a minor refactor on mgmt plane code (#3149)

Author: ArcturusZhang

src/AutoRest.CSharp/Mgmt/Generation/MgmtClientBaseWriter.cs

@@ -506,9 +506,6 @@ protected virtual void WriteMethod(MgmtClientOperation clientOperation, bool isA
         protected Dictionary<string, WriteMethodDelegate> _customMethods = new Dictionary<string, WriteMethodDelegate>();
         private WriteMethodDelegate GetMethodDelegate(MgmtClientOperation clientOperation)
         {
-            if (clientOperation.IsLongRunningOperation && clientOperation.IsPagingOperation)
-                throw new NotImplementedException($"Pageable LRO is not implemented yet, please use `remove-operation` directive to remove the following operationIds: {string.Join(", ", clientOperation.Select(o => o.OperationId))}");
-
             if (!_customMethods.TryGetValue($"Write{clientOperation.Name}Body", out var function))
             {
                 function = GetMethodDelegate(clientOperation.IsLongRunningOperation, clientOperation.IsPagingOperation);
@@ -523,9 +520,14 @@ protected virtual WriteMethodDelegate GetMethodDelegate(bool isLongRunning, bool
                 (true, false) => WriteLROMethodBody,
                 (false, true) => WritePagingMethodBody,
                 (false, false) => WriteNormalMethodBody,
-                _ => throw new InvalidOperationException("Unknown method combination"),
+                (true, true) => WritePagingLROMethodBody,
             };
 
+        private void WritePagingLROMethodBody(MgmtClientOperation clientOperation, Diagnostic diagnostic, bool isAsync)
+        {
+            throw new NotImplementedException($"Pageable LRO is not implemented yet, please use `remove-operation` directive to remove the following operationIds: {string.Join(", ", clientOperation.Select(o => o.OperationId))}");
+        }
+
         protected IDisposable WriteCommonMethod(MgmtClientOperation clientOperation, bool isAsync)
         {
             _writer.Line();