/Source/DafnyCore/Backends/CSharp/CsharpCodeGenerator.cs
+++ b/Source/DafnyCore/Backends/CSharp/CsharpCodeGenerator.cs
@@ -185,15 +185,10 @@ protected override void EmitBuiltInDecls(SystemModuleManager systemModuleManager
     //   }
     // They aren't in any namespace to make them universally accessible.
     private void EmitFuncExtensions(SystemModuleManager systemModuleManager, ConcreteSyntaxTree wr) {
-      // An extension for this arity will be provided in the Runtime which has to be linked.
-      var omitAritiesBefore16 = !Options.IncludeRuntime && Options.SystemModuleTranslationMode is not CommonOptionBag.SystemModuleMode.OmitAllOtherModules;
-      var name = omitAritiesBefore16 ? "FuncExtensionsAfterArity16" : "FuncExtensions";
-      var funcExtensions = wr.NewNamedBlock("public static class " + name);
+      var funcExtensions = wr.NewNamedBlock("internal static class FuncExtensions");
+      wr.WriteLine("// end of class FuncExtensions");
       foreach (var kv in systemModuleManager.ArrowTypeDecls) {
         int arity = kv.Key;
-        if (omitAritiesBefore16 && arity <= 16) {
-          continue;
-        }
 
         List<string> TypeParameterList(string prefix) {
           var l = arity switch {
diff --git a/Source/DafnyCore/DafnyGeneratedFromDafnyPost.py b/Source/DafnyCore/DafnyGeneratedFromDafnyPost.py
index f493acfaea3..af5eadec414 100644
--- a/Source/DafnyCore/DafnyGeneratedFromDafnyPost.py
+++ b/Source/DafnyCore/DafnyGeneratedFromDafnyPost.py
@@ -67,6 +67,18 @@
 
     print(f"File generated: {file_path}")
 
+  # Special-case the FuncExtensions class, which isn't declared inside a namespace
+  func_extensions_pattern = re.compile(r'(internal\s+static\s+class\s+FuncExtensions\s*{[\s\S]*?}\s*//\s*end\s*of\s*class\s*FuncExtensions)')
+  match = func_extensions_pattern.search(content)
+  func_extensions_content = match[0]
+
+  file_content = f"{prelude}\n\n{func_extensions_content}"
+  file_path = f"{output}/FuncExtensions.cs"
+  with open(file_path, 'w') as file:
+    file.write(file_content)
+
+  print(f"File generated: {file_path}")
+
 # Now delete the file output.cs
 os.remove(output + '.cs')
 print("File deleted: " + output + '.cs')
diff --git a/Source/DafnyCore/GeneratedFromDafny/FuncExtensions.cs b/Source/DafnyCore/GeneratedFromDafny/FuncExtensions.cs
new file mode 100644
index 00000000000..39894fcdf07
--- /dev/null
+++ b/Source/DafnyCore/GeneratedFromDafny/FuncExtensions.cs
@@ -0,0 +1,38 @@
+// Dafny program the_program compiled into C#
+// To recompile, you will need the libraries
+//     System.Runtime.Numerics.dll System.Collections.Immutable.dll
+// but the 'dotnet' tool in net6.0 should pick those up automatically.
+// Optionally, you may want to include compiler switches like
+//     /debug /nowarn:162,164,168,183,219,436,1717,1718
+
+using System;
+using System.Numerics;
+using System.Collections;
+#pragma warning disable CS0164 // This label has not been referenced
+#pragma warning disable CS0162 // Unreachable code detected
+#pragma warning disable CS1717 // Assignment made to same variable
+
+internal static class FuncExtensions {
+  public static Func<U, UResult> DowncastClone<T, TResult, U, UResult>(this Func<T, TResult> F, Func<U, T> ArgConv, Func<TResult, UResult> ResConv) {
+    return arg => ResConv(F(ArgConv(arg)));
+  }
+  public static Func<UResult> DowncastClone<TResult, UResult>(this Func<TResult> F, Func<TResult, UResult> ResConv) {
+    return () => ResConv(F());
+  }
+  public static Func<U1, U2, UResult> DowncastClone<T1, T2, TResult, U1, U2, UResult>(this Func<T1, T2, TResult> F, Func<U1, T1> ArgConv1, Func<U2, T2> ArgConv2, Func<TResult, UResult> ResConv) {
+    return (arg1, arg2) => ResConv(F(ArgConv1(arg1), ArgConv2(arg2)));
+  }
+  public static Func<U1, U2, U3, UResult> DowncastClone<T1, T2, T3, TResult, U1, U2, U3, UResult>(this Func<T1, T2, T3, TResult> F, Func<U1, T1> ArgConv1, Func<U2, T2> ArgConv2, Func<U3, T3> ArgConv3, Func<TResult, UResult> ResConv) {
+    return (arg1, arg2, arg3) => ResConv(F(ArgConv1(arg1), ArgConv2(arg2), ArgConv3(arg3)));
+  }
+  public static Func<U1, U2, U3, U4, UResult> DowncastClone<T1, T2, T3, T4, TResult, U1, U2, U3, U4, UResult>(this Func<T1, T2, T3, T4, TResult> F, Func<U1, T1> ArgConv1, Func<U2, T2> ArgConv2, Func<U3, T3> ArgConv3, Func<U4, T4> ArgConv4, Func<TResult, UResult> ResConv) {
+    return (arg1, arg2, arg3, arg4) => ResConv(F(ArgConv1(arg1), ArgConv2(arg2), ArgConv3(arg3), ArgConv4(arg4)));
+  }
+  public static Func<U1, U2, U3, U4, U5, U6, U7, UResult> DowncastClone<T1, T2, T3, T4, T5, T6, T7, TResult, U1, U2, U3, U4, U5, U6, U7, UResult>(this Func<T1, T2, T3, T4, T5, T6, T7, TResult> F, Func<U1, T1> ArgConv1, Func<U2, T2> ArgConv2, Func<U3, T3> ArgConv3, Func<U4, T4> ArgConv4, Func<U5, T5> ArgConv5, Func<U6, T6> ArgConv6, Func<U7, T7> ArgConv7, Func<TResult, UResult> ResConv) {
+    return (arg1, arg2, arg3, arg4, arg5, arg6, arg7) => ResConv(F(ArgConv1(arg1), ArgConv2(arg2), ArgConv3(arg3), ArgConv4(arg4), ArgConv5(arg5), ArgConv6(arg6), ArgConv7(arg7)));
+  }
+  public static Func<U1, U2, U3, U4, U5, UResult> DowncastClone<T1, T2, T3, T4, T5, TResult, U1, U2, U3, U4, U5, UResult>(this Func<T1, T2, T3, T4, T5, TResult> F, Func<U1, T1> ArgConv1, Func<U2, T2> ArgConv2, Func<U3, T3> ArgConv3, Func<U4, T4> ArgConv4, Func<U5, T5> ArgConv5, Func<TResult, UResult> ResConv) {
+    return (arg1, arg2, arg3, arg4, arg5) => ResConv(F(ArgConv1(arg1), ArgConv2(arg2), ArgConv3(arg3), ArgConv4(arg4), ArgConv5(arg5)));
+  }
+}
+// end of class FuncExtensions
\ No newline at end of file
diff --git a/Source/DafnyRuntime/DafnyRuntimeSystemModule.cs b/Source/DafnyRuntime/DafnyRuntimeSystemModule.cs
index 2c19c6eacb4..b5b6988b139 100644
--- a/Source/DafnyRuntime/DafnyRuntimeSystemModule.cs
+++ b/Source/DafnyRuntime/DafnyRuntimeSystemModule.cs
@@ -503,7 +503,7 @@ public static T[] InitNewArray1<T>(T z, BigInteger size0) {
     }
   }
 } // end of namespace Dafny
-public static class FuncExtensions {
+internal static class FuncExtensions {
   public static Func<U, UResult> DowncastClone<T, TResult, U, UResult>(this Func<T, TResult> F, Func<U, T> ArgConv, Func<TResult, UResult> ResConv) {
     return arg => ResConv(F(ArgConv(arg)));
   }
@@ -556,6 +556,7 @@ public static Func<U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14,
     return (arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11, arg12, arg13, arg14, arg15, arg16) => ResConv(F(ArgConv1(arg1), ArgConv2(arg2), ArgConv3(arg3), ArgConv4(arg4), ArgConv5(arg5), ArgConv6(arg6), ArgConv7(arg7), ArgConv8(arg8), ArgConv9(arg9), ArgConv10(arg10), ArgConv11(arg11), ArgConv12(arg12), ArgConv13(arg13), ArgConv14(arg14), ArgConv15(arg15), ArgConv16(arg16)));
   }
 }
+// end of class FuncExtensions
 #endif
 namespace _System {