Write default values by default in Json (#23542)

src/SignalR/common/Protocols.Json/src/Protocol/JsonHubProtocol.cs

@@ -773,7 +773,7 @@ namespace Microsoft.AspNetCore.SignalR.Protocol
                 WriteIndented = false,
                 ReadCommentHandling = JsonCommentHandling.Disallow,
                 AllowTrailingCommas = false,
-                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault,
+                DefaultIgnoreCondition = JsonIgnoreCondition.Never,
                 IgnoreReadOnlyProperties = false,
                 PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
                 PropertyNameCaseInsensitive = true,

src/SignalR/common/SignalR.Common/test/Internal/Protocol/JsonHubProtocolTests.cs

@@ -28,7 +28,7 @@ namespace Microsoft.AspNetCore.SignalR.Common.Tests.Internal.Protocol
             {
                 PayloadSerializerOptions = new JsonSerializerOptions()
                 {
-                    IgnoreNullValues = ignoreNullValues,
+                    DefaultIgnoreCondition = ignoreNullValues ? System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault : System.Text.Json.Serialization.JsonIgnoreCondition.Never,
                     PropertyNamingPolicy = useCamelCase ? JsonNamingPolicy.CamelCase : null,
                     Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
                 }

src/SignalR/common/SignalR.Common/test/Internal/Protocol/JsonHubProtocolTestsBase.cs

@@ -314,6 +314,33 @@ namespace Microsoft.AspNetCore.SignalR.Common.Tests.Internal.Protocol
             }, streamItemMessage.Item);
         }
 
+        [Fact]
+        public void DefaultValuesAreWrittenByDefault()
+        {
+            var obj = new CustomObject()
+            {
+                ByteArrProp = new byte[] { 2, 4, 6 },
+                IntProp = default,
+                DoubleProp = 1.1,
+                StringProp = "test",
+                DateTimeProp = default
+            };
+            var expectedOutput = Frame("{\"type\":1,\"invocationId\":\"123\",\"target\":\"Target\",\"arguments\":[{\"stringProp\":\"test\",\"doubleProp\":1.1,\"intProp\":0,\"dateTimeProp\":\"0001-01-01T00:00:00\",\"nullProp\":null,\"byteArrProp\":\"AgQG\"}]}");
+
+            var writer = MemoryBufferWriter.Get();
+            try
+            {
+                JsonHubProtocol.WriteMessage(new InvocationMessage("123", "Target", new object[] { obj }), writer);
+                var json = Encoding.UTF8.GetString(writer.ToArray());
+
+                Assert.Equal(expectedOutput, json);
+            }
+            finally
+            {
+                MemoryBufferWriter.Return(writer);
+            }
+        }
+
         public static IDictionary<string, MessageSizeTestData> MessageSizeData => new[]
         {
             new MessageSizeTestData("InvocationMessage_WithoutInvocationId", new InvocationMessage("Target", new object[] { 1 }), 45),