Implement Activity.AddLink

src/libraries/System.Diagnostics.DiagnosticSource/ref/System.Diagnostics.DiagnosticSourceActivity.cs

@@ -50,6 +50,7 @@ public string? Id
         public string? TraceStateString { get { throw null; } set { } }
         public System.Diagnostics.Activity AddBaggage(string key, string? value) { throw null; }
         public System.Diagnostics.Activity AddEvent(System.Diagnostics.ActivityEvent e) { throw null; }
+        public System.Diagnostics.Activity AddLink(System.Diagnostics.ActivityLink link) { throw null; }
         public System.Diagnostics.Activity AddTag(string key, string? value) { throw null; }
         public System.Diagnostics.Activity AddTag(string key, object? value) { throw null; }
         public System.Diagnostics.Activity SetTag(string key, object? value) { throw null; }

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs

@@ -517,6 +517,21 @@ public Activity AddEvent(ActivityEvent e)
             return this;
         }
 
+        /// <summary>
+        /// Add an <see cref="ActivityLink"/> to the <see cref="Links"/> list.
+        /// </summary>
+        /// <param name="link">The <see cref="ActivityLink"/> to add.</param>
+        /// <returns><see langword="this" /> for convenient chaining.</returns>
+        public Activity AddLink(ActivityLink link)
+        {
+            if (_links != null || Interlocked.CompareExchange(ref _links, new DiagLinkedList<ActivityLink>(link), null) != null)
+            {
+                _links.Add(link);
+            }
+
+            return this;
+        }
+
         /// <summary>
         /// Update the Activity to have baggage with an additional 'key' and value 'value'.
         /// This shows up in the <see cref="Baggage"/> enumeration as well as the <see cref="GetBaggageItem(string)"/>

src/libraries/System.Diagnostics.DiagnosticSource/tests/ActivityTests.cs

@@ -1597,6 +1597,31 @@ public void TestEvent()
             Assert.Equal(0, activity.Events.ElementAt(1).Tags.Count());
         }
 
+        [Fact]
+        public void AddLinkTest()
+        {
+            ActivityContext c1 = new ActivityContext(ActivityTraceId.CreateRandom(), ActivitySpanId.CreateRandom(), ActivityTraceFlags.None);
+            ActivityContext c2 = new ActivityContext(ActivityTraceId.CreateRandom(), ActivitySpanId.CreateRandom(), ActivityTraceFlags.None);
+
+            ActivityLink l1 = new ActivityLink(c1);
+            ActivityLink l2 = new ActivityLink(c2, new ActivityTagsCollection()
+            {
+                new KeyValuePair<string, object?>("foo", 99)
+            });
+
+            Activity activity = new Activity("LinkTest");
+            Assert.True(ReferenceEquals(activity, activity.AddLink(l1)));
+            Assert.True(ReferenceEquals(activity, activity.AddLink(l2)));
+
+            ActivityLink[] links = activity.Links.ToArray();
+            Assert.Equal(2, links.Length);
+            Assert.Equal(c1, links[0].Context);
+            Assert.Equal(c2, links[1].Context);
+            KeyValuePair<string, object> tag = links[1].Tags.Single();
+            Assert.Equal("foo", tag.Key);
+            Assert.Equal(99, tag.Value);
+        }
+
         [Fact]
         public void TestIsAllDataRequested()
         {