Add a pre-visit create request hook

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/RestClientProvider.cs

@@ -81,21 +81,45 @@ protected override ScmMethodProvider[] BuildMethods()
             {
                 var operation = serviceMethod.Operation;
                 var method = BuildCreateRequestMethod(serviceMethod);
-                methods.Add(method);
-                MethodCache[operation] = method;
+                method = VisitCreateRequestMethod(method, serviceMethod);
+
+                if (method != null)
+                {
+                    methods.Add(method);
+                    MethodCache[operation] = method;
+                }
 
                 // For paging operations with next link, also generate a CreateNextXXXRequest method
                 if (serviceMethod is InputPagingServiceMethod { PagingMetadata.NextLink: not null })
                 {
                     var nextMethod = BuildCreateRequestMethod(serviceMethod, isNextLinkRequest: true);
-                    methods.Add(nextMethod);
-                    NextMethodCache[operation] = nextMethod;
+                    nextMethod = VisitCreateRequestMethod(nextMethod, serviceMethod);
+
+                    if (nextMethod != null)
+                    {
+                        methods.Add(nextMethod);
+                        NextMethodCache[operation] = nextMethod;
+                    }
                 }
             }
 
             return [.. methods];
         }
 
+        private ScmMethodProvider? VisitCreateRequestMethod(ScmMethodProvider method, InputServiceMethod serviceMethod)
+        {
+            ScmMethodProvider? result = method;
+            foreach (var visitor in ScmCodeModelGenerator.Instance.Visitors)
+            {
+                if (visitor is ScmLibraryVisitor scmVisitor)
+                {
+                    result = scmVisitor.VisitCreateRequestMethod(serviceMethod, this, result);
+                }
+            }
+
+            return method;
+        }
+
         private ScmMethodProvider BuildCreateRequestMethod(InputServiceMethod serviceMethod, bool isNextLinkRequest = false)
         {
             var options = ScmKnownParameters.RequestOptions;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/ScmLibraryVisitor.cs

@@ -17,6 +17,14 @@ public abstract class ScmLibraryVisitor : LibraryVisitor
             return methodProviderCollection;
         }
 
+        protected internal virtual ScmMethodProvider? VisitCreateRequestMethod(
+            InputServiceMethod serviceMethod,
+            RestClientProvider enclosingType,
+            ScmMethodProvider? createRequestMethodProvider)
+        {
+            return createRequestMethodProvider;
+        }
+
         protected internal virtual ClientProvider? Visit(InputClient client, ClientProvider? clientProvider)
         {
             return clientProvider;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/InputLibraryVisitorTests.cs

@@ -67,5 +67,45 @@ public void PreVisitsClients()
 
             _mockVisitor.Protected().Verify<ClientProvider?>("Visit", Times.Once(), inputClient, ItExpr.IsAny<ClientProvider?>());
         }
+
+        [Test]
+        public void PreVisitsCreateRequestMethods()
+        {
+            _mockGenerator.Object.AddVisitor(_mockVisitor.Object);
+
+            var inputOperation = InputFactory.Operation("testOperation", responses: [InputFactory.OperationResponse(bodytype: InputPrimitiveType.Any)]);
+            var inputServiceMethod = InputFactory.BasicServiceMethod("test", inputOperation);
+            var inputClient = InputFactory.Client("fooClient", methods: [inputServiceMethod]);
+            _mockInputLibrary.Setup(l => l.InputNamespace).Returns(InputFactory.Namespace(
+                "Sample",
+                clients: [inputClient]));
+
+            var clientProvider = new ClientProvider(inputClient);
+            var restClient = clientProvider.RestClient;
+            _ = restClient.Methods;
+
+            _mockVisitor.Protected().Verify<ScmMethodProvider?>("VisitCreateRequestMethod", Times.Once(), inputServiceMethod, ItExpr.IsAny<RestClientProvider>(), ItExpr.IsAny<ScmMethodProvider?>());
+        }
+
+        [Test]
+        public void PreVisitsCreateNextRequestMethods()
+        {
+            _mockGenerator.Object.AddVisitor(_mockVisitor.Object);
+
+            var inputOperation = InputFactory.Operation("testOperation", responses: [InputFactory.OperationResponse(bodytype: InputPrimitiveType.Any)]);
+            var pagingMetadata = InputFactory.NextLinkPagingMetadata(["items"], ["nextLink"], InputResponseLocation.Body);
+            var inputServiceMethod = InputFactory.PagingServiceMethod("test", inputOperation, pagingMetadata: pagingMetadata);
+            var inputClient = InputFactory.Client("fooClient", methods: [inputServiceMethod]);
+            _mockInputLibrary.Setup(l => l.InputNamespace).Returns(InputFactory.Namespace(
+                "Sample",
+                clients: [inputClient]));
+
+            var clientProvider = new ClientProvider(inputClient);
+            var restClient = clientProvider.RestClient;
+            _ = restClient.Methods;
+
+            // Verified twice because it is called for CreateRequest and CreateNextRequest
+            _mockVisitor.Protected().Verify<ScmMethodProvider?>("VisitCreateRequestMethod", Times.Exactly(2), inputServiceMethod, ItExpr.IsAny<RestClientProvider>(), ItExpr.IsAny<ScmMethodProvider?>());
+        }
     }
 }