Simpler detect oversized batch (#840)

src/Transport/Sending/MessageDispatcher.cs

@@ -1,5 +1,6 @@
 namespace NServiceBus.Transport.AzureServiceBus
 {
+    using System;
     using System.Collections.Generic;
     using System.Threading.Tasks;
     using System.Transactions;
@@ -8,6 +9,8 @@
 
     class MessageDispatcher : IDispatchMessages
     {
+        const int MaxMessageThresholdForTransaction = 100;
+
         readonly MessageSenderRegistry messageSenderRegistry;
         readonly string topicName;
 
@@ -27,6 +30,8 @@ public Task Dispatch(TransportOperations outgoingMessages, TransportTransaction
             var unicastTransportOperations = outgoingMessages.UnicastTransportOperations;
             var multicastTransportOperations = outgoingMessages.MulticastTransportOperations;
 
+            AssertBelowMaxMessageThresholdForTransaction(unicastTransportOperations, multicastTransportOperations, committableTransaction);
+
             var tasks = new List<Task>(unicastTransportOperations.Count + multicastTransportOperations.Count);
 
             foreach (var transportOperation in unicastTransportOperations)
@@ -66,6 +71,40 @@ public Task Dispatch(TransportOperations outgoingMessages, TransportTransaction
             return tasks.Count == 1 ? tasks[0] : Task.WhenAll(tasks);
         }
 
+        /// <summary>
+        /// Throws an exception if attempting to send more than 100 messages in a transaction.
+        /// This prevents the transport from experiencing this issue https://github.com/Azure/azure-sdk-for-net/issues/37265
+        /// </summary>
+        static void AssertBelowMaxMessageThresholdForTransaction(List<UnicastTransportOperation> unicastTransportOperations, List<MulticastTransportOperation> multicastTransportOperations, Transaction transaction)
+        {
+            var totalNumberOfOperations = unicastTransportOperations.Count + multicastTransportOperations.Count;
+            if (transaction == null || totalNumberOfOperations <= MaxMessageThresholdForTransaction)
+            {
+                return;
+            }
+
+            var numberOfTransactionalOperations = 0;
+            foreach (var transportOperation in unicastTransportOperations)
+            {
+                if (transportOperation.RequiredDispatchConsistency == DispatchConsistency.Default)
+                {
+                    numberOfTransactionalOperations++;
+                }
+            }
+            foreach (var transportOperation in multicastTransportOperations)
+            {
+                if (transportOperation.RequiredDispatchConsistency == DispatchConsistency.Default)
+                {
+                    numberOfTransactionalOperations++;
+                }
+            }
+
+            if (numberOfTransactionalOperations > MaxMessageThresholdForTransaction)
+            {
+                throw new Exception($"The number of outgoing messages ({numberOfTransactionalOperations}) exceeds the limits permitted by Azure Service Bus ({MaxMessageThresholdForTransaction}) in a single transaction");
+            }
+        }
+
         static async Task DispatchOperation(ServiceBusSender sender, ServiceBusMessage message, CommittableTransaction transactionToUse)
         {
             using (var scope = transactionToUse.ToScope())