IAsyncEnumerableOfTConverter<TAsyncEnumerable, TElement> throws OutOfMemoryException when custom JsonConverter involved

[kikaragyozov]

If a consumer has defined a custom JsonConverter for an interface, to allow polymorphism, for example something like this:

public class CustomInterfaceConverter : JsonConverter<ICustomInterface>
{
	public override ICustomInterface? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
	{
		// Convert the C# property name to the expected JSON property name based on JsonSerializerOptions
		string typePropertyName = options.PropertyNamingPolicy?.ConvertName("Type") ?? "Type";

		using (JsonDocument doc = JsonDocument.ParseValue(ref reader))
		{
			JsonElement root = doc.RootElement;
			JsonElement typeProp;

			// Check for property name, considering case insensitivity
			if (options.PropertyNameCaseInsensitive)
			{
				if (!TryGetPropertyCaseInsensitive(root, typePropertyName, out typeProp))
				{
					throw new JsonException($"Type property '{typePropertyName}' is missing");
				}
			}
			else
			{
				if (!root.TryGetProperty(typePropertyName, out typeProp))
				{
					throw new JsonException($"Type property '{typePropertyName}' is missing");
				}
			}

			CustomInterfaceType type = (CustomInterfaceType)typeProp.GetInt32();
			return type switch
			{
				CustomInterfaceType.One => JsonSerializer.Deserialize<One>(root.GetRawText(), options),
				CustomInterfaceType.Two => JsonSerializer.Deserialize<Two>(root.GetRawText(), options),
				CustomInterfaceType.Three => JsonSerializer.Deserialize<Three>(root.GetRawText(), options),
				_ => throw new JsonException("Unknown type."),
			};
		}
	}

	public override void Write(Utf8JsonWriter writer, ICustomInterface value, JsonSerializerOptions options)
	{
		JsonSerializer.Serialize(writer, value, value.GetType(), options);
	}

	private static bool TryGetPropertyCaseInsensitive(JsonElement element, string name, out JsonElement value)
	{
		foreach (JsonProperty prop in element.EnumerateObject())
		{
			if (string.Equals(prop.Name, name, StringComparison.OrdinalIgnoreCase))
			{
				value = prop.Value;
				return true;
			}
		}

		value = default;
		return false;
	}
}

And you then try to return it as an IAsyncEnumerable<ICustomInterface>, you end up getting an OutOfMemoryException because the content is never flushed by the IAsyncEnumerableOfTConverter<TAsyncEnumerable, TElement>.

Workarounds

Using IAsyncEnumerable<object> without a custom JsonConverter<> works as expected. You can then use the custom JsonConverter<> in the client code to consume it as an interface type, like so:

await foreach (var message in JsonSerializer.DeserializeAsyncEnumerable<ICustomInterface>(stream, jsonOptions)
{
   // Your logic here.
}

Is this expected?

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[eiriktsarpalis]

Presumably each individual element is too large to fit in-memory? This is a known restriction of custom converters that do not support streaming, and is not directly related to IAsyncEnumerable. This is being tracked by this issue #63795

[kikaragyozov]

Presumably each individual element is too large to fit in-memory? This is a known restriction of custom converters that do not support streaming, and is not directly related to IAsyncEnumerable. This is being tracked by this issue #63795

Each individual element is not too large to fit into memory. But rather, ShouldFlush never returns true and the collection keeps getting bigger in the buffer, never being flushed out to the caller.

[eiriktsarpalis]

Can you share a repro?

[kikaragyozov]

Can you share a repro?

I'll be able to provide one later tomorrow. I'll ping you as soon as I got one running!

[eiriktsarpalis]

Thanks, would appreciate if it could be made as minimal as possible, preferably a small console app.

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[BrennanConroy]

You mentioned ShouldFlush but also show using DeserializeAsyncEnumerable which are unrelated. ShouldFlush is only used for Serializing, not Deserializing.

Ignoring that discrepancy... from your code you have a similar pattern in your Write method to a similar issue #66102 which has been fixed in 9.0.

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