From 3c39996f5ef6a6de0ade93019878de9b8d4d6f4c Mon Sep 17 00:00:00 2001
From: Peter Butzhammer <peter.butzhammer@evopro-ag.de>
Date: Wed, 7 Feb 2024 13:33:50 +0100
Subject: [PATCH] Make CreateNotification for nonexistent variable fail with
 Exception

---
 Sharp7.Rx/Sharp7Plc.cs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/Sharp7.Rx/Sharp7Plc.cs b/Sharp7.Rx/Sharp7Plc.cs
index 974619d..0265a80 100644
--- a/Sharp7.Rx/Sharp7Plc.cs
+++ b/Sharp7.Rx/Sharp7Plc.cs
@@ -85,8 +85,14 @@ public class Sharp7Plc : IPlc
             var disposeableContainer = multiVariableSubscriptions.GetOrCreateObservable(variableName);
             disposeableContainer.AddDisposableTo(disposables);
 
-            var observable = disposeableContainer.Observable
-                .Select(bytes => S7ValueConverter.ConvertToType<TValue>(bytes, address));
+            var observable =
+                // Directly read variable first.
+                // This will propagate any errors due to reading from invalid addresses.
+                Observable.FromAsync(() => GetValue<TValue>(variableName))
+                    .Concat(
+                        disposeableContainer.Observable
+                            .Select(bytes => S7ValueConverter.ConvertToType<TValue>(bytes, address))
+                    );
 
             if (transmissionMode == TransmissionMode.OnChange)
                 observable = observable.DistinctUntilChanged();