poc for reuse

Author: dadhi

src/FastExpressionCompiler/TestTools.cs

@@ -591,7 +591,7 @@ public static implicit operator TestContext(TestRun t)
     public bool Fail(string testName, int sourceLineNumber, AssertKind assertKind, string message)
     {
 #if DEBUG
-        // When debugging raise the fail immediately as excpetion to avoid false sense of security
+        // When debugging raise the fail immediately as exception to avoid false sense of security
         throw new AssertionException($"`{testName}` failed at line {sourceLineNumber}:{NewLine}{message}{NewLine}");
 #else
         TestRun.Failures.Add(new TestFailure(testName, sourceLineNumber, assertKind, message));

test/FastExpressionCompiler.IssueTests/EmitHacksTest.cs

@@ -28,7 +28,47 @@ public int Run()
 
         void TryToReuseTheDynamicMethod(TestContext t)
         {
-            t.Fail("TryToReuseTheDynamicMethod is not implemented yet");
+            // Let's say there is a DynamicMethod of Func<int, int> returning its argument, 
+            // at the end I want to reuse the created DynamicMethod for another Func<int, int> returning arg + 42
+
+            var dynMethod = new DynamicMethod(string.Empty,
+                typeof(int),
+                [typeof(ExpressionCompiler.ArrayClosure), typeof(int)], 
+                typeof(ExpressionCompiler.ArrayClosure), 
+                true);
+            
+            var il = dynMethod.GetILGenerator();
+
+            il.Emit(OpCodes.Ldarg_1);
+            il.Emit(OpCodes.Ret);
+
+            var func = (Func<int, int>)dynMethod.CreateDelegate(typeof(Func<int, int>), ExpressionCompiler.EmptyArrayClosure);
+            t.AreEqual(41, func(41));
+
+            // Reset the DynamicMethod internals that we need to reuse:
+            // _ilGenerator = null;
+            // _initLocals = true;
+            // _methodHandle = null;
+            var dynMethodType = dynMethod.GetType();
+            var ilGeneratorField = dynMethodType.GetField("_ilGenerator", BindingFlags.Instance | BindingFlags.NonPublic);
+            var initLocalsField = dynMethodType.GetField("_initLocals", BindingFlags.Instance | BindingFlags.NonPublic);
+            var methodHandleField = dynMethodType.GetField("_methodHandle", BindingFlags.Instance | BindingFlags.NonPublic);
+
+            Console.WriteLine($"{{_ilGenerator: {ilGeneratorField?.GetValue(dynMethod) ?? "null"}, _initLocals: {initLocalsField.GetValue(dynMethod)}, _methodHandle: {methodHandleField?.GetValue(dynMethod) ?? "null"}}}");
+            ilGeneratorField.SetValue(dynMethod, null);
+            initLocalsField.SetValue(dynMethod, true);
+            // methodHandleField.SetValue(dynMethod, null); // breaks the CLR
+
+            var il2 = dynMethod.GetILGenerator();
+
+            il2.Emit(OpCodes.Ldarg_1);
+            il2.Emit(OpCodes.Ldc_I4, 42);
+            il2.Emit(OpCodes.Add);
+            il2.Emit(OpCodes.Ret);
+            t.AreEqual(41, func(41));
+
+            var func2 = (Func<int, int>)dynMethod.CreateDelegate(typeof(Func<int, int>), ExpressionCompiler.EmptyArrayClosure);
+            t.AreEqual(83, func2(41));
         }
 
         public void DynamicMethod_Emit_Hack()