Fix ReadOnlyMemory/Memory deserialization during stream continuation with null elements

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/MemoryConverter.cs

@@ -18,7 +18,7 @@ internal override bool OnTryRead(
             scoped ref ReadStack state,
             out Memory<T> value)
         {
-            if (reader.TokenType is JsonTokenType.Null)
+            if (reader.TokenType is JsonTokenType.Null && state.Current.ReturnValue is null)
             {
                 value = default;
                 return true;

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Collection/ReadOnlyMemoryConverter.cs

@@ -18,7 +18,7 @@ internal override bool OnTryRead(
             scoped ref ReadStack state,
             out ReadOnlyMemory<T> value)
         {
-            if (reader.TokenType is JsonTokenType.Null)
+            if (reader.TokenType is JsonTokenType.Null && state.Current.ReturnValue is null)
             {
                 value = default;
                 return true;

src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.Memory.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Xunit;
 
@@ -137,5 +139,39 @@ public async Task DeserializeMemoryByteClassAsync()
             MemoryOfTClass<byte> memoryOfByteClass = await Serializer.DeserializeWrapper<MemoryOfTClass<byte>>(json);
             AssertExtensions.SequenceEqual<byte>(s_testData.AsSpan(), memoryOfByteClass.Memory.Span);
         }
+
+        [Fact]
+        public async Task DeserializeReadOnlyMemoryFromStreamWithManyNulls()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/118346
+            if (StreamingSerializer is null)
+            {
+                return;
+            }
+
+            // Create an array of ~200 null elements to force resumptions with smallest buffer size
+            string json = $"[{string.Join(",", Enumerable.Repeat("null", 200))}]";
+            using var stream = new Utf8MemoryStream(json);
+            ReadOnlyMemory<string?> result = await StreamingSerializer.DeserializeWrapper<ReadOnlyMemory<string?>>(stream);
+
+            Assert.Equal(200, result.Length);
+        }
+
+        [Fact]
+        public async Task DeserializeMemoryFromStreamWithManyNulls()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/118346
+            if (StreamingSerializer is null)
+            {
+                return;
+            }
+
+            // Create an array of ~200 null elements to force resumptions with smallest buffer size
+            string json = $"[{string.Join(",", Enumerable.Repeat("null", 200))}]";
+            using var stream = new Utf8MemoryStream(json);
+            Memory<string?> result = await StreamingSerializer.DeserializeWrapper<Memory<string?>>(stream);
+
+            Assert.Equal(200, result.Length);
+        }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/CollectionTests.cs

@@ -461,6 +461,8 @@ public async Task DeserializeAsyncEnumerable()
         [JsonSerializable(typeof(StackWrapper))]
         [JsonSerializable(typeof(ClassWithRecursiveCollectionTypes))]
         [JsonSerializable(typeof(MemoryOfTClass<byte>))]
+        [JsonSerializable(typeof(ReadOnlyMemory<string>))]
+        [JsonSerializable(typeof(Memory<string>))]
         [JsonSerializable(typeof(ReadOnlyMemoryOfTClass<byte>))]
         [JsonSerializable(typeof(MemoryOfTClass<int>))]
         [JsonSerializable(typeof(ReadOnlyMemoryOfTClass<int>))]
@@ -883,6 +885,8 @@ public CollectionTests_Default()
         [JsonSerializable(typeof(StackWrapper))]
         [JsonSerializable(typeof(ClassWithRecursiveCollectionTypes))]
         [JsonSerializable(typeof(MemoryOfTClass<byte>))]
+        [JsonSerializable(typeof(ReadOnlyMemory<string>))]
+        [JsonSerializable(typeof(Memory<string>))]
         [JsonSerializable(typeof(ReadOnlyMemoryOfTClass<byte>))]
         [JsonSerializable(typeof(MemoryOfTClass<int>))]
         [JsonSerializable(typeof(ReadOnlyMemoryOfTClass<int>))]