From 0232e47f4daeff2523edd345279f4acd53f6c40a Mon Sep 17 00:00:00 2001
From: Sipke Schoorstra <sipkeschoorstra@outlook.com>
Date: Tue, 5 Oct 2021 17:36:15 +0200
Subject: [PATCH] Backwards compatibility for older workflow instances

---
 src/core/Elsa.Abstractions/Models/WorkflowInstance.cs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/core/Elsa.Abstractions/Models/WorkflowInstance.cs b/src/core/Elsa.Abstractions/Models/WorkflowInstance.cs
index c3c864087..7121e5dd5 100644
--- a/src/core/Elsa.Abstractions/Models/WorkflowInstance.cs
+++ b/src/core/Elsa.Abstractions/Models/WorkflowInstance.cs
@@ -34,7 +34,14 @@ namespace Elsa.Models
         public WorkflowInputReference? Input { get; set; }
         public WorkflowOutputReference? Output { get; set; }
         public IDictionary<string, IDictionary<string, object?>> ActivityData { get; set; } = new Dictionary<string, IDictionary<string, object?>>();
-        public IDictionary<string, object?> Metadata { get; set; } = new Dictionary<string, object?>();
+
+        // To prevent NRE when old workflow instances are deserialized. 
+        private IDictionary<string, object?>? _metadata;
+        public IDictionary<string, object?> Metadata
+        {
+            get { return _metadata ??= new Dictionary<string, object?>(); }
+            set => _metadata = value ?? new Dictionary<string, object?>();
+        }
 
         public HashSet<BlockingActivity> BlockingActivities
         {
@@ -43,7 +50,6 @@ namespace Elsa.Models
         }
 
         public object? GetMetadata(string key) => Metadata.TryGetValue(key, out var value) ? value : default;
-
         public void SetMetadata(string key, object? value)
         {
             Metadata[key] = value;