SseItem response in Results.ServerSentEvents

[WeihanLi]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Results.ServerSentEvents implemented on #56172

When I use IAsyncEnumerable<SseItem<DateTimeOffset>> as the response type, it seems the response is not as expected

The response I got now with .NET 10 RC 2:

event: heartbeat
data: {"data":"2025-10-28T21:41:13.2483569+08:00","eventType":"message","eventId":"019a2b0d25a07d958cfe5ad7bbfb747a","reconnectionInterval":null}

event: heartbeat
data: {"data":"2025-10-28T21:41:14.2626512+08:00","eventType":"message","eventId":"019a2b0d299676098e504358dd0aa140","reconnectionInterval":null}

The message on Chrome Dev Tool > Network > EventStream panel

Expected Behavior

According to the spec from https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation

It should be as follows:

id: 019a2b0d25a07d958cfe5ad7bbfb747a
event: heartbeat
data: 2025-10-28T21:41:13.2483569+08:00

id: 019a2b0d299676098e504358dd0aa140
event: heartbeat
data: 2025-10-28T21:41:14.2626512+08:00

Steps To Reproduce

var app = WebApplication.Create();
app.MapGet("/sse", (CancellationToken cancellationToken) => 
            Results.ServerSentEvents(GetSseItem(cancellationToken), "heartbeat"));
await app.RunAsync();

static async IAsyncEnumerable<SseItem<DateTimeOffset>> GetSseItem(
    [EnumeratorCancellation]CancellationToken cancellationToken
)
{
    while (!cancellationToken.IsCancellationRequested)
    {
        await Task.Delay(1000, cancellationToken);
        yield return new SseItem<DateTimeOffset>(DateTimeOffset.Now)
        {
            EventId = Guid.CreateVersion7().ToString("N"),
            // ReconnectionInterval = TimeSpan.FromSeconds(10)
        };
    }
}

Exceptions (if any)

No response

.NET Version

10.0.100-rc.2.25502.107

Anything else?

ASP.NET Core 10 RC 2

dotnet --info output:

.NET SDK:
 Version:           10.0.100-rc.2.25502.107
 Commit:            89c8f6a112
 Workload version:  10.0.100-manifests.4d32cd9e
 MSBuild version:   18.0.0-preview-25502-107+89c8f6a11

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.26200
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\10.0.100-rc.2.25502.107\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
  Version:      10.0.0-rc.2.25502.107
  Architecture: x64
  Commit:       89c8f6a112

.NET SDKs installed:
  6.0.428 [C:\Program Files\dotnet\sdk]
  8.0.206 [C:\Program Files\dotnet\sdk]
  8.0.415 [C:\Program Files\dotnet\sdk]
  9.0.111 [C:\Program Files\dotnet\sdk]
  9.0.205 [C:\Program Files\dotnet\sdk]
  9.0.306 [C:\Program Files\dotnet\sdk]
  10.0.100-rc.2.25502.107 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.10 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.0-rc.2.25502.107 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.21 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.0-rc.2.25502.107 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.21 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.10 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 10.0.0-rc.2.25502.107 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  DOTNET_CLI_TELEMETRY_OPTOUT              [1]
  DOTNET_PRINT_TELEMETRY_MESSAGE           [false]

global.json file:
  Not found

Learn more:
  https://aka.ms/dotnet/info

Download .NET:
  https://aka.ms/dotnet/download

[WeihanLi]

@captainsafia could you please help on this, since it's a new feature, think we should keep it aligned with the spec on GA(not sure if we can make it for .NET 10 GA, hope we can make it)

[WeihanLi]

Oh, my bad, I'm using it in the wrong way.

When adding the eventType, it calls the IAsyncEnumerable<T> overload.
When using SseItem, we should specify the eventType when constructing the SseItem

works as expected when moving the eventType to the SseItem constructor

closing now