MVC doesn't buffer IAsyncEnumerable types when using System.Text.Json

In ASP.NET Core 5, MVC added support for output formatting xref:System.Collections.Generic.IAsyncEnumerable%601 types by buffering the sequence in memory and formatting the buffered collection. In ASP.NET Core 6, when formatting using xref:System.Text.Json?displayProperty=fullName, MVC no longer buffers xref:System.Collections.Generic.IAsyncEnumerable%601 instances. Instead, MVC relies on the support that xref:System.Text.Json?displayProperty=fullName added for these types.

In most cases, the absence of buffering would not be observable by the application. However, some scenarios may have inadvertently relied on the buffering semantics to correctly serialize. For example, returning an xref:System.Collections.Generic.IAsyncEnumerable%601 that's backed by an Entity Framework query on a type with lazy-loaded properties can result in concurrent query execution, which might not be supported by the provider.

This change does not affect output formatting using Newtonsoft.Json or with XML-based formatters.

Version introduced

ASP.NET Core 6.0

Old behavior

xref:System.Collections.Generic.IAsyncEnumerable%601 instances returned from an MVC action as a value to be formatted using xref:System.Data.Objects.ObjectResult or a xref:System.Web.Mvc.JsonResult are buffered before being serialized as a synchronous collection.

New behavior

When formatting using xref:System.Text.Json?displayProperty=fullName, MVC no longer buffers xref:System.Collections.Generic.IAsyncEnumerable%601 instances.

Reason for change

xref:System.Text.Json?displayProperty=fullName added support for streaming xref:System.Collections.Generic.IAsyncEnumerable%601 types. This allows for a smaller memory footprint during serialization.

Recommended action

If your application requires buffering, consider manually buffering the xref:System.Collections.Generic.IAsyncEnumerable%601 object:

// Before
public IActionResult Get()
{
    return Ok(dbContext.Blogs);
}

// After
public async Task<IActionResult> Get()
{
    return Ok(await dbContext.Blogs.ToListAsync());
}