[http-client-csharp] use recursion for statement flattening (microsoft#5268)

contributes to microsoft#5017

Author: jorgerangel-msft

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/Statements/MethodBodyStatement.cs

@@ -28,24 +28,19 @@ internal override void Write(CodeWriter writer)
 
         public IEnumerable<MethodBodyStatement> Flatten()
         {
-            Queue<MethodBodyStatement> queue = new();
-            queue.Enqueue(this);
-
-            while (queue.Count > 0)
+            if (this is MethodBodyStatements statements)
             {
-                MethodBodyStatement current = queue.Dequeue();
-
-                if (current is MethodBodyStatements statements)
+                foreach (var statement in statements.Statements)
                 {
-                    foreach (var subStatement in statements.Statements)
+                    foreach (var subStatement in statement.Flatten())
                     {
-                        queue.Enqueue(subStatement);
+                        yield return subStatement;
                     }
                 }
-                else
-                {
-                    yield return current;
-                }
+            }
+            else
+            {
+                yield return this;
             }
         }
 

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/Statements/StatementTests.cs

@@ -518,6 +518,33 @@ public void TestFlatten()
             Assert.AreEqual(ifTrueStatement.Body.ToDisplayString(), body[0].ToDisplayString());
         }
 
-
+        [Test]
+        public void TestFlatten_CorrectNestedOrder()
+        {
+            var statement1 = new IfStatement(True) { Return(True) };
+            var statement2 = new IfStatement(False) { Return(False) };
+            var nestedStatement1 = new IfStatement(False) { Return(Literal("Foo")) };
+            var nestedStatement2 = new IfStatement(True) { Return(Literal("Bar")) };
+            var methodBodyStatements = new MethodBodyStatements(
+            [
+                statement1,
+                new MethodBodyStatements(
+                [
+                    nestedStatement1,
+                    nestedStatement2
+                ]),
+                statement2
+            ]);
+
+            var result = methodBodyStatements.Flatten();
+            var expectedOrder = new List<MethodBodyStatement>
+            {
+                statement1,
+                nestedStatement1,
+                nestedStatement2,
+                statement2
+            };
+            Assert.AreEqual(expectedOrder, result);
+        }
     }
 }