From 2d8dfb1ddcff40cfb5da4386d1ed11aa8fd91db0 Mon Sep 17 00:00:00 2001
From: Andrew Arnott <andrew.arnott@microsoft.com>
Date: Fri, 17 May 2024 14:48:49 -0600
Subject: [PATCH] Fix System.Text.Json formatter's support for named arguments
 object

Fixes #1028
---
 src/StreamJsonRpc/SystemTextJsonFormatter.cs |  3 +--
 test/StreamJsonRpc.Tests/JsonRpcTests.cs     | 13 +++++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/StreamJsonRpc/SystemTextJsonFormatter.cs b/src/StreamJsonRpc/SystemTextJsonFormatter.cs
index 5e41a5fc..54893f80 100644
--- a/src/StreamJsonRpc/SystemTextJsonFormatter.cs
+++ b/src/StreamJsonRpc/SystemTextJsonFormatter.cs
@@ -487,9 +487,8 @@ public override ArgumentMatchResult TryGetTypedArguments(ReadOnlySpan<ParameterI
             using (this.formatter.TrackDeserialization(this, parameters))
             {
                 // Support for opt-in to deserializing all named arguments into a single parameter.
-                if (parameters.Length == 1 && this.formatter.ApplicableMethodAttributeOnDeserializingMethod?.UseSingleObjectParameterDeserialization is true)
+                if (parameters.Length == 1 && this.formatter.ApplicableMethodAttributeOnDeserializingMethod?.UseSingleObjectParameterDeserialization is true && this.JsonArguments is not null)
                 {
-                    Assumes.NotNull(this.JsonArguments);
                     typedArguments[0] = this.JsonArguments.Value.Deserialize(parameters[0].ParameterType, this.formatter.massagedUserDataSerializerOptions);
                     return ArgumentMatchResult.Success;
                 }
diff --git a/test/StreamJsonRpc.Tests/JsonRpcTests.cs b/test/StreamJsonRpc.Tests/JsonRpcTests.cs
index 4a795c5c..799598b0 100644
--- a/test/StreamJsonRpc.Tests/JsonRpcTests.cs
+++ b/test/StreamJsonRpc.Tests/JsonRpcTests.cs
@@ -1453,6 +1453,13 @@ public async Task InvokeWithSingleObjectParameter_SendingExpectedObjectAndCancel
         Assert.Equal(7, sum);
     }
 
+    [Fact]
+    public async Task InvokeWithSingleObjectParameter_SupplyNoArgument()
+    {
+        int sum = await this.clientRpc.InvokeWithParameterObjectAsync<int>(nameof(Server.MethodWithSingleObjectParameterWithDefaultValue), cancellationToken: this.TimeoutToken);
+        Assert.Equal(-1, sum);
+    }
+
     [Fact]
     public async Task InvokeWithSingleObjectParameter_SendingExpectedObjectAndCancellationToken_InterfaceMethodAttributed()
     {
@@ -3337,6 +3344,12 @@ public static int MethodWithSingleObjectParameterAndCancellationToken(XAndYPrope
             return fields.x + fields.y;
         }
 
+        [JsonRpcMethod(UseSingleObjectParameterDeserialization = true)]
+        public static int MethodWithSingleObjectParameterWithDefaultValue(XAndYProperties? arg = null)
+        {
+            return arg is not null ? arg.x + arg.y : -1;
+        }
+
         [JsonRpcMethod("test/MethodWithSingleObjectParameterWithProgress", UseSingleObjectParameterDeserialization = true)]
         public static int MethodWithSingleObjectParameterWithProgress(XAndYPropertiesWithProgress fields)
         {