Update SseItem's ctor eventType argument to be nullable

src/libraries/System.Net.ServerSentEvents/ref/System.Net.ServerSentEvents.cs

@@ -12,7 +12,7 @@ public readonly partial struct SseItem<T>
         private readonly T _Data_k__BackingField;
         private readonly object _dummy;
         private readonly int _dummyPrimitive;
-        public SseItem(T data, string eventType) { throw null; }
+        public SseItem(T data, string? eventType) { throw null; }
         public T Data { get { throw null; } }
         public string EventType { get { throw null; } }
     }

src/libraries/System.Net.ServerSentEvents/src/System/Net/ServerSentEvents/SseItem.cs

@@ -7,19 +7,22 @@ namespace System.Net.ServerSentEvents
     /// <typeparam name="T">Specifies the type of data payload in the event.</typeparam>
     public readonly struct SseItem<T>
     {
+        /// <summary>The event's type.</summary>
+        internal readonly string? _eventType;
+
         /// <summary>Initializes the server-sent event.</summary>
         /// <param name="data">The event's payload.</param>
         /// <param name="eventType">The event's type.</param>
-        public SseItem(T data, string eventType)
+        public SseItem(T data, string? eventType)
         {
             Data = data;
-            EventType = eventType;
+            _eventType = eventType;
         }
 
         /// <summary>Gets the event's payload.</summary>
         public T Data { get; }
 
         /// <summary>Gets the event's type.</summary>
-        public string EventType { get; }
+        public string EventType => _eventType ?? SseParser.EventTypeDefault;
     }
 }

src/libraries/System.Net.ServerSentEvents/tests/SseItemTests.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace System.Net.ServerSentEvents.Tests
+{
+    public partial class SseItemTests
+    {
+        [Fact]
+        public void SseItem_Roundtrips()
+        {
+            SseItem<string> item;
+
+            item = default;
+            Assert.Null(item.Data);
+            Assert.Equal(SseParser.EventTypeDefault, item.EventType);
+
+            item = new SseItem<string>("some data", null);
+            Assert.Equal("some data", item.Data);
+            Assert.Equal(SseParser.EventTypeDefault, item.EventType);
+
+            item = new SseItem<string>("some data", "eventType");
+            Assert.Equal("some data", item.Data);
+            Assert.Equal("eventType", item.EventType);
+        }
+    }
+}

src/libraries/System.Net.ServerSentEvents/tests/SseParserTests.cs

@@ -47,20 +47,6 @@ public async Task Parse_Async_SupportsOnlyOneEnumeration_Throws()
             Assert.Throws<InvalidOperationException>(() => e.MoveNext());
         }
 
-        [Fact]
-        public void SseItem_Roundtrips()
-        {
-            SseItem<string> item;
-
-            item = new SseItem<string>();
-            Assert.Null(item.EventType);
-            Assert.Null(item.Data);
-
-            item = new SseItem<string>("some data", "eventType");
-            Assert.Equal("eventType", item.EventType);
-            Assert.Equal("some data", item.Data);
-        }
-
         [Theory]
         [MemberData(nameof(NewlineTrickleAsyncData))]
         public async Task Parse_Empty_NoItems(string newline, bool trickle, bool useAsync)

src/libraries/System.Net.ServerSentEvents/tests/System.Net.ServerSentEvents.Tests.csproj

@@ -5,11 +5,12 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="SseItemTests.cs" />
     <Compile Include="SseParserTests.cs" />
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\src\System.Net.ServerSentEvents.csproj"/>
+    <ProjectReference Include="..\src\System.Net.ServerSentEvents.csproj" />
     <ProjectReference Include="$(LibrariesProjectRoot)System.Text.Json\gen\System.Text.Json.SourceGeneration.Roslyn4.4.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" SetTargetFramework="TargetFramework=netstandard2.0" />
   </ItemGroup>