revision check implemented

StudentEnrollment03.Esdb/Program.cs

@@ -87,34 +87,42 @@
 var settings = EventStoreClientSettings.Create(connectionString);
 var client = new EventStoreClient(settings);
 
+// Append the initial batch of events.
 await client.AppendToStreamAsync(
     streamId,
     StreamState.Any,
     new[] { created, enrolled, enrolled2, enrolled3, emailChanged },
     cancellationToken: default
 );
 
-await client.AppendToStreamAsync(
-    streamId,
-    new StreamRevision(), // StreamState.StreamExists, // TODO
-    new[] {  withdrawn },
-    cancellationToken: default
-);
-
+// Read from the stream we just appended to.
 var streamResult = client.ReadStreamAsync(
     Direction.Forwards,
     streamId,
     StreamPosition.Start,
     cancellationToken: default
 );
 
+// Optional safety check, but here we're ensuring the stream was found.
 if (await streamResult.ReadState is ReadState.StreamNotFound)
 {
     Console.WriteLine($"The fetched stream (id: {streamId}) that was read was in StreamNotFound state");
     return;
 }
 
+// Okay, taking that StreamResult we're going to make it a List of ResolvedEvents.
 var eventStream = await streamResult.ToListAsync();
+// Get the last event's event number.
+var lastEventInStream = eventStream.Last().Event.EventNumber.ToUInt64();
+
+// Append another event. This time let's make sure no one has appended to (AKA updated) the stream.
+await client.AppendToStreamAsync(
+    streamId,
+    new StreamRevision(lastEventInStream), // checking against expected revision number
+    new[] {  withdrawn },
+    cancellationToken: default
+);
+
 var student = new Student();
 
 Console.WriteLine($"Events (total: {eventStream.Count}) from selected stream: ");